Population genetics of harbour porpoise in Swedish warters ... · Report 5419 – Population...

Population genetics of harbour porpoise

in Swedish waters– a literature review

REPORT 5419 • NOVEMBER 2004

Efter den 1 juli 2011 ansvarar Havs- och vattenmyndigheten för denna publikation.Telefon 010-698 60 [email protected]/publikationer

Population genetics of harbourporpoise in Swedish waters

– a literature review

Anna Palmé, Linda Laikre, and Nils Ryman

Department of ZoologyDivision of Population Genetics

Stockholm UniversityS-106 91 Stockholm, Sweden

THE SWEDISH ENVIRONMENTAL PROTECTION AGENCYReport 5419 – Population genetics of harbour porpoise in Swedish waters

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Cover photo: Florian Granér/NaturbildISBN: 91-620-5419-8.pdfISSN: 0282-7298

Electronic publication

„ The Swedish EnvironmentalProtection Agency 2004


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Preface

This report is aimed at reviewing current knowledge on the genetic population structureof harbour porpoise (Phocoena phocoena) in Swedish and adjacent waters (here definedto include the Baltic, Kattegat, and Skagerrak Seas). The review was initiated by theSwedish Environmental Protection Agency to provide conservation and managementguidelines for the harbour porpoise.

We thank Per Berggren for valuable information, discussions, and comments on themanuscript. We are indebted to Liselotte W. Andersen, Julia Creek, Sture Hansson,Christina Lockyer, Annika Tidlund, and Håkan Westerberg for various types ofinformation, and to Stefan Palm, Carl André, and an anonymous reviewer for commentson a previous version of this text.

The study was financed by the Swedish Environmental Protection Agency. Supportfrom the Swedish Research Council and the Swedish Research Council for Environment,Agricultural Sciences and Spatial Planning to Nils Ryman and Linda Laikre is alsogreatly acknowledged.

Stockholm in November 2004

Anna Palmé, Linda Laikre, Nils RymanDepartment of ZoologyDivision of Population GeneticsStockholm UniversitySweden


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Contents

Summary 6

1 Introduction 9

1.2 The organization of this report 9

2 The harbour porpoise - brief background 11

2.1 Distribution 112.2 Population status 122.3 Intrinsic growth rate 122.4 Threats to harbour porpoise populations 12

Potential threats in Swedish waters 132.5 International agreements 14

3 Biological diversity on the gene level - brief background 15

Nuclear and mitochondrial DNA 15Selection, migration, mutation 15

3.1 Loss of genetic diversity 16Effective population size 17

3.2 Methods for studying gentic variation. 17

4 Literature review 19

4.1 Genetic divergence among major distribution areas 194.2 Population differentiation within major distribution areas 20

North Pacific Ocean 20North Atlantic 21

5 Population genetic structure in Swedish and adjacent waters 23

5.1 Inconsistent definitions of geographic areas 235.2 Population genetic studies 24

6 Discussions and conclusions 26

Genetic structure 26Number of populations in Swedish and adjacent waters 26Implications for management 28

7 References 29

Tables and figures 38

APPENDIX 53


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Summary

This report is aimed at reviewing current knowledge on the genetic population structureof harbour porpoise (Phocoena phocoena) in Swedish and adjacent waters (here definedto include the Baltic, Belt, Kattegat, and Skagerrak Seas). Population genetic informationis necessary for effective conservation and management of the species, and one of the keyquestions refers to how many genetically distinct populations of porpoises that exist inthis region.

The review was conducted within the framework of ongoing efforts of the SwedishEnvironmental Protection Agency to provide conservation and management guidelinesfor the harbour porpoise. No new data has been collected; the review is based exclusivelyon previously published information.

The harbour porpoise is regarded as threatened in most parts of its distribution range,which includes the three major geographic areas of the North Pacific Ocean, the NorthAtlantic, and the Black Sea/Sea of Azov. The global threat status of the species is"vulnerable". In Sweden, the abundance has declined during the second half of the 20thcentury, and the population reduction is considered particularly dramatic within the BalticSea, where the census size is estimated to only a few hundred individuals.

We collected previously published scientific studies that directly or indirectly addressquestions concerning the genetic population structure of the harbour porpoise throughsearch of literature databases. We summarize and discuss the conclusions of those studiesfocusing on papers that are based on clearcut genetic data obtained by means of variousmolecular techniques. Several workers address the issue of population differentiationthrough studying characters for which the genetic basis is not fully understood (e.g., skullor tooth morphology, time for reproduction, etc.), and it is therefore not clear to whatextent the variation of such characters reflect divergence on the gene level. Some of thosestudies are referred to in this report, but our discussion is based exclusively on the resultsof the "direct" genetic studies.

The report also includes background sections on harbour porpoise biology and popula-tion status, as well as basic information on biological diversity on the gene level (geneticdiversity), processes that affects this diversity, and methods for studying populationgenetic variability patterns.

Studies of population structure

We found 50 publications dealing with population differentiation in harbour porpoises.Those papers include all the 16 reports using "direct" genetic data that we have been ableto identify. They also include all the 17 papers using morphological data that we found,16 examples of studies using various types of ecological information, and one study usingbiogeography/ distribution data (Table 1).

The majority (11 out of 16) of the genetic studies have analyzed genetic variation ofthe mtDNA, only seven peer reviewed studies use nuclear DNA markers (microsatellitesand allozymes). Thus, most of the present knowledge of the genetic population structureof harbour porpoise on a global scale is based on variation of mtDNA that is maternallyinherited and provides no information on the composition of the nuclear genome. Of the


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16 studies based on "direct" molecular genetic markers, seven include samples fromSwedish and adjacent waters.

On a world wide geographic scale, the mtDNA differences among porpoises from thethree major distribution basins (North Pacific, North Atlantic, and Black Sea/Sea ofAzarov) are so large that it might be justified to classify those groups as differentsubspecies. There is also genetic heterogeneity within major basins with statisticallysignificant allele or haplotype frequency differences among many, or most, of the regionssampled within basins. The amount of genetic differentiation observed between areaswithin major basins is generally quite small. For the North Atlantic, pairwise FST-valuesbetween regions such as West Greenland, Norway, Ireland, and "IDW" are typicallyaround 0.01 or less for nuclear genes. The overall genetic population structure of thespecies is by no means resolved.

Number of populations in Swedish and adjacent waters

There are seven reports on population genetic structure of harbour porpoises in Swedishand adjacent waters. Two studies are based on mtDNA (Tiedemann et al. 1996; Wang &Berggren 1997), three on microsatellites (Andersen et al. 1995, 1997, 2001), two useallozymes (Andersen 1993, Andersen et al. 1997), and one study deals with RAPDvariation (Stensland 1997). Five of the seven studies report significant genetic differencesamong sampling areas within Swedish and adjacent waters, although the samplinglocations and/or the patterns of differentiation are not necessarily the same.

There are considerable difficulties to compare information from the seven studies.First, the definition of geographic areas varies among authors, frequently reflecting ageneral lack of strictness of the nomenclature used for various parts of the water bodiessurrounding Sweden and Denmark. Second, in several cases the exact location for samplecollection is not clearly presented; the geographic origin of analyzed specimens is sometimes given only as an ambiguous location name rather than as geographic coordinates.Further, some studies are based on rather small sample sizes (tens of individuals), and thestatistical evaluation and/or presentation of the results is in some cases incomplete ormisleading.

Five studies report small but statistically significant genetic differences betweenharbour porpoises of the North Sea and Skagerrak on one hand and the Katte-gat/Belt/Baltic area (east of Skagerrak) on the other. Only two studies include samplesthat make it possible to address the issue of genetic heterogeneity within the Katte-gat/Belt/Baltic area east of the North Sea, and neither of those studies provide unambigu-ous evidence for the existence of a substructure within this region.

On the basis of presently published information there seems to be consensus on theexistence of a minimum of two populations in Swedish and adjacent waters. The areaaround Skagerrak appears to hold a population of porpoises (that may extend into theNorth Sea, and perhaps into Kattegat) that is genetically distinct from porpoises furthereast.

With respect to the question of the existence of more than two populations, it appearsthat this issue cannot be settled from presently reported information. There are indicationsof genetic heterogeneities within the area comprising the Baltic, Belt, and Katte-gat/Skagerrak Seas, but the underlying biology reflected by these heterogeneities is notclear.


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We do not think that further sampling represents the most urgent line of action, rather,it appears that additional statistical evaluation of available genetic data may be an easierway of providing further information on the population genetic structure within Swedishand adjacent waters. For example, the question of isolation by distance should beexamined, e.g. through testing for associations between geographic and genetic distance.

Conclusions

The main conclusions of this review include:

• A minimum of two porpoise populations appear to inhabit Swedish and adjacentwaters. Porpoises in the Skagerrak area, are genetically different from those furthereast.

• Genetic heterogeneity appears to occur within the Kattegat, Belt, and Baltic Seas, butlocal populations have not been identified.

• There are shortcomings, particularly with respect to the statistical treatment, inseveral of the published studies on population genetic structure in Swedish and adja-cent waters.

• Inconsistent definitions of geographic areas among authors confuse interpretation ofresults.

• Important questions that have not been addressed in sufficient detail include:- Is the genetic structure of the harbour porpoise characterized by discretesubpopulations, or are there "clines" of continuous genetic change withoutdistinct boundaries?- Do the observed genetic heterogeneities reflect true spatial variation, or dothey only reflect "evolutionary noise" that is caused by e.g. differencesamong age classes or by temporal allele frequency change resulting in diffe-rences between samples collected several years apart?

• Until it has been clarified whether the area east of the North Sea and Skagerrakconsists of multiple distinct subpopulations, one homogenous population, or a clineof continuous genetic change the precautionary principle implies the following:

1. The North Sea and Skagerrak harbour porpoises should be treated as ge-netically distinct from those of the Kattegat/Belt/Baltic regions east of theNorth Sea and Skagerrak.2. The area east of the North Sea and Skagerrak should be managed as ifcomprising multiple subpopulations.3. The Baltic should be treated as if representing a distinct population.


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1 Introduction

This report is aimed at reviewing current knowledge on the genetic population structureof harbour porpoise (Phocoena phocoena) in Swedish and adjacent waters. The reviewwas conducted within the framework of ongoing efforts of the Swedish EnvironmentalProtection Agency to provide conservation and management guidelines for the species.No new data have been collected for the present review that is based exclusively onpreviously published information. Swedish and adjacent waters are defined here toinclude the Baltic, Belt, Kattegat, and Skagerrak Seas (including the Kiel and Mecklen-burg Bays).

The harbour porpoise is regarded as threatened in most parts of its distribution range(e.g. Berggren 1994; IUCN 1996; Hammond et al. 2002), primarily due to fisheriesby-catches (Perrin et al. 1994). In Sweden the abundance has declined during the secondhalf of the 20th century, and the species is currently classified as "vulnerable" in Swedenas well as globally (Ahlén & Tjernberg 1996; www.redlist.org). The total population sizein Swedish waters and adjacent waters is approximated to about 36 000 individuals(Hammond et al. 2002). The population decline is considered particularly dramatic withinthe Baltic Sea, where the census size some years ago was estimated to only 599individuals (Hiby & Lovell 1996), and a more recently conducted survey indicates aconsiderably smaller population (Dr. Per Berggren, pers. comm.).

Information on the genetic population structure of the harbour porpoise is crucial foreffective management and conservation. For instance, it is necessary to know whetherone or more genetically distinct population exist within Swedish and adjacent waters.Lack of such knowledge may result in severe reduction, and even extinction, of individualpopulations. Similarly, information on the amount of genetic exchange between potentialpopulations, as well as the rate of inbreeding and genetic drift within those populations, isof great importance for the development of conservation management strategies.

We have collected previously published scientific studies that directly or indirectlyaddress questions concerning the genetic population structure of the harbour porpoisethrough search of literature databases. Here, we summarize and discuss the conclusions ofthose studies focusing on papers that are based on clearcut genetic data obtained bymeans of various molecular techniques (studying variation at e.g., mitochondrial DNA,allozyme, or microsatellite loci). Several workers address the issue of populationdifferentiation studying characters for which the genetic basis is not fully understood(e.g., skull or tooth morphology, time for reproduction, etc.), and it is therefore not clearto what extent the variation of such characters reflect divergence on the gene level. Someof those studies are summarized in this report, but our discussion is based exclusively onthe results of the "direct" genetic studies.

1.2 The organization of this report

Some sections of elementary background material included in this report provide basicinformation on the harbour porpoise (2) and on biological diversity on the gene level andbasic conservation genetic issues (3). Readers familiar with those topics may skip theseparts. The methods and results are presented in sections 4-6. Seven genetic studies


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including samples from Swedish and adjacent waters have been identified in the literaturesearch, and the main observations of those studies are summarized separately in Figures3a-g and Table 2, as well as in the text (5.2).


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2 The harbour porpoise – brief background

There are six species of porpoises (order Cetacea, suborder Odontoceti - toothed whales,family Phocoenidae), and the harbour porpoise (Phocoena phocoena) is one of threespecies of the genus Phocoena (Read 1999; MacDonald 2001; www.iwcoffice.org ). Asindicated by the name, the harbour porpoise is found primarily in relatively shallowcoastal zones, and it exists in much of the temperate zone of the Northern Hemisphere(Gaskin 1984; Rosel et al. 1995; Read 1999). The species is the only cetacean regularlyoccurring in the Swedish parts of the Skagerrak, Kattegat and Baltic Seas (Berggren &Arrhenius 1995a).

An adult harbour porpoise is typically around two meters long and weighs around 50kg (Read 1999). It becomes sexually mature at an age of 3-4 years and seldom liveslonger than 15 years (Berggren 1995; IWC 2000). The female typically gives birth to asingle calf each or every second year (Read 1990), and mortality among newborn calvesis high (Lindahl et al. 2003). Females with calves are usually fairly stationary during thesummer season, whereas males and young individuals may migrate long distances (Wanget al. 1996).

The harbour porpoise preferably feeds on small fish (10-30 cm). The prey compositionvaries between areas, seasons, years, reproductive status, ontogeny, and sex (Read 1999and references therein; Lockyer & Andreasen 2004). In the Baltic Sea harbour porpoisesfeed mainly on clupeoids, such as herring and sprat (Lindroth 1962; Read 1999), whereasthe most important prey in the Skagerrak and Kattegat are gadids (cod-type fish) andgobiids (Lockyer & Andreasen 2004), as well as herring and Atlantic hagfish (Börjessonet al. 2003). It is not known to what degree the porpoise is opportunistic with respect toprey selection (Koschinski 2002), but Lockyer & Andreasen (2004) suggest that there hasbeen a dietary switch in the Baltic Sea because of changed prey availability, withcod-type fish now being the most important prey. Porpoises feed on both pelagic andbottom living fish; in the latter case they compete for food with, e.g., cod, haddock, andwhiting (Berggren 1995).

2.1 Distribution

The harbour porpoise occurs in three major geographical areas, namely the North PacificOcean, the North Atlantic, and the Black Sea/Sea of Azov (Figure 1), but it appears that ithas practically disappeared from the Sea of Azov (Gaskin 1984). There is strong evidencethat no, or minor, genetic exchange occurs between these three major basins, and thisconclusion is based on biogeographic, morphological, as well as mitochondrial DNA(mtDNA) data (e.g., Gaskin 1984; Yurick & Gaskin 1987; Rosel et al. 1995, 1999b;Wang et al. 1996). No observations of porpoises have been made in the MediterraneanSea during the past 15 years, further indicating that the Black Sea/Sea of Azov populationis currently isolated (Read 1999).

In the North Pacific Ocean harbour porpoises are found in the Bering's Strait, in thecoastal waters of northern Japan and from Alaska to southern California. The NorthAtlantic basin comprises two different areas holding harbour porpoises, namely thewestern and eastern North Atlantic (Figure 1; Gaskin 1984).


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In northern European waters the harbour porpoise occurs around Iceland, Great Britain(but not in the Channel), along the German coast, in Danish waters including the BeltSeas, and along the western and northern coasts of Norway. In Sweden the porpoise nowexists along the western and southern coasts, and in the Baltic up to the Gotland area, andoccasionally even further north (Gaskin 1984; Berggren & Arrhenius 1995a; IWC 1996;Wang & Berggren 1997; Read 1999; Hammond et al. 2002).

2.2 Population status

There is a general lack of information on the status and size of most harbour porpoisepopulations (Berggren & Arrhenius 1995a; Read 1999). Information on global abundanceis presented in Figure 1, and estimates for Swedish and adjacent waters are given inFigure 2a (in cases where more than one estimate of population size is available for aparticular area, all estimates are presented).

There is a general notion that the harbour porpoise population of the Baltic Sea hasdeclined dramatically during the past century and that population sizes during the earlypart of the 1900s were considerably larger than presently (e.g. Andersen 1982; Berggren& Arrhenius 1995a). Strict population estimates from that period are lacking, however.The number of sightings of harbour porpoises in the Baltic Sea is currently very small(Berggren & Arrhenius 1995b).

2.3 Intrinsic growth rate

Population growth rate is low for whales in general. For the harbour porpoise there is anobvious lack of demographic information, and only few studies report estimates ofpopulation growth rates. Woodley and Read (1991) estimated the intrinsic populationgrowth rate to be in the order of 5%, while Barlow and Boveng (1991) arrived at anestimate of 9.4% under more optimistic conditions. The ASCOBANS (Agreement on theConservation of Small Cetaceans of the Baltic and North Seas) Working Group of theInternational Whaling Commission (IWC) has agreed to consider 4% as the maximumrate of intrinsic growth for the harbour porpoise (IWC 2000) to avoid that the potentialrate of increase is overestimated.

2.4 Threats to harbour porpoise populations

The harbour porpoise is considered to be strongly affected by human activities (Tolley etal. 2001). The species is classified as "Vulnerable" on a global scale (IUCN criterionVUA1cd; IUCN 1996; www.redlist.org).

In Sweden the porpoise was included in the national list of protected species in 1973(Ahlén & Tjernberg 1996). Despite the listing, no increase of the number of porpoises hasbeen reported during the past decades (Berggren & Arrhenius 1995a). The porpoisepopulation of the Baltic Sea is regarded particularly sensitive, and in 1996 the WorldConservation Union (IUCN) classified it as a "vulnerable geographical population"(ASCOBANS: Jastarnia Plan 2002). The Red Data Book of the Baltic Region alsoclassifies the the species as "Vulnerable" (Ingelög et al. 1993)


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Potential threats in Swedish waters

Historically, there appears to have been an extensive hunting of harbour porpoises in theBaltic and adjacent waters (Andersen 1982; Berggren 1994; Kinze 1995). Porpoises werecaught primarily for the blubber, which was used as oil for lighting (Andersen 1982).During the hunts, pods of porpoises were driven into shallow areas where they wereenclosed by nets and pulled ashore (Møhl-Hansen 1954; Kinze 1995). The average annualtake in Danish waters may have comprised 1000-2000 individuals (Andersen 1982; Kinze1995). There are no records on porpoise hunting in Swedish waters (Berggren 1994;www.scb.se), and it is not clear to what extent hunting occurred in Sweden that lacks thetype of shallow bays typically used for porpoise harvesting (Dr. Håkan Westerberg, pers.comm.).

Hunting pressure, in combination with four winters with particularly severe ice condi-tions in the Baltic during the first half of the 20th century, has been suggested to be theprimary cause for the population decline in the Baltic area (Skora et al. 1988). Heavy icecover may result in porpoises drowning (Møhl-Hansen 1954) or migrating long distancesto ice free waters (Kinze 1985). Presently, the main threats to the porpoise in Sweden arethought to include by-catch, pollution, vessel traffic/tourism, diseases/parasites,demographic and genetic stochasticity, and possibly food depletion.

By-catch: By-catch in commercial fisheries is currently considered the primary globalthreat to the harbour porpoise (e.g. Berggren & Arrhenius 1995a; Carlström & Berggren1996; Wang et al. 1996; Wang & Berggren 1997), and the occurrence of by-catch hasalso been confirmed in Swedish waters (Berggren 1995). According to Berggren et al.(2002) current levels of by-catch exceed the limit for sustainable anthropogenic mortality.

Pollution: We have found no studies demonstrating a detrimental impact of chemicalpollutants on cetaceans. However, high levels of organochlorines/contaminants arereported for Swedish and other Scandinavian waters (Kleivane et al. 1995; Berggren et al.1999), and those substances have been shown to cause reproduction problems in othermammalian Baltic top predators such as the grey seal (Halichoerus grypus; Helle et al.1976). It appears likely that such pollutants may also be detrimental to porpoises (Gaskin1984; Aguilar & Borrell 1995; Kleivane et al. 1995).

Vessel traffic and tourism: Increasing vessel traffic and tourism may have negativeeffects on the porpoises, and are regarded as potential threats (Berggren 1995).

Diseases and parasites: The harbour porpoise has a relatively large parasite fauna(Møhl-Hansen 1954; Tomilin 1967). Most adult porpoises carry many parasites withoutany apparent health problems (Read 1999), but diseases and heavy loads of parasites mayhave a negative influence on harbour porpoise populations (Lindahl et al. 2003).

Food depletion: Commercial fisheries for several of the species that the harbourporpoise feed on have increased dramatically over the past century, and decreasing fishstocks may have a negative effect on porpoise survival (Börjesson & Berggren 1997).Overall biomass production has increased in the Baltic over the past decades, however,and it therefore appears unlikely that harbour porpoises in that area would lack food(Dr. Sture Hansson, pers. comm.). The issue of whether food depletion constitutes athreat to porpoises is addressed only briefly in the literature, and information appears tobe lacking.

Demographic and genetic stochasticity: Demographic and genetic stochasticity arepotentially severe threats towards harbour porpoise populations. If individual populationsare small (as indicated by i.e., the population size estimates for the Baltic) pure chance


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events affecting mortality and reproduction of individual specimens may have asignificant effect on population persistence. Similarly, if populations are isolated,inbreeding and genetic drift may lead to loss of genetic diversity and reduced populationviability (see below).

2.5 International agreements

Several political measures have been taken in response to the increased awareness of theneed for conservation and sustainable use of natural resources, and some of thosemeasures directly concern small cetaceans such as the harbour porpoise. Sweden, togetherwith seven other countries (Belgium, Denmark, Germany, Netherlands, Finland, Poland,and the United Kingdom), has ratified an agreement about small cetaceans in the Balticand the North Seas (ASCOBANS: Agreement on the Conservation of Small Cetaceans ofthe Baltic and North Seas) under the UN Bonn Convention in 1994. The aim ofASCOBANS is "to restore and/or maintain biological or management stocks of smallcetaceans at the level they would reach when there is the lowest possible anthropogenicinfluence". The temporary goal for ASCOBANS is "to restore populations to, or maintainthem at, 80% or more of carrying capacity".

The population structure of harbour porpoise in Swedish and adjacent waters has beenaddressed in several studies (eg. Koschinski 2002; see also Andersen 2003 for anextensive review). On the basis of some of those studies, the IWC-ASCOBANS WorkingGroup (IWC 2000) suggested the existence of five local stocks in the Baltic and NorthSeas: 1) Baltic Sea, 2) Kattegat, inner Danish waters, and German Baltic Sea, 3) northernNorth Sea, 4) central and southern North Sea, and 5) Celtic Shelf. ASCOBANS clearlystates, however, that it is difficult to define potential local populations within this area(ASCOBANS: Jastarnia Plan 2002).


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3 Biological diversity on the gene level– brief background

Biological diversity is frequently referred to as variation on the three levels of ecosy-stems, species, and genes (e.g., the UN Convention on Biological Diversity, Rio deJaneiro 1992). In reality, however, there are no strict "borders" between these levels -they rather represent points along a continuum that originates in differences at the genelevel.

Intraspecific genetic variation is based on sequence differences of the DNA molecule.Particular parts of a DNA molecule have specific functions - they contain the sequenceinformation for individual genes (so-called loci). For a specific gene (locus), the DNAsequence may differ somewhat between different copies of the gene - the nucleotidebuilding blocks do not always follow exactly the same order - resulting in differentvariants of the same gene (so-called alleles). This variation on the gene level constitutesthe basis for the biological evolution on our planet.

Most higher plants and animals are diploid - they have two copies of each gene. Onewas inherited from the mother and one from the father, i.e., each parent provides half thegenes to each of its offspring.

Nuclear and mitochondrial DNA

The DNA molecule is located in the nucleus of every cell, i.e., nuclear DNA (nDNA). Inaddition, a small amount of DNA also exists in the mitochondrial organelles in the cellcytoplasm outside the nucleus (mitochondrial DNA or mtDNA). MtDNA has severalcharacteristics that differ from those of the nuclear DNA. MtDNA consists of a single,circular DNA-molecule, implying that the mitochondrial genome is haploid, i.e., there isonly one copy of each gene. The genes of the mtDNA molecule are inherited together.Differences in DNA sequence in the mitochondrial genome result in the occurrence ofdifferent haplotypes (cf. alleles of the nuclear genome). Because only the eggs containcytoplasm (not the sperms) - and thus mitochondrial organelles - mtDNA is maternallyinherited. The maternal inheritance implies that the observed genetic structuring usingmtDNA markers reflects the evolutionary forces acting on the female segment of thepopulation(s).

Selection, migration, mutation

The genetic diversity of a species is distributed within populations - as differencesbetween individuals of the same populations with respect to the variants of a specificgene they carry - and between populations. Different alleles may, or may not, occur in allpopulations of a species, and when occurring, their frequency may differ betweenpopulations. The presence of genetic variation within species is essential for theirpotential to survive and for successfully evolving in response to both short-term andlong-term environmental changes.

In a particular environment a specific variant of a gene (allele) may be more or lessadvantageous for the individual carrying it. Through the process of natural selectionindividuals that carry advantageous alleles in a specific environment are "favored", i.e.,


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they are more likely to survive and reproduce than individuals that do not carry theseparticular alleles.

A population acquires new alleles primarily through immigration of individuals fromsurrounding populations (i.e., gene flow or genetically effective migration), and throughthe process of random change of the DNA sequence (mutation). In addition to these twoprocesses, the number of alleles and their frequency is determined by the size of thepopulation and the selective forces that particular alleles are subjected to. Mutationstypically occur very seldom, and this process for recreating genetic variation that hasbeen lost can usually be ignored within the time frames typically surveyed by humanactivities (tens or hundreds of years). From an evolutionary perspective (thousands ofyears), however, mutations are very important, and the only way in which new geneticvariability can be created.

The genetic composition of natural populations can be studied by the use of differentlaboratory techniques which can identify alleles at particular loci. A brief presentation ofsome of the methods for distinguishing such genetic markers is given in section 3.2.

3.1 Loss of genetic diversity

Contrary to biodiversity on the levels of ecosystems and species, diversity on the genelevel may be more difficult to recognize directly. The reason is that the genes themselvesare "invisible" to the human eye. Typically, only a small fraction of them have beenvisualized by means of advanced molecular genetic techniques. The loss of geneticdiversity may therefore go undetected, unless coupled with loss of diversity on the levelsof species or ecosystem.

Three phenomena cause loss of genetic variation, namely population extinction,hybridization, and limited population size. Population extinction results in loss of withinspecies diversity if the population that went extinct was genetically different from otherpopulations of the same species. Hybridization may occur both between species andbetween genetically divergent populations within a species. The process of hybridizationdoes not necessarily imply the loss of individual alleles but may result in rearrangementsof previously existing combinations of alleles at different loci. This may, in turn, result inbreakdown of adaptations to particular environmental conditions, and, in a next step, topopulation decline.

Populations of restricted size always lose genetic variation due to chance alone; not allalleles carried by the individuals of a particular generation are transferred to the next. Theprocess of random genetic change of allele frequencies due to a restricted population sizeis called genetic drift; the smaller the population, the larger the temporal frequency shiftsbecome. Similarly, the rate of inbreeding (i.e., matings between close relatives) isassociated with the size of the population. If a population is small and isolated, inbreedingis inevitable. In many species inbreeding is coupled with reduced viability and reproduc-tion, and increased occurrence of diseases and defects (so-called inbreeding depression).To our knowledge, the effects of inbreeding has not been studied in cetaceans - butvirtually all other mammals that have been studied suffer from inbreeding depression inone way or another (e.g., Ralls & Ballou 1983; Ralls et al. 1988; Lacy et al. 1993; Laikre1999).


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Effective population size

The rate of genetic drift and inbreeding is not determined by the actual, census, popula-tion size, but by a parameter denoted effective population size or Ne. Effective populationsize is one of the most important concepts in population genetics with respect toconservation biology, and it refers to the size of an "ideal" population that would have thesame rate of drift and inbreeding as the observed, actual population.

The effective population size of a natural population is typically much smaller than thecensus size and depends on factors such as sex ratio, variance in family size (i.e.,variability on numbers of offspring per individual), temporal fluctuations in the numberof breeding individuals, the degree to which generations overlap, etc. Minimum effectivepopulation sizes of 50 to 5000 per generation have been suggested by various workers asbeing necessary to avoid significant loss of genetic variability over various periods oftime (Allendorf & Ryman 2002).

3.2 Methods for studying genetic variation

During the past few decades the development of molecular genetics has been rapid, andmany laboratory techniques are now available for the accumulation of population geneticdata on harbour porpoise and other species.

Allozymes: Electrophoresis of allelic variants at protein coding loci of the nuclearDNA, so called allozymes (e.g. Utter et al. 1987; May 1992) is a widely used method forstudying genetic population structure of natural animal and plant populations. However,for harbour porpoises only two protein coding loci showing variation have been reported(Andersen 1993; Andersen et al. 1997).

Microsatellites: Microsatellite loci consist of non-coding regions of the genome wherevery short DNA segments are repeated in so-called tandem repeats (e.g., Estoup &Angers 1998; Goldstein & Schlötterer 1998). Different alleles at microsatellite loci differwith respect to the number of tandem repeats, and thus in size. This type of loci aregenerally highly variable and each locus typically exhibits many more alleles than e.g.,allozyme loci. A total of 24 microsatellite loci have so far been identified and analyzed inthe harbour porpoise (Andersen et al 1995, 1997, 2001; Rosel et al. 1999a; Chivers et al.2002; Duke 2003).

Mitochondrial DNA: Analyses of genetic variation of the mtDNA is conducted on thetotal mtDNA genome or on particular parts of the mtDNA molecule which are analyzedfor DNA sequence differences, using a set of restriction enzymes (so-called RestrictionFragment Length Polymorphism or RFLP analysis) or through determining the exactsequence (so-called sequencing). MtDNA is so far the most frequently used geneticmarker in harbour porpoise studies. As stated above, mtDNA variability patterns does notnecessarily reflect that of the nuclear genome (that represents the vast majority - 99% ormore - of the genetic information of an individual; e.g., Avise 1994).

Selective neutrality: The genetic markers that have been employed to study the geneticstructure of the harbour porpoise (as well as most other species) are considered to beselectively neutral. Selective neutrality implies that the fitness of an individual isindependent of its genotype at that particular locus. The genetic structure observed usingselectively neutral loci is expected to be affected by genetic drift and migration, but notby selection. Therefore, studies using neutral markers do not provide information on the


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extent to which genetically distinct populations are adapted to local environmentalconditions.

Genetic differentiation: Genetic data obtained through either of the above type ofmethods may be used to address issues of, for example, the amount of genetic similari-ty/difference between harbor porpoises collected in different areas. Various types ofstatistical techniques may be applied in this context. A frequently used measure of geneticdifferentiation within species is FST (Nei 1987; Weir 1996), which is estimated amongtwo or more samples. FST-values may range from 0-1 (negative values are interpreted aszero), and an FST of zero indicates no genetic differentiation between samples (identicalallele frequencies at all loci). An FST of 1 typically implies that the samples are perfectlygenetically differentiated, and fixed for alternate alleles (but see Hedrick 1999 for furtherdiscussion of this topic). The statistical significance of FST-values may be evaluated usinga variety of different procedures.

There are several ways of computing/defining FST, but the difference between thoseapproaches are typically quite small or negligible. For most species the majority of FST-values recently presented in the scientific literature were obtained using the method ofWeir & Cockerham (1984; see also Weir 1996). This is also the technique that we haveused when computing FST-values and associated significance levels from published alleleand/or haplotype frequencies in cases were such measures are not presented in theoriginal paper (using the software GENEPOP version 3.3; Raymond & Rousset 1995).


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4 Literature review

We searched the literature for publications on harbour porpoise population structure usingthe BIOSIS computerized database. We also collected references from the papersretrieved which were not obtained through BIOSIS.

The publications found have been numbered and are listed in Table 1. They includestudies that are based exclusively on molecular genetic markers as well as reports usingother types of data (i.e., morphological and ecological data; Table 1). For addressing thepresent question of genetic population structure we consider studies based on "direct"genetic information most appropriate; the results/conclusions from "indirect" data areincluded for comprehensiveness, but they are only discussed briefly.

We found 50 publications dealing with population differentiation in harbour porpoises.Those papers include all the 16 reports using "direct" genetic data that we have been ableto identify (Table 1: 1-8, 26-33). They also include all the 17 papers using morphologicaldata that we found (Table 1: 9-19, 34-39), 16 examples of studies using various types ofecological information (Table 1: 20-25, 40-49), and one study using biogeo-graphy/distribution data (Table 1: 50). There are additional studies that consider thepresent issue of population divergence from some kind of ecological perspective (e.g.,accumulation of pollutants, time for reproduction), but we have not included that entirebody of non-genetic literature in this presentation.

Half (Table 1: 1-25) of the 50 papers include porpoise samples from the geographicarea targeted in the present review. Of the 16 studies based on "direct" molecular geneticmarkers, seven include samples from the target area addressing the issue of populationdifferentiation within this area (Table 1: 1-4, 6-8; Table 2). The majority (11 out of 16) ofthe genetic studies have analyzed genetic variation of the mtDNA (Table 1: 5, 7, 8, 26-33), only seven peer reviewed studies use nuclear DNA markers (microsatellites andallozymes; Table 1: 1-4, 26, 27, 29). Thus, most of our present knowledge of the geneticpopulation structure of harbour porpoise on a global scale is based on variation ofmtDNA that is maternally inherited and provides no information on the composition ofthe nuclear genome.

4.1 Genetic divergence among major distribution areas

Two genetic studies address the issue of differentiation among the three major basins inwhich the harbour porpoise occurs (the North Pacific Ocean, the North Atlantic, and theBlack Sea/Sea of Azov). Both these studies are based on mtDNA and indicate that nohaplotype overlap occurs among the three basins (Rosel et al. 1995; Wang et al. 1996).Wang et al. (1996) estimate that the mtDNA lineages they observe in the eastern NorthPacific and the western North Atlantic were separated 1-5 million years ago. This degreeof mtDNA divergence is large for conspecific populations, and may justify a basis foraddressing the systematic status of the porpoises of these three major basins; should theyreally be considered the same species? Unfortunately, there appear to be no studiescomparing the three basins using nuclear genetic markers such as allozymes or microsa-


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tellites, so information is lacking on the amount of differentiation at loci potentiallyreflecting levels of gene flow also for the male segment of the population.

The results of the seven studies based on morphology or ecology are congruent withthe notion of substantial differentiation among the basins (Table 1: 15, 19, 34, 35, 37-39),and several workers discuss the porpoises of the three basins in terms of three differentsubspecies P.p relicta (Black sea/Sea of Azov), P.p. phocoena (North Atlantic), and P.p.vomerina (North Pacific; Tomilin 1967; Amano & Miyazaki 1992; Rosel et al. 1995;Wang et al. 1996).

4.2 Population differentiation within major distribution areas

We have identified seven studies on population differentiation within the North Pacific(Table 1: 15, 26, 28, 34, 37, 43, 50), two of which are based on genetic data (Table 1: 26,28). For the North Atlantic the corresponding number of studies are 43 and 14, respecti-vely (Table 1: 1-25, 27, 29-34, 36, 40-42, 44-50; 1-8, 27, 29-33). We found no studieslooking into subdivision of the isolated population of the Black Sea/Sea of Azov.Collectively, the genetic studies within the Atlantic and Pacific Oceans indicate that thereis genetic differentiation within ocean basins. The amount of divergence (FST) amonglocations are typically not very large, however, suggesting that migration amongpopulations may be substantial.

A review by Gaskin (1984) of the sub-structure within the Pacific Ocean and the NorthAtlantic has largely influenced the current view of harbour porpoise populationdifferentiation. Gaskin's work was based on biogeography and distribution data, and didnot include genetic information. Within the Pacific, Gaskin identified threesub-populations: Bering Sea, Sea of Okhotsk, and Gulf of Alaska to Los Angeles harbour.

For the North Atlantic Gaskin (1984) suggests 14 regional sub-populations, andrevision of this work by the International Whaling Commission (IWC) in 1996 resulted inprovisional identification of 13 sub-populations in the North Atlantic, four of whichbelong to the Western North Atlantic (Gulf of Maine-Bay of Fundy, Gulf of St.Lawrence, Newfoundland and Labrador, and Greenland), and nine to the Eastern NorthAtlantic (Iceland, Faroe Islands, North Norway and Barents Sea, North Sea, Kattegat andadjacent waters, Baltic Sea, Ireland and western United Kingdom, Iberia and Bay ofBiscaya, and Northwest Africa). Recent studies, however, stress the need for a revision ofthis division (e.g., Walton 1997; Andersen et al. 2001, Andersen 2003).

North Pacific Ocean

Neither of the two genetic studies focusing on genetic population differentiation withinthe North Pacific Ocean (i.e. Rosel et al. 1995; Chivers et al. 2002) have tested thegenetic justification of identifying three sub-populations within this area as suggested byGaskin (1984). Both studies focus on samples collected along the North American Pacificwest coast (i.e. within Gaskin's third subgroup - Gulf of Alaska to Los Angeles harbor)and report significant heterogeneity among sampling localities on the basis of thevariation of mtDNA and at microsatellite loci. It may be mentioned that three papersusing morphological data have compared samples from two of Gaskin's proposed threegroupings within the North Pacific, and they report significant differences (Miyazaki etal. 1987; Amano & Miyazaki 1992; Koopman & Gaskin 1994).


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North Atlantic

Three genetic studies (based on mtDNA or two allozyme loci) concern differentiationbetween the eastern and western North Atlantic (Andersen 1993; Rosel et al. 1999b;Tolley et al. 2001) and the results indicate differentiation between these two areas. Twoestimates of FST between these two regions are available, and both are based on mtDNAdata. Tolley et al. (2001) present an FST of 0.04, whereas an estimate calculated from thehaplotype frequencies presented by Rosel et al. (1999b) yields an FST of 0.13, and thelatter value appears to reflect considerable divergence between the eastern and westernNorth Atlantic. Significantly lower levels of mtDNA variation are detected within theeastern part of the North Atlantic than in the western one (Wang & Berggren 1997; Roselet al. 1999b).

Western North Atlantic: Three genetic studies deal with population structure within thewestern North Atlantic (Wang et al. 1996 (mtDNA); Rosel et al. 1999a (mtDNA andmicrosatellites); Tolley et al. 2001 (mtDNA)). Rosel et al. (1999a) analyzed samples fromfour of the western North Atlantic sub-groupings proposed by Gaskin/IWC, and founddifferences among most of them. Tolley et al. (2001) sampled the same four areas as wellas two additional ones arriving at a similar result, as did Wang et al. (1996) who analyzeddata from three of the areas.

None of these three studies provide overall quantitative measures of the amount ofdivergence among the areas sampled. For two of the studies the data presented permitcalculation of FST-values, and based on the mtDNA information of Rosel et al. (1999a)we computed an average FST among five areas of 0.03 (P<0.01). Similarly, mtDNA datafrom Wang et al. (1996) yield an overall FST estimate of 0.02 (P<0.001).

Eastern North Atlantic including the Baltic Sea: Eleven (Table 1: 1-4, 6-8, 27, 30, 32,33) of 30 studies (Table 1: 1-4, 6-14, 16-25, 27, 30, 32-33, 40-41, 46) on populationstructure in the eastern North Atlantic are based on genetic marker data. Half of thesestudies use mtDNA, and half microsatellites or allozymes (reflecting variation at thenuclear genome), the most comprehensive one (Andersen et al. 2001) being based onmicrosatellites.

There appears to have been no systematic attempts to confirm genetically the existenceof the nine Gaskin/IWC suggested subpopulations within the Eastern North Atlanticincluding the Baltic Sea, but all the eleven studies include samples from at least two ofthe nine areas. We have tried to interpret and summarize how these eleven studies refer tothe nine Gaskin/IWC subpopulations (Table 2). There were several difficulties associatedwith this procedure. For example, the definitions of the Gaskin/IWC subgroupings areunclear. Further, the exact geographic location of samples collected by various investi-gators are frequently not presented. Our interpretation of how specific sampling localitiesshould be classified with respect to the Gaskin/IWC subgroupings may therefore deviatefrom the view of the respective authors. The confusion and lack of consistency withrespect to the definition of geographic areas is particularly pronounced for the Baltic Seaand the waters around Denmark and southern Sweden.

As indicated in Table 2, it appears that the porpoises of the North Sea and the Kattegatand adjacent waters are, by comparison, relatively well studied genetically, whereas threeof the Gaskin/IWC areas (Iberia and Bay of Biscaya, Northwest Africa, and Faroe Island)do not seem to have been studied separately. Other areas such as the waters aroundIceland, Ireland and western UK, as well as the Baltic Sea are relatively poorly examined.


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Our impression is that the information is scattered with respect to both the regions studiedand the techniques applied, and that a general synthesis/consensus is lacking regardingthe overall population genetic structure among and within regions of the eastern NorthAtlantic. Most studies, however, report statistically significant heterogeneity both amongand within regions representing the different Gaskin/IWC groupings. Thus, the existenceof genetic substructuring within the eastern North Atlantic is generally recognized, butthe details of this structure are not clear. With respect to the present target area, however,it appears that most investigators consider the porpoises of the North Sea to be geneticallydistinct from those inhabiting the Baltic Sea and the areas around the Belt and KattegatSeas (see below).

Although most studies report significant genetic heterogeneity among locations, theamount of divergence is generally quite small, and FST-values are typically in the range of0.002-0.01 (Andersen et al. 2001; Duke 2003), and 0.02-0.08 (Tiedemann et al. 1996;Walton 1997; Wang & Berggren 1997; Tolley et al. 1999, 2001) for estimates based onnuclear genes and mtDNA, respectively. Estimates based exclusively on mtDNA aregenerally expected to produce FST-values that overestimate the amount of divergence forthe genome as a whole, because the rate of mtDNA drift is determined by the femaleeffective size rather than by the total effective size (cf. Laikre et al. 1998). For porpoisesthe observation of larger FST-values for mtDNA is also in line with the notion that femaleporpoises have a tendency to migrate over shorter distances than males (Andersen et al1997; Walton 1997; Tolley et al. 1999).


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5 Population genetic structure in Swedish andadjacent waters

As mentioned above, there are seven reports on population genetic structure of harbourporpoises in Swedish and adjacent waters (Table 3), defined to include the Baltic, Belt,Kattegat, and Skagerrak Seas. Two studies are based on mtDNA (Tiedemann et al. 1996;Wang & Berggren 1997), three on microsatellites (Andersen et al. 1995, 1997, 2001), twouse allozymes (Andersen 1993, Andersen et al. 1997), and one study deals with RAPDvariation (Stensland 1997).

Five of the seven studies (Table 1: 1, 3-4, 7-8) report significant genetic differencesamong sampling areas within Swedish and adjacent waters, although the samplinglocations and/or the patterns of differentiation are not necessarily the same. There areconsiderable difficulties to compare information from these studies. First, the definitionof geographic areas varies among authors, frequently reflecting a general lack ofstrictness of the nomenclature used for various parts of the water bodies surroundingSweden and Denmark. Second, in several cases the exact location for sample collection isnot clearly presented; the geographic origin of analyzed specimens is frequently givenonly as an ambiguous location name rather than as geographic coordinates. A specialproblem in this context refers to the samples collected as "stranded" porpoises because itis unclear where these porpoises actually originated. Further, some studies are based onrather small sample sizes (tens of individuals), and the statistical evaluation and/orpresentation of the results is in some cases incomplete or misleading.

5.1 Inconsistent definitions of geographic areas

The following water bodies are inconsistently defined in the literature.

The Baltic Sea: The critical question when defining the Baltic in the present context refersto the western border. Some authors use the strict hydrogeographic definition of theLimhamn and Darss under-water ridges (Figure 2b; Dr. Annika Tidlund, pers. comm) asthe western border line, whereas others also seem to include parts of the Belt Seas and/orthe Kattegat when referring to the Baltic. We recommend that the strict definition of theLimhamn and Darss under-water ridges is applied to define the Baltic.

Inner Danish Waters (IDW): This concept is not strictly defined geographically and doesnot appear on general maps. However, the term is frequently used in the literature on theharbour porpoise, but it is not always clear exactly what water bodies are included.Further, there seems to be inconsistencies in the application of this term between differentpublications. For example, in some cases IDW appears to include the Öresund, theKattegat, and the Belt Seas, whereas in other cases IDW is said to include the Kattegat,Belt, and Baltic Seas (cf. Figure 2b).


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The Kattegat/Skagerrak: The assignment of sampling locations to the Kattegat orSkagerrak Seas sometimes appears ambiguous.

5.2 Population genetic studies

Because of the inconsistencies regarding the definition of particular geographic areaswithin Swedish and adjacent waters we have summarized each of the seven populationgenetic reports that include samples from this region in separate figures (Figures 3a-g),and discuss briefly the findings of each of these publications.

Andersen 1993 (Figure 3a) genotyped a total of 185 porpoises representing IDW (hereincluding the Öresund, Belt, and Kattegat Seas) and the North Sea using two polymorphicallozyme loci. No statistically significant difference could be found when analyzing thetotal material (FST=0), but when the samples were classified with respect to season ofcollection (summer vs. winter), significant differences were observed between the IDWand the North Sea samples within both seasons (FST=0.06 and FST=0.04, respectively).

Andersen et al. 1995 (Figure 3b) conducted a preliminary study using two microsatel-lite loci, including samples from the North Sea, IDW, and West Greenland (total n=143).They detected no allele frequency differences between harbour porpoises from the IDW(area not defined in the original publication) and the North Sea, but reported an overallgenetic heterogeneity when including the sample from Greenland.

Tiedemann et al. 1996 (Figure 3c) report genetic differences of mtDNA haplotypefrequencies between the Baltic and the North Sea when analyzing two fairly smallsamples (n=20 and 19, respectively). FST is estimated to 0.08.

Andersen et al. 1997 (Figure 3d) used two allozyme and three microsatellite loci andobserved significant allele frequency differences between the IDW (here defined toinclude Öresund, the Belt and Kattegat Seas; n=53), and the North Sea (n=33) whencomparing porpoise samples collected during summer. No significant heterogeneity wasobserved when combining summer and winter samples, however.

Stensland 1997 (Figure 3e) found no significant difference between porpoises from theSwedish Baltic Sea (n=24) and the Skagerrak Sea (n=24) using a RAPD technique. It isnot clear how Stensland defines the western border of the Baltic, nor is the exact locationof her samples.

Wang & Berggren 1997 (Figure 3f) compared samples from the Swedish Baltic (n=27;defined according to the above mentioned under-water ridges), the Kattegat-Skagerrakarea (n=25), and the Norwegian west coast (n=13) using mtDNA RFLP analysis. Theyreport statistically significant differences of haplotype frequency distributions for allpairwise comparisons. For the comparison between the Baltic and the Kattegat-Skagerraksamples, however, recalculation does not fully support the notion of a statisticalsignificance; the appropriate P-value for this comparison is about 0.095 (exact calcula-tion), rather than 0.035 as reported by Wang & Berggren (see Appendix). The differenceis not very large, and strict application of the 0.05 limit for defining statistical heteroge-neity should be exercised with caution. Nevertheless, the result of the statistical re-analysis implies that this set of data does not lend strong support to the idea of a Balticpopulation that is genetically distinct from that of the Kattegat-Skagerrak area. Wecalculated an overall FST of 0.04 from the haplotype frequencies presented (P < 0.01),including their Baltic, Kattegat/Skagerrak, and Norwegian west coast samples.


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Andersen et al. 2001 (Figure 3g) is the most comprehensive population genetic studyof harbour porpoises in Swedish and adjacent waters (as well as world wide). It comprisesa total of 807 individuals, and 12 microsatellite loci were scored. The authors classifytheir samples into six different regions (IDW n=169, Danish North Sea n=151, BritishNorth Sea n=131, Ireland n=105, Norway n=49, West Greenland n=150), and in additionthey analyze a sample from the Netherlands (n=52). They report statistically significantallele frequency heterogeneity for all pairwise comparisons (the Netherland sample notincluded), the pairwise FST-values ranging in the interval 0.002-0.014.

Andersen and co-workers refer all their porpoises from the Baltic, the Belt, and theKattegat Seas to the region IDW (Inner Danish Waters), but their conclusion regardingthe degree of population structuring within this area is not perfectly clear. On one handthey state that "samples within regions showing no substantial evidence of heterogeneitywere pooled in the subsequent tests among regions", implying that the IDW exhibits nosubstantial evidence of substructuring.

On the other hand, they report significant deviations from Hardy-Weinberg expecta-tions (heterozygote deficiency) within the IDW. They present FIS-values for each of thetwelve loci (their Table 4), and for the IDW they report heterozygote deficiency at nine ofthese loci, two of them being statistically significant (no overall FIS is presented, and itcannot be calculated from the information provided). As noted by the authors, heterozy-gote deficiency is a potential indicator of population subdivision, but they leave thisobservation with the general statement that "these results suggest that deviations fromHWE are likely minor and they likely result from a Wahlund effect or non randommating, though we cannot exclude the possibility that they may reflect inbreeding or thepresence of null alleles". Further, when comparing allele frequencies within their IDWregion they find statistically significant differences among some of their subsamples.Andersen et al. (2001) have 32 porpoises from the Baltic Sea, 27 of which are identical tothe ones examined by Wang & Berggren (1997; Dr. Per Berggren, pers. comm.).Andersen and co-workers do not, however, report results from comparisons of allelefrequencies between their Baltic sample and other locations (in all the tests they present,the Baltic sample has been pooled with one or more other locations).


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6 Discussion and conclusions

Genetic structure

It is obvious that genetic structuring exists in the harbour porpoise. On a worldwide geographic scale, the mtDNA differences among porpoises from the three majordistribution basins (North Pacific, North Atlantic, and Black Sea/Sea of Azarov) are solarge that it might be justified to classify those groups as different subspecies (Amano &Miyazaki 1992; Rosel et al. 1995, Wang et al. 1996) or maybe even species, but studiesof the nuclear genome are necessary to further clarify this issue.

There is also genetic heterogeneity within major basins with statistically significantallele or haplotype frequency differences among many, or most, of the regions sampledwithin basins. The overall genetic population structure for the species is by no meansresolved, however. For instance, the North Atlantic sub-population structure suggested byGaskin (1984) and IWC (1996) has not been unambiguously verified by genetic studies.Similarly, the existence of the five local stocks as proposed by the IWC-ASCOBANSWorking Group (IWC 2000) has not yet been confirmed genetically.

The amount of genetic differentiation observed between areas within major basins isgenerally quite small. For the North Atlantic, for example, pairwise FST-values betweenregions such as West Greenland, Norway, Ireland, and "IDW" are typically around 0.01or less for nuclear genes (Andersen et al. 2001).

Low FST-values at nuclear loci have also been observed for some other marine mam-mals such as the grey seal (Halichoerus grypus; Allen et al. 1995), whereas considerabledivergence has been reported for, e.g., the Eastern Atlantic harbour seal (Phoca vitulinavitulina; Goodman 1998). In fishes, there is a general tendency for marine species toexhibit considerably less differentiation (smaller FST-values) than freshwater resident andanadromous ones, and the common explanation to this phenomenon is that geographicbarriers are frequently lacking in the marine environment, thereby permitting gene flowover large geographic distances (e.g. Hauser & Ward 1998). The low FST-values observedfor harbour porpoise are by no means incompatible with the potential existence ofseparate populations or of a more continuous genetic change over geographic space (so-called isolation by distance), but they indicate that the amount of divergence may not bevery large, potentially reflecting considerable gene flow.

Number of populations in Swedish and adjacent waters

Five studies (Andersen 1993; Andersen et al. 1997, 2001; Tiedemann et al. 1996; Wang& Berggren 1997) report small but statistically significant genetic differences betweenharbour porpoises of the North Sea and Skagerrak on one hand and the Katte-gat/Belt/Baltic area (east of Skagerrak) on the other. Only two studies (Wang & Berggren1997; Andersen et al. 2001) include samples that make it possible to address the issue ofgenetic heterogeneity within the Kattegat/Belt/Baltic area east of the North Sea, andneither of those studies provide unambiguous evidence for the existence of a substructurewithin this region.

On the basis of presently published information, how many genetically distinct popu-lations occur in Swedish and adjacent waters? There seems to be consensus on the


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existence of a minimum of two populations, although there appears to be some disagree-ment on how to classify the geographic areas inhabited by these two populations. Asmentioned above, Andersen et al. (2001) consider the Baltic, Belt, and Kattegat Seas tobe inhabited by one population which is distinct from that of the North Sea, and theyinclude Skagerrak in their definition of the North Sea. In contrast, Wang & Berggren(1997) consider the Baltic porpoises to represent one population separated from a distinctgroup in the Kattegat/Skagerrak area (although recalculation using their published datadoes not provide statistical significance). Thus, the area around Skagerrak appears to holda population of porpoises (that may extend into the North Sea, and perhaps into Kattegat)that is genetically distinct from porpoises further east.

With respect to the question of the existence of more than two populations, it appearsthat this issue cannot be settled from presently reported information. There are indicationsof genetic heterogeneities within the area comprising the Baltic, Belt, and Katte-gat/Skagerrak Seas, but the underlying biology reflected by these heterogeneities is notclear.

We do not think that further sampling currently represents the most urgent line ofaction to separate among the above alternatives. Rather, it appears that additionalstatistical evaluation of available genetic data may provide further information on thepopulation genetic structure within Swedish and adjacent waters. For example, thequestion of isolation by distance should be examined, e.g. through testing for associationsbetween geographic and genetic distance. Of course, analysis of more samples is alwaysvaluable, but it is important and cost effective that as much information as possible isgenerated from the material already available.

Important questions that currently published information does not appear to addressinclude:

• Is the genetic structure of the harbour porpoise characterized by discrete subpopula-tions, or are there "clines" of continuous genetic change without distinct boundaries?

• Do the observed genetic heterogeneities reflect true spatial variation, or do they onlyreflect "evolutionary noise" that is caused by e.g. differences among age classes or bytemporal allele frequency change resulting in differences between samples collectedseveral years apart?

Similarly, it may be informative to test for spatial heterogeneity after "re-grouping" partof the material. For example, most of the samples analyzed for mtDNA variation byWang & Berggren (1997) were also included in the microsatellite study by Andersen etal. (2001). As mentioned above, Wang & Berggren (1997) use a "strict" definition of theBaltic Sea, whereas Andersen et al. (2001) lump the same samples with those from theBelt and Kattegat Seas and refer them to "IDW". Thus, Wang & Berggren address thequestion of the existence of a distinct Baltic population corresponding to the suggestionfrom Gaskin/IWC, whereas Andersen et al. (2001) state that they have tested for geneticdifferentiation within the Kattegat/Belt/Baltic area, but that they did not find anystatistical significances (data not presented in the publication). At the same time,however, they report several indications of heterogeneity within the area (see above).Using the data of Andersen and co-workers to test exactly the same hypothesis as that ofWang & Berggren might shed some additional light on the issue of the possible existence


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of a genetically distinct Baltic population. Similarly, with respect to the samples that wereused by both Wang & Berggren and by Andersen et al. it appears that the geneticinformation has not been used to its full capacity. For the individuals included in bothstudies there is genotypic data on both mtDNA and microsatellites, but the analysesreported so far utilize these data sets separately, without exploiting the potential ofcombining them.

Estimates of population size in Swedish and adjacent waters indicate that the numberof porpoises in the Baltic Sea is only about 500 or less, whereas about 30 000 individualsappear to exist in the area of the Belt, Kattegat, and Skagerrak Seas. Because there are noapparent geographic barriers preventing migration into the Baltic, the large difference indensity almost provides the impression of some other form of biological "fence", but wehave seen no discussions of this issue in the literature. It seems to us that one possibleexplanation for such a "fence" could be that the Baltic environment is perceived as so"unattractive" that porpoises typically avoid to enter the region. Alternatively, the Balticrequires genetic adaptation to particular environmental conditions. If the latter is correct,there may be a genetically distinct Baltic population that is currently on the verge ofextinction. A third possibility implies continuous immigration into the Baltic, but thatmortality rates (e.g. due to by-catches) are so high that the population size does notincrease.

Implications for management

Clearly, there are a number of issues that are not unambiguously resolved with respect tothe genetic structure of the harbour porpoise in Swedish and adjacent waters. Until it hasbeen clarified whether the area east of the North Sea and Skagerrak consists of severaldistinct subpopulations, one homogenous population, or a cline of continuous geneticchange the precautionary principle implies the following:

1. The North Sea and Skagerrak harbour porpoises should be treated as geneticallydistinct from those of the Kattegat/Belt/Baltic regions east of the North Sea andSkagerrak.

2 . The area east of the North Sea and Skagerrak should be managed as if comprisingmultiple subpopulations.

3. The Baltic should be treated as if representing a distinct population.


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7 References

Aguilar, A. and Borrell, A. 1995. Pollution and harbour porpoises in the eastern NorthAtlantic: a review. Rep. Int. Whal. Commn., 16: 231-242.

Ahlén, I. and Tjernberg, M. (eds.) 1996. Rödlistade arter i Sverige – Artfakta. [SwedishRed Data Book of Vertebrates 1996]. ArtaDatabanken, SLU, Uppsala.

Allen, P.J., Amos, W., Pomeroy, P.P., and Twiss, S.D. 1995. Microsatellite variation ingrey seals (Halichoerus grypus) shows evidence of genetic differentiation between twoBritish breeding colonies. Molecular Ecology, 4: 653-662.

Allendorf, F.W. and Ryman, N. 2002. The role of genetics in population viabilityanalysis. In: Beissinger, S.R. and McCullough, D.R. (eds.), Population Viability Analysis.University of Chicago Press, Chicago.

Amano, M. and Miyazaki, N. 1992. Geographic variation in skulls of the harbourporpoise, Phocoena phocoena. Mammalia, 56: 133-144.

Andersen, S.H. 1972. Status over den danske hvalbestand. In: Status over den danskedyrverden, pp. 239-242. Zoologisk Museum, Copenhagen.

Andersen, S.H. 1982. Change in occurrence of the harbour porpoise, Phocoena phocoena,in Danish waters as illustrated by catch statistics from 1834-1970. Reprinted fromMammals in the Seas, FAO Fisheries Series No. 5, 4: 131-133.

Andersen, L.W. 1993. The population structure of the harbour porpoise, Phocoenaphocoena, in Danish waters and part of the North Atlantic. Marine Biology, 116: 1-7.

Andersen, L.W. 2003. Harbour porpoises (Phocoena phocoena) in the North Atlantic:Distribution and genetic population structure. NAMMCO Sci. Submitted.

Andersen, L.W., Holm, L-E, Clausen, B., and Kinze, C.C. 1995. Preliminary results of aDNA-microsatellite study of the population and social structure of the harbour porpoise.In: Blix, A.S., Walløe, L. and Ulltang, Ø (eds.), Whales, seals, fish and man. ElsevierScience B.V. pp. 119-127.

Andersen, L.W., Holm, L-E., Siegismund, H.R., Clausen, B., Kinze, C.C., and Lo-eschcke, V. 1997. A combined DNA-microsatellite and isozyme analysis of thepopulation structure of the harbour porpoise in Danish waters and West Greenland.Heredity, 78: 270-276.

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37

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Personal communication:

Dr. Per Berggren, 2003. Department of Zoology, Stockholm University.Dr. Sture Hansson, 2003. Department of Systems Ecology, Stockholm University.Dr. Annika Tidlund, 2003. Stockholm Marine Research Center, Stockholm University.Dr. Håkan Westerberg, 2003. Swedish National Board of Fisheries, Gothenburg.

Websites:

www.iwcoffice.org (2003-06-25)www.redlist.org (2003-01-08)www.scb.se (2003-09-30)


38

Table 1 (a and b). Studies concerning population differentiation in the harbour porpoise.All the studies using genetic and/or morphologic data that were found are included, aswell as examples of studies based on ecological data. The studies are separated withrespect to genetic, morphologic, and ecological data. n = number of animals. Table 1apresents reports including Swedish and/or adjacent waters, whereas Table 1b refers tostudies not including Swedish and/or adjacent waters.

Table 1a. Studies including Swedish and/or adjacent waters.No. Reference n

(total)n (BalticSea)

Num-ber ofareas

Material Marker/Character

Geographic areasstudied

1 Andersen (1993) 303 - 5 By-catchStranded

Allozymes Inner Danish waters,North Sea, WestGreenland, Dutchcoast, Canada

2 Andersen et al. (1995) 143 57 1)

(IDW)3 By-catch

StrandedMicrosat. Inner Danish waters,

North Sea, WestGreenland

3 Andersen et al. (1997) 124 53(IDW)

3 By-catchStranded

AllozymesMicrosat.

Inner Danish waters,North Sea, WestGreenland

4 Andersen et al. (2001) 807 32 (IDW= 169)

6 Mostlystranded

Microsat. Inner Danish waters,Danish North Sea-Skagerrak, BritishNorth Sea, Ireland,Norwegian westcoast, WestGreenland

5 Rosel et al. (1999b) 329 20 2 By-catchStranded

mtDNA Western and easternNorth Atlantic

6 Stensland (1997) 48 24 2 By-catch RAPD Baltic Sea, Skagerrak7 Tiedemann et al.

(1996)39 20 2 Stranded mtDNA Baltic Sea, North sea

2)

8 Wang & Berggren(1997)

65 27 3 By-catch mtDNA Baltic Sea, Kattegat-Skagerrak Seas,Norwegian westcoast

9 Andersen (unpubl.,cited in Andersen1972)

? ? ? ? Skullcharact.

Baltic Sea, North Sea

10 BMBF (1997, cited inKoschinski 2002)

? ? 4 ? Skullcharact.

German North andBaltic Seas

11 Börjesson & Berggren(1997)

103 31 2 By-catchStranded

Skullcharact.

Baltic Sea, Kattegat-Skagerrak Seas

12 Huggenberger et al.(2000)

242 ? 3 By-catchStranded

Skullcharact.

Baltic Sea, transitionarea, German Bight

13 Huggenberger et al.(2002)

242 ? 3 By-catchStranded

Skullcharact.

Baltic Sea, transitionarea, German Bight

14 Kinze (1985) 548 ? 3 ? Skullcharact.

Inner Danish waters,North Sea, Dutchcoast

15 Koopman & Gaskin(1994)

393 224(Danishwaters)

5 By-catchStrandedHuntedLive

Pigment. Denmark, Japan, Bayof Fundy, Gulf of St.Lawrence, BritishColumbia


39

No. Reference n(total)

n (BalticSea)

Num-ber ofareas



16 Lockyer (1999) 863 41 14 ? Toothcharact.

e.g. Inner Danishwaters, Skagerrak,Baltic Sea, NorthSea, West Greenland

17 van Utrecht (1960) 30 11 2 ? Morph.data


18 van Utrecht (1978) 488 389 2 By-catchStrandedHunted

Morph.data


19 Yurick & Gaskin(1987)

473 6 3 ? Skullcharact.

Eastern NorthPacific, western andeastern NorthAtlantic

20 Angerbjörn et al.(2002)

24 12 2 By-catchStranded

Isotopes Baltic Sea, Kattegat-Skagerrak Seas

21 Berggren et al. (1999) 47 17 3 By-catch Pollutants Baltic Sea, Kattegat-Skagerrak Seas,Norwegian westcoast

22 Bruhn et al. (1999) 40 19 3 By-catchStranded

Pollutants Baltic Sea, NorthSea, West Greenland

23 Börjesson & Read(2003)

183 95 4 - Time forreproduct.

Baltic Sea, Kattegat-Skagerrak Seas,North Sea, Bay ofFundy-Gulf of Maine

24 Kleivane et al. (1995) 34 - 3 By-catch Pollutants North Sea, Kattegat,Barents Sea

25 Tielmann et al. (inpress)

52 1 2 Satellite “Homerange”

Inner Danish waters,Danish North Sea-Skagerrak

1) IDW not defined. Unclear if porpoises from the Baltic Sea are included.2) The Baltic Sea sample may include specimens from the Belt Seas.


40

Table 1b. Studies not including Swedish and/or adjacent waters.No. Reference n

(total)Numberofareas



26 Chivers et al. (2002) 226 9 By-catchStrandedBiopsy

mtDNAMicrosat.

Eastern North Pacific

27 Duke (2003) 306 9 By-catchStranded

mtDNAMicrosat.

Iceland, Ireland,Norwegian waters

28 Rosel et al. (1995) 105 3 By-catchStranded

mtDNA Eastern North Pacific,North Atlantic, BlackSea

29 Rosel et al. (1999a) 253 4 By-catch mtDNAMicrosat.

Western North Atlantic

30 Walton (1997) 327 5 By-catchStranded

mtDNA Great Britain andadjacent waters

31 Wang et al. (1996) 204 5 Mostlyby-catch

mtDNA Eastern North Pacific,western North Atlantic

32 Tolley et al. (1999) 145 3 By-catchStranded

mtDNA North Sea, Norwegianwestcoast

33 Tolley et al. (2001) 370 6 By-catchStranded

mtDNA Western and easternNorth Atlantic

34 Amano & Miyazaki(1992)

108 4 ? Skull charact. Western and easternNorth Pacific, westernand eastern NorthAtlantic

35 Gao & Gaskin (1996a) 456 5 ? Skull charact. Eastern North Pacific,western North Atlantic

36 Gao & Gaski (1996b) 433 4 ? Skull charact. Western North Atlantic37 Miyazaki et al. (1987) 402 3 By-catch Morf. data Japanese waters,

eastern North Pacific,North Sea

38 Noldus & de Klerk (1984) 160 2 Stranded Morf. data Eastern North Pacific,eastern North Atlantic

39 Tomilin (1967) 76 3 ? Skull charact. North Pacific, NorthAtlantic, Black Sea

40 Berrow et al. (1998) 25 4 By-catchStranded

Pollutants British North Sea, IrishSea, Celtic Sea, westcoast of Ireland

41 Bjørge & Øien (1995) 429(sight.)

- Sightings Distr. andabund.

Norwegian waters

42 Calambokidis (1986) 36 3 ? Pollutants Western North Atlantic43 Calambokidis & Barlow

(1991)45 3 Stranded Pollutants Eastern North Pacific

44 Gaskin et al. (1983) 102 3 By-catchShooting

Pollutants Western North Atlantic

45 Read & Westgate (1997) 9 2 Satellite Satellite data Western North Atlantic46 Tolley & Heldal (2002) 36 3 By-catch Pollutants Norwegian waters47 Westgate & Johnston

(1995)197 3 By-catch Pollutants Western North Atlantic

48 Westgate & Tolley (1999) 188 3 By-catch Pollutants Western North Atlantic49 Westgate et al. (1997) 196 3 By-catch

StrandedPollutants Western North Atlantic

50 Gaskin (1984) - - - BiogeographyDistributiondata

-

41

THE SWEDISH ENVIRONMENTAL PROTECTION AGENCY Report 5419 – Population genetics of harbour porpoise in Swedish waters

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. Not

e th

at n

eith

er o

f th

e th

ree

subp

opul

atio

ns F

aroe

Isl

and,

Ibe

ria

and

Bay

of

Bis

caya

, and

Nor

thw

est A

fric

a ha

ve b

een

inve

stig

ated

sep

arat

ely.

Gas

kin/

IWC

ar

eas

And

erse

n 19

93A

nder

sen

et a

l. 19

95 A

nder

sen

et a

l. 19

97A

nder

sen

et a

l. 20

01D

uke

2003

Sten

slan

d 19

97T

iede

man

n et

al.

1996

Tol

ley

et a

l. 19

99T

olle

y et

al.

2001

Wal

ton

1997

Wan

g &

B

ergg

ren

1997

Faro

e Is

land

sIb

eria

and

Bay

of

Bis

caya

Nor

thw

est

Afr

ica

Icel

and

Mic

ros.

mtD

NA

Nor

th N

orw

ay

and

Bar

ents

Sea

Mic

ros.

(?)

Mic

ros.

mtD

NA

mtD

NA

Nor

th S

eaA

llozy

me

Mic

ros.

Mic

ros.

Allo

zym

eM

icro

s.m

tDN

Am

tDN

Am

tDN

Am

tDN

A

Kat

tega

t and

ad

jace

nt w

ater

sA

llozy

me

(?)

Mic

ros.

(?)

Mic

ros.

Allo

zym

eM

icro

s.R

AD

Pm

tDN

A

Bal

tic S

eaM

icro

s. (

?)R

APD

mtD

NA

mtD

NA

Irel

and

and

wes

tern

U.K

.M

icro

s.M

icro

s.m

tDN

A

42


Tab

le 3

. Gen

etic

stu

dies

con

cern

ing

popu

latio

n st

ruct

ure

in h

arbo

ur p

orpo

ises

in S

wed

ish

and

adja

cent

wat

ers.

n =

num

ber

of a

nim

als.

F ST

- a

nd P

-val

ues

not g

iven

by

the

auth

ors

have

bee

n ca

lcul

ated

with

GE

NE

POP

usin

g al

lele

/hap

loty

pe f

requ

enci

es p

rese

nted

in th

e or

igin

al p

ublic

atio

n; th

ose

valu

es a

re g

iven

in it

alic

s.

Ref

eren

cen (t

otal

)n (B

alti

c Se

a)N

umbe

r of

are

asM

ater

ial

Mar

ker

Num

ber

of lo

ciN

umbe

r of

alle

les

/ ha

plot

ypes

Ove

rall

hete

roge

neit

yG

eogr

aphi

c ar

eas

stud

ied

FST

1)P

-val

ue

And

erse

n (1

993)

303

-2

(5)

By-

catc

hSt

rand

edA

llozy

mes

25

-0.0

03 2)

0.60

8 2)

Inne

r D

anis

h w

ater

s, N

orth

Se

a +

Wes

t Gre

enla

nd, D

utch

co

ast,

Can

ada

And

erse

n et

al.

(199

5)14

357

3)

(ID

W)

3B

y-ca

tch

Stra

nded

Mic

rosa

t.2

130.

008

<0.0

07 4)

Inne

r D

anis

h w

ater

s, N

orth

Se

a, W

est G

reen

land

And

erse

n et

al.

(199

7)12

453

(ID

W)

3B

y-ca

tch

Stra

nded

Allo

zym

esM

icro

sat.

2/3

5/32

0.03

1<0

.05

5)In

ner

Dan

ish

wat

ers,

Nor

th

Sea,

Wes

t Gre

enla

ndA

nder

sen

et a

l. (2

001)

807

32 (

IDW

=

169)

6M

ostly

st

rand

edM

icro

sat.

1214

70.

002-

0.01

4 6)

<0.0

5 7)

Inne

r D

anis

h w

ater

s, D

anis

h N

orth

Sea

-Ska

gerr

ak, B

ritis

h N

orth

Sea

, Ire

land

, Nor

weg

ian

wes

tcoa

st, W

est G

reen

land

Sten

slan

d (1

997)

4824

2B

y-ca

tch

RA

PD8

-0.

001

>0.0

5B

altic

Sea

, Ska

gerr

akT

iede

man

n et

al.

(199

6)39

20 8)

2St

rand

edm

tDN

A1

90.

077

0.00

6B

altic

Sea

, Nor

th S

ea

Wan

g &

B

ergg

ren

(199

7)65

273

By-

catc

hm

tDN

A1

110.

044

0.00

2 9)

Bal

tic S

ea, K

atte

gat-

Skag

erra

k Se

as, N

orw

egia

n w

estc

oast

1) F

ST o

r eq

uiva

lent

.2) R

efer

s to

ID

W v

s. N

orth

Sea

. (ID

W(s

umm

er)

vs. N

orth

Sea

(sum

mer

): F

ST =

0.0

57, P

= 0

.003

. ID

W(w

inte

r) v

s. N

orth

Sea

(win

ter)

: FST

= 0

.037

, P =

0.0

1.)

3) I

DW

not

defi

ned.

Unc

lear

if a

nim

als

from

the

Bal

tic S

ea a

re in

clud

ed.

4) N

o si

gnifi

cant

dif

fere

ntia

tion

in p

airw

ise

com

pari

son

betw

een

the

Bal

tic S

ea a

nd th

e N

orth

Sea

.5) N

o si

gnifi

cant

dif

fere

nce

betw

een

the

Bal

tic a

nd K

atte

gat S

eas.

Sig

nific

ant d

iffe

renc

e be

twee

n ID

W a

nd D

anis

h N

orth

Sea

-Ska

gerr

ak.

6) N

o ov

eral

l FST

-est

imat

e is

pre

sent

ed. V

alue

s re

fer

to p

airw

ise

F ST -

valu

es b

etw

een

six

regi

ons.

7) N

o si

gnifi

cant

dif

fere

nce

betw

een

the

Bal

tic a

nd K

atte

gat S

eas.

Sig

nific

ant d

iffe

rent

iatio

n be

twee

n ID

W a

nd D

anis

h N

orth

Sea

-Ska

gerr

ak. D

evia

tion

from

H

ardy

-Wei

nber

g ex

pect

atio

ns (

hete

rozy

gote

defi

cien

cy)

and/

or a

llele

fre

quen

cy d

iffe

renc

es o

ccur

with

in I

DW

and

the

Dan

ish

Nor

th S

ea-S

kage

rrak

. 8) T

he B

altic

Sea

sam

ple

may

incl

ude

spec

imen

s fr

om th

e B

elt S

eas.

9) R

e-ca

lcul

atio

n yi

elds

P>0

.05

(non

-sig

nific

ant)

for

the

pair

wis

e co

mpa

riso

n be

twee

n th

e B

altic

Sea

and

the

Kat

tega

t-Sk

ager

rak

Seas

.

43


Black Sea-Sea of Azovn = 13 000

Celtic Sean = 36 000

West Greenlandn = 10-15 000

Washington tosouthern California(Monterey Bay region only)n = c. 600

Gulf of Alaskan = c. 1 000

Britisheast coastn = 17 000

Gulf of Maine-Bay of Fundyn = 47 000 Iceland

n = 27 000

Dutch watersn = 750

EnglishChanneln = 0

East Scotland-Shetland Islandsn = 61 000

Fig. 1. Geographic distribution of harbour porpoise in the early part of the 1900s. Black areas: known consistent occurrence. Stippled areas: occasional, peripheral, or probable range. Note that during recent years the species has reduced in abundance or disappeared from several of these areas, including the English Channel, the Mediterranean, the west coast of Spain, and the east coast of Greenland (Read 1999). n = abundance (from Gaskin (1984), Ivashin & Borodin (1992), Donovan & Bjørge (1995), Hammond et al. (2002), and references therein). See Fig. 2a for abundance estimates around Scandinavia. This figure is a modification of Figure 4 in Gaskin (1984) and is presented with kind permission from the IWC Office.

44


Fig. 2a. Harbour porpoise abundance around Scandinavia. For some areas more than one estimate is available.

BalticSea

Katteg

at

BeltSeas

NorthSea

Barents Sea

Skagerra

k

Sea

Lofoten

SWEDENSouthern Norway-

Northern North Sea

GermanBight

English C

hannel

Area Abundance ReferenceBaltic Sea 599 Hiby & Lovell (1996)Baltic Sea 93 Per Berggren (2003, pers. comm.)Skagerrak and Kattegat Seas 36 046 Hammond et al. (2002)Skagerrak and Kattegat (Swedish economical zone)

9 000 Lindahl et al. (2003) and references therein

Great Belt 516 Heide-Jørgensen et al. (1993)Great Belt 1 526 Donovan & Bjørge (1995)Great Belt 5 262 Hammond et al. (2002)Little Belt 91 Heide-Jørgensen et al. (1993)Little Belt 588 Hammond et al. (2002)Kiel Bight 87 Heide-Jørgensen et al. (1993)Northern Fyn 502 Heide-Jørgensen et al. (1993)Isle of Sylt 217 Heide-Jørgensen et al. (1993)North Sea (total) 268 452 Hammond et al. (2002)Southern Norway-Northern North Sea 82 619 Bjørge & Øien (1995)Lofoten-Barents Sea 10 994 Bjørge & Øien (1995)

45


Isle ofSylt

Kiel Bight

Great Belt

GERMANY POLAND

SWEDEN

DENMARK

FYN

Little Belt

A

B

Fig. 2b. Detail of Swedish and adjacent waters constituting the target area of the present report. The Limhamn and Darss under-water ridges (A and B) are frequently used to definethe western border of the Baltic Sea.

Baltic Sea

Kattegat

Skagerrak

Öresund

Fig. 2b. Detail of Swedish and adjacent waters constituting the target area of the present report. The Limhamn and Darss under-water ridges (A and B) are frequently used to define the western border of the Baltic Sea.

46


Fig. 3a. Sampling areas; Andersen 1993. Here IDW refers to Öresund and the Belt and Kattegat Seas. FST- and P-values are not given in the original paper, but were calculated from allele frequencies reported in the original publication. In addition to the 185 porpoises from the North Sea and IDW, the study also included specimens from West Greenland (n = 66), Canada (n = 12), and from the Dutch coast (n = 40).

Andersen (1993)n = 1852 allozyme loci

n (IDW) = 93

IDW vs. North Sea P>0.05IDW (summer) vs. North Sea (summer) P<0.01IDW (winter ) vs. North Sea (winter) P<0.01

FST = -0.003 (P >0.05)

North Sean = 92

IDWn = 93

Andersen (1993)n = 1852 allozyme loci

n (IDW) = 93

IDW vs. North Sea P>0.05IDW (summer) vs. North Sea (summer) P<0.01IDW (winter ) vs. North Sea (winter) P<0.01

FST = -0.003 (P >0.05)

North Sean = 92

IDWn = 93

47


Fig.

3b.

Sam

plin

g lo

calit

ies;

And

erse

n et

al.

1995

. Her

e, th

e ar

ea o

f ID

W is

not

defi

ned.

West

Greenland

n=37

NorthSea

n=49

IDW

n=57

Andersenetal.(1995)

n=143

2microsatelliteloci

n(IDW)=57

IDW

vs.N

orthSea

vs.W

estGreenlandP<0.05

IDW

vs.N

orthSea

P>0.05

OverallFST=0.008(P<0.007)

48


Fig. 3c. Sampling localities; Tiedemann et al. 1996. The Baltic Sea sample may include specimens from the Belt Seas. FST- and P-values are not given in the original paper, but were calculated from haplotype frequencies reported in the original publication.

North Sean = 19

Baltic Sean = 20

Tiedemann et al. (1996)n = 391 mtDNA locus; 9 alleles/haplotypes

n (Baltic Sea) = 20

Baltic Sea vs. North Sea FST = 0.077 (P<0.01)

49


West

Greenland

n=38

NorthSea

n=33

IDW

n=53

Andersenetal.(1997)

n=124

3microsatelliteloci

2allozymeloci

n(IDW)=53

IDW

(summer)vs.N

orthSea

(summer)P<0.05

IDW

(summer)vs.W

estGreenlandP<0.05

OverallheterogeneityFST=0.031(P<0.05)

Fig.

3d.

Sam

plin

g lo

calit

ies;

And

erse

n et

al.

1997

. Her

e, ID

W re

fers

to th

e Ö

resu

nd a

nd th

e B

elt a

nd K

atte

gat S

eas.

Sam

plin

g ar

ea

spec

ifica

tions

are

not

pre

sent

ed in

the

orgi

nal p

ublic

atio

n, b

ut w

ere

give

n by

Kos

chin

ski (

2002

).

West

Greenland

n=38

NorthSea

n=33

IDW

n=53

Andersenetal.(1997)

n=124

3microsatelliteloci

2allozymeloci

n(IDW)=53

IDW

(summer)vs.N

orthSea

(summer)P<0.05

IDW

(summer)vs.W

estGreenlandP<0.05

OverallheterogeneityFST=0.031(P<0.05)

50


Baltic Sean = 24

Stensland (1997)n = 48RAPD 8 markers

n (Baltic Sea) = 24

“FST” = 0.0012 (P>0.05)

SkagerrakSean = 24

Fig. 3e. Sampling localities; Stensland 1997. RAPD markers are dominant, and “FST” refers to a variance component calculated by means of AMOVA technique.

Baltic Sean = 24

Stensland (1997)n = 48RAPD 8 markers

n (Baltic Sea) = 24

“FST” = 0.0012 (P>0.05)

SkagerrakSean = 24

51


West coastof Norwayn = 13

Kattegat/SkagerrakSeasn = 25

Baltic Sean = 27

Wang and Berggren (1997)n = 651 mtDNA locus; 11 alleles/haplotypes

n (Baltic Sea) = 27

Baltic Sea vs. Kattegat and Skagerrak SeasP<0.05aBaltic Sea vs. Norway P<0.05

Overall heterogeneity FST = 0.044 (P = 0.01)

a Re-calculation yields P>0.05

Fig. 3f. Sampling localities; Wang and Berggren 1997. Note that the statistical significance (P<0.05; Baltic Sea vs. Kattegat/Skagerrak Seas) reported in the original paper cannot be confirmed when checking the calculation. FST- and P-values are not given in the original paper, but were calculated from haplotype frequencies reported in the original publication.

52


Fig.

3g.

Sam

plin

g lo

calit

ies;

And

erse

n et

al.

2001

. Her

e, ID

W re

fers

to th

e K

atte

gat,

Bel

t, an

d Sw

edis

h B

altic

Sea

s. Th

e ov

eral

l FST

-val

ue

cann

ot b

e ca

lcul

ated

from

the

data

giv

en in

the

orig

inal

pub

licat

ion.

No

estim

ate

of F

ST is

pro

vide

d fo

r ID

W v

s. N

ethe

rland

s in

the

orig

inal

pa

per,

and

it ca

nnot

be

calc

ulat

ed fr

om th

e da

ta p

rese

nted

.

Westcoast

ofNorway

n=49

Ireland

n=105

IDW

n=169

Andersenetal.(2001)

n=807

12microsatelliteloci

n(BalticSea)=32;n(IDW)=169

IDW

vs.S

kagerrak/DanishNorthSea

FST=0.005(P<0.05)

IDW

vs.B

ritish

NorthSea

FST=0.009(P<0.05)

IDW

vs.IrelandFST=0.012(P<0.05)

IDW

vs.N

orway

FST=0.014(P<0.05)

IDW

vs.W

estGreenlandFST=0.01

(P<0.05)

British

NorthSea

n=131

Skagerrak

+Danish

NorthSea

n=151

Netherlands

n=52

West

Greenland

n=150


53

Appendix

Wang and Berggren 1997 compared samples from the Swedish Baltic (n=27), theKattegat-Skagerrak area (n=25), and the Norwegian west coast (n=13) using mtDNARFLP analysis. They report statistically significant differences of haplotype frequencydistributions for all pairwise comparisons using the Monte program of the RestrictionEnzyme Analysis Package (REAP) version 4.1 (McElroy et al. 1992). Based on the datapresented in their Table 2 we re-analyzed the haplotype frequency difference between theSwedish Baltic area on one hand and the Kattegat-Skagerrak area on the other, whereWang and Berggren report a significant P-value of 0.035. They observed the followinghaplotype frequencies (modified from their Table 2, excluding haplotypes that were notfound in the Baltic or the Kattegat-Skagerrak):

Haplotype HaplotypeNo.

Baltic Sea Kattegat-SkagerrakSeas

AAAABAAAA 2 24 20AAAAAAAEA 5 0 2HAAAACAAA 53 0 1AABCAADBH 54 0 1AAAABAAQA 55 0 1AAAABAAAD 56 1 0AAAAAANAA 58 2 0

We used several statistical softwares (including REAP´s Monte, version 4.1) applyingboth exact calculations and simulation approaches, but never obtained a P-value of lessthan 0.05. The null hypothesis tested is that the haplotype frequency distribution is thesame in the Baltic and the Kattegat-Skagerrak Seas. We accept this null hypothesis if,under the null hypothesis, the probability of obtaining an observation as likely as, or lesslikely than, the present one exceeds 0.05.

GENEPOP (version 3.1c, subprogram STRUC, 100,000 iterations; Raymond &Roussett 1995) provides P=0.09087 (S.E. 0.00326). REAP´s (McElroy et al. 1992) MonteCarlo simulation performed by Monte (200 simulations with 10,000 iterations persimulation) yields P=0.08121. StatXact-3 (CYTEL 1997) calculates an exact P of 0.0956,and a Monte Carlo-estimate implemented in the same software also results in P=0.0956(1,000,000 simulations). A "regular" contingency chi-square test (e.g. Sokal & Rohlf1981) yields χ2=8.299, df=6, and P=0.2170. In no case did we obtain a P-value less thanthe 0.05 significance limit, and we therefore accept the null hypothesis of identicalhaplotype frequencies in the Baltic and the Kattegat-Skagerrak Seas.

This is a review of the current knowledge on the genetic

population structure of harbour porpoise (Phocoena

phocoena) in Swedish and adjacent waters. Informa-

tion on the population genetics of the species in other

geographic areas is also included.

On the basis of presently published information

there seems to be general consensus on the existence of

a minimum of two populations in Swedish and adjacent

waters. The area around Skagerrak appears to hold a

population of porpoises (that may extend into the

North Sea, and perhaps into Kattegat) that is genetically

distinct from porpoises further east.

With respect to the question of the existence of more

than two populations, it appears that this issue cannot

be settled from presently reported data. There are indi-

cations of genetic heterogeneities within the area com-

prising the Baltic, Belt, and Kattegat/Skagerrak Seas, but

the underlying biology reflected by these heterogeneities

is not clear. There are no unambiguous indications of

a genetically distinct Baltic population. More in depth

statistical analyses of available genetic information

might provide further information on the genetic

structure within this geographic region.

Population genetics of harbour porpoise in Swedish waters– a literature review

REPORT 5419

SWEDISH ENVIRONMENTAL

PROTECTION AGENCY

ISBN 91-620-5419-8

ISSN 0282-7298

swedish environmentalprotection agency

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