The Southern Ocean is a highly productive areawith a diverse biogeography (Ducklow et al. 2007). Asignificant feature lying between the east Pacific andAtlantic sector of the Southern Ocean is the WestAntarctic Peninsula (WAP), which extends theAntarctic continent to the north. The area is charac-terised by multiple islands directly on or close to theshelf, including the South Orkney Islands, ElephantIsland and the South Shetland Islands within the Sco-tia Arc. These geological features create dynamicslopes on the otherwise relatively flat sea floor shelf,which leads to local, nutrient-rich upwellings(Prézelin et al. 2000, Dinniman & Klinck 2004).
While there have been various fisheries-relatedstudies around the western flank of the WAP underthe auspices of the Commission for the Conservationof Antarctic Marine Living Resources (CCAMLR)
and cetacean surveys by the International WhalingCommission (IWC), most of these studies date backto the 1990s. More recent studies used fixed strip sur-veys to produce population estimates for a number ofmarine species (e.g. Joiris & Dochy 2013). There isstill very little knowledge on abundance of fin whalesBalaenoptera physalus in the Southern Hemisphere.Having suffered substantially from commercialwhaling activities in the 20th century (with casualtiestotalling >700 000 animals; Clapham & Baker 2002),fin whales are still listed by the International Unionfor Conservation of Nature as Critically Endangered(Reilly et al. 2013). There is only sparse informationon their population status, ecology, migration pat-terns and ecological role within the Southern Ocean,and, with the exception of analysis of the circumpolarInternational Decade of Cetacean Research/South-ern Ocean Whale and Ecosystem Research (IDCR-SOWER) datasets collected between 1978/1979 and
Mid-summer abundance estimates of fin whales Balaenoptera physalus around the
South Orkney Islands and Elephant Island
Sacha Viquerat, Helena Herr*
Institute for Terrestrial and Aquatic Wildlife Research, University of Veterinary Medicine Hannover, Foundation,25761 Büsum, Germany
ABSTRACT: A line-transect distance sampling survey for fin whales Balaenoptera physalus wasconducted around Elephant Island and the South Orkney Islands on board a CCAMLR fishing sur-vey for fin fish in January and February 2016. Collected data were used for model-based abun-dance estimates of fin whales in 2 strata. The minimum average (± SE) density of fin whales wasestimated at 0.0268 ± 0.0183 ind. km−2 in a 19 750 km2 area around Elephant Island, resulting in aminimum abundance estimate of 528 ± 362 fin whales. In a 13 550 km2 area around the SouthOrkney Islands, we estimated a minimum density of 0.0588 ± 0.0381 ind. km−2 and a minimumabundance of 796 ± 516 ind. The results of this study confirm a westerly extension of a recentlydescribed high-density area for fin whales in the West Antarctic Peninsula region. In the light ofincreasing krill fisheries in the local region, we suggest this area for further studies to assess thepotential for conflict between recovering whale populations and emerging industrial interests.
KEY WORDS: West Antarctic Peninsula · Southern Ocean · Population status · Baleen whales
OPENPEN ACCESSCCESS
Endang Species Res 32: 515–524, 2017
2004 (Branch & Butterworth 2001, Ensor et al. 2006,2007, Leaper & Miller 2011) under the auspices of theIWC, there are no recent population estimates for theSouthern Hemisphere.
Increasing numbers of fin whale sightings in thearea around the WAP over the past few years (Joiris& Dochy 2013, Santora et al. 2014) have indicatedthat the Southern Hemisphere population is poten-tially recovering. The WAP seems to be a key areawhere fin whales now appear to aggregate duringthe austral summer months (Herr et al. 2016). Thiscontrasts with a decade ago, when only a few sight-ings of fin whales were reported from the same area(Scheidat et al. 2011).
A dedicated aerial survey conducted in 2013 withinthe Bransfield Strait and Drake Passage producedthe first density estimates for fin whales in that areaand found minimum (i.e. without correcting for avail-ability) densities of 0.117 ind. km−2 (95% CI: 0.053−0.181). A minimum abundance of fin whales withinan approximately 42 000 km2 area in the Drake Pas-sage was estimated at 4898 (95% CI: 2221−7575) ind.(Herr et al. 2016). The most re cent circumpolar abun-dance estimate for fin whales south of 60 °S was 5445(95% CI: 2000−14 500) ind., based on an analysis ofIDCR/SOWER survey data collected between 1978/1979 and 2004 (Leaper & Miller 2011). These findingssuggest that either a major proportion of SouthernHemisphere fin whales use the area west of the WAPat least temporarily or that there has been a consider-able growth in the total abundance of fin whales inthe Southern Hemisphere since the IDCR/SOWERcruises.
At the same time, effort by the industrial krill fish-ery is increasing, especially in the area around theWAP (Nicol et al. 2012), and a declining krill stockhas been reported (Atkinson et al. 2004). Becausekrill fisheries and whales are competing for the sameresource, there is a need for information on fin whaledistribution and abundance in the wider area aroundthe WAP to assess the potential for conflict and facil-itate management between the demands of industryand the needs of a recovering population.
Ship time on research vessels in the SouthernOcean is limited. However, commercial vessels pro-vide more extensive platforms of opportunity forcetacean research. With modern modelling tech-niques, cetacean surveys do not depend on a fixedsurvey design but instead can collect data followingestablished methodologies along a random cruisetrack (Hedley & Buckland 2004, Paxton et al. 2009,Campbell et al. 2015, Gilles et al. 2016, Herr et al.2016).
In this study, we conducted a distance samplingsurvey for model-based abundance estimation of finwhales around the South Orkney Islands and Ele-phant Island, using a commercial fisheries vesselchartered for a CCAMLR fin fish survey as a platformof opportunity for an ad hoc survey design.
MATERIALS AND METHODS
Data collection
We conducted a line transect distance-samplingsurvey with a single observer from the bridge (11.2 melevation above sea level) of the Chilean fishing ves-sel ‘Cabo de Hornos’ from 27 January until 7 Febru-ary 2016 around the South Orkney Islands and Ele-phant Island. The ‘Cabo de Hornos’ was charteredfor a CCAMLR fin fish survey following a scientificsurvey protocol approved by CCAMLR (see https://www.ccamlr.org/en/wg-fsa-15/10-0). The observerwas stationed centrally with an unobstructed field ofview of 60° to each side of the ship. Observationswere restricted to 60° to each side to enable a singleobserver to cover the field of view, minimising per-ception bias. Data were gathered on transits betweenfishing trawl sites. There was neither interaction noractive approach toward animals, and no animalswere deliberately harmed or stressed at any time byour work.
Using a survey computer hooked up to a GPSdevice to record survey parameters and detections,the observer recorded all sightings within the field ofview, focussing on the 90° sector around the transectline (45° to each side of the transect). After initialnaked eye detection, binoculars with reticule displayfor distance measurements (Fujinon MTRC-SX) wereused to measure the distance to the sighting and, ifnecessary, to confirm species identification. Observershifts were limited to a maximum of 1.5 h stretches ofcontinuous effort (depending on environmental con-ditions and ship activities). After a continuous 1.5 hshift, at least a 0.5 h break was enforced to preventobserver fatigue. Total effort time within any 24 h pe-riod was limited to 8 h of effort to allow the single ob-server to rest. The environmental parameters seastate (measured in the Beaufort scale), swell, ice cov-erage and glare were judged by the observer andrecorded at the beginning of each effort period andwhenever any change occurred. In addition, subjec-tive sighting conditions (a compound variable whichdescribes the overall ease of detecting fin whales dependent on weather and ambient light conditions,
516
Viquerat & Herr: Fin whales around the South Orkneys and Elephant Island 517
using 4 levels: ‘good’, ‘moderate’, ‘poor’ and ‘unaccept-able’) were assessed separately for each side by theobserver. In case of ‘unacceptable’ conditions (usuallydue to fog, very strong glare etc.), observation of thatside was discontinued. To limit the distraction fromthe target species, no seals or birds were recorded.
Information collected for each cetacean sightingincluded the horizontal angle and the radial distanceto the sighting (measured using calibrated reticulesthat are specific for the binoculars in use), the speciesidentification (allowing for unidentified animals),group size estimate and, if measurable, the generalswim direction.
Horizontal angles were measured using an angleboard in relation to the ships heading. The perpendi-cular distance to the track line was then calculatedusing:
dperp = sin θ × drad (1)
where dperp is the perpendicular distance to the trackline, θ is the horizontal angle, and drad is the radialdistance, calculated by converting δ (the declinationangle measured in reticules).
Data analyses
From the collected sighting data, we producedmultiple covariate detection functions for fin whales(MCDS; see Buckland et al. 2004). Half-normal mod-els of fin whale sightings, including the environmen-tal parameters sea state, swell, ice coverage, glare,group size and subjective sighting conditions, weretested against models without covariates. The bestde tection function model was chosen based on theAkaike Information Criterion (AIC; see Akaike1974).
The dataset was then segmented into continuouseffort stretches of approximately 5 km. With theeffective half strip width (esw) for the left (esw l) andright (eswr) side derived from the detection function,we calculated the effectively covered area per 5 kmsegment using:
Aeff = (eswl + eswr) × Lsegment (2)
where Aeff is the effectively covered area within thesegment, esw is the effective half strip width for finwhales, and Lsegment is the total effort within the seg-ment. In case of unfavourable survey conditions onone side that did not allow observations on that side(fog, glare, etc.), the corresponding esw was set to 0for that segment, reducing total effort for that seg-ment by one half.
Using the number of fin whale group sightings andthe average group size for each stratum, we thenestimated the density of fin whales for each segmentas follows:
(3)
where is the density of fin whales per km2,Gsegment is the number of recorded fin whale groupsalong the segment, Aeff is the effectively coveredarea of the segment, and sstratum is the average groupsize of fin whales within the stratum.
We used generalised additive models to producea density surface model based on the segment -ed dataset associated with environmental co vari -ates (see Table 1 for a summary of the tested covariates).
A Tweedie error distribution (Tweedie 1956) wasused in all models to compensate for overdispersion,typically encountered in cetacean surveys (e.g.Williams et al. 2011, Miller et al. 2013). We used the
DG
As
segmentsegment
effstratum
ˆ= ×
Dsegment
Covariate Name Source
Bathymetry (m) Depth IBCSO (Arndt et al. 2013)Aspect of seafloor Aspect Calculated in R (R Development Core Team 2015) from IBSCO (Arndt et al. (angular degree) 2013) using the terrain function from package raster (Hijmans 2015)
Slope of seafloor Slope Calculated in R (R Development Core Team 2015) from IBSCO (Arndt et al. (angular degree) 2013) using the terrain function from package raster (Hijmans 2015)
Distance to southern dist2sBACC CCAMLR online GIS repository (http://gis.ccamlr.org/home)boundary of Antarctic Circumpolar Current (km)
Coordinates of (x, y) Coordinate of segment midpoint in WGS84 / Antarctic Polar Stereographic segment midpoint (m) projection (EPSG: 3031)
Table 1. Covariates used in the modelling process. Name denotes the identifier of the respective covariate used in the model-ling and is used as a substitute in the text; source denotes the origin of the data
Endang Species Res 32: 515–524, 2017
decadal log of the segment length as sample weightsfor the modelling stage. The dimensions of the thinplate smoothing functions (Wood 2003) were re stric -ted to 4 dimensions in each covariate to avoid unreal-istic overfitting of the data. The best model was cho-sen based on the Restricted Maximum Likelihoodscore (REML).
To estimate abundance in the survey area, wedefined 2 strata post-survey by assigning a 20 kmbuffer zone around the covered track lines sepa-rately for both island groups (Fig. 1). A 20 km bufferwas considered a narrow stretch around the actuallysurveyed area and chosen to avoid extrapolatingmuch beyond the survey boundaries. These stratawere used in the modelling step as areas for pre-dicting densities and abundances within the bufferzones.
The model was applied to prediction grids in5 × 5 km resolution (for the 2 strata, South OrkneyIslands and Elephant Island, respectively) to producedistribution maps as well as density and abundanceestimates for fin whales in each stratum.
All analyses were performed in R 3.2.2 (R Develop-ment Core Team 2015) using the packages Distance(Miller 2015), rgdal (Bivand et al. 2015), rgeos(Bivand & Rundel 2015), maptools (Bivand & Lewin-Koh 2015), raster (Hijmans 2015) and mgcv (Wood2011).
RESULTS
The stratum around Elephant Island was surveyedbetween 27 January and 1 February 2016. A total of
299 km of track line were observedon-effort within this stratum, with27 group sightings of 29 fin whales,averaging a group size of 1.07 finwhales per group. The South Ork -ney Islands stratum was surveyedbetween 2 February and 7 February2016. A total of 164 km of track lineswere observed on effort, with 17group sightings of 32 fin whales,averaging a group size of 1.88 finwhales per group (Table 2, Fig. 2).The only other cetaceans sighted oneffort were humpback whales Mega -ptera novaeangliae (28 sightings, 39individuals including 1 calf) in bothstrata and a single sei whale Bal-aenoptera borealis in the South Ork -ney Islands stratum.
Only data recorded at sea states≤4 and in moderate or good condi-tions on at least 1 side of the transectwere included in the final dataset.After right truncation at 2500 m
518
Fig. 1. Overview of cruise track and surveyeffort between 27 January and 7 Febru-ary 2016. The thick red line indicates theship track; the light green line marks sur-vey effort periods; the shaded polygonsmark the strata defined for later analysis:Elephant Island stratum (yellow) andSouth Orkney Island stratum (purple);ACC fronts: boundaries of the Antarcticcircumpolar currents system (data source:CCAMLR GIS repo sitory). Backgroundbathymetry from ETOPO (Amante &
Eakins 2009)
Viquerat & Herr: Fin whales around the South Orkneys and Elephant Island
from the track line, 38 fin whalegroups were available for the de -tection function modelling step. Astraightforward detection function us-ing no additional covariates (fw1) waschosen as the best model and used forthe subsequent (Table 3, Fig. 3).
The segmentation process of the sur-vey dataset yielded 119 individualsegments. The results of the additivemodelling are given in Table 4. Thebest model was m14 including a spatialsmoother (x, y), explaining 66.23% ofthe observed deviance. Introducingadditional covariates in flated predic-tions unrealistically and did not con-tribute substantially to the robustnessand scope of the models, as indicatedby REML score and explained de -viance (see Table 4). While models in -cluding slope and depth yielded betterscores, we opted for the simpler modelm14 because the information gainedfrom the more complex models did notbenefit or change the study results.
Based on model m14, the averagedensity of fin whales was predicted at0.0268 ± 0.0183 (95% CI: 0−0.0627)ind. km−2 for the Elephant Island stra-tum and at 0.0588 ± 0.0381 (95% CI:0−0.1334) ind. km−2 for the South Ork -ney Islands stratum. Abundance wasestimated at 528 ± 362 (95%CI: 0−1238) ind. around Elephant Island and796 ± 516 (95% CI: 0−1807) ind. aroundthe South Orkney Islands (Table 5).
The highest density of fin whaleswas predicted within the South Ork -ney Islands stratum, along the shelfedge about 50 km south-west of theshoreline (Fig. 4).
DISCUSSION
Our study provides mid-summerminimum density estimates for finwhales around Elephant Island andthe South Orkney Islands based on adedicated cetacean line-transect dis-tance-sampling survey from a plat-form of opportunity. The only other re -cently published information on fin
519
Stratum Effort Fin whales Humpback whales(km) G I C s G I C s
Table 2. Summary of fin and humpback whale records on effort. G: numberof cetacean groups; I: total number of individuals; C: number of calves recorded
on effort for each species; s: average group size
Fig. 2. Positions of cetacean records in 2 strata (a) Elephant Island; (b) theSouth Orkney Islands and (c) complete survey period. Effort stretches aremarked in green. ACC: Antarctic Circumpolar Current; see Fig. 1 for moredetails. PF: Polar Front; sACCf: Southern Antarctic Circumpolar CurrentFront; sBACC: southern boundary of the Antarctic Circumpolar Current;
ACC: Antarctic Circumpolar Current; SAF: Subantarctic Front
Endang Species Res 32: 515–524, 2017
whales in the same area is based on a mixed-speciesstrip-transect count, which produced local densityestimates of 0.03 ind. km−2 near Elephant Islandbetween March and April 2012 (Joiris & Dochy 2013),supporting the order of magnitude of fin whale den-sities in the area found in this study (0.0268 ± 0.0183ind. km−2; 95% CI: 0 to 0.0627). Densities around theSouth Orkney Islands from our study were predictedto be even higher (0.0588 ± 0.0381 ind. km−2; 95% CI:0 to 0.1334), but no estimates for comparison are cur-rently available.
Even higher fin whale densities (0.114 ind. km−2;95% CI: 0.053 to 0.181) were recently de scribed for
the area of the South Shetland Islands, based on anaerial survey conducted in February and March2013, suggesting a newly emerged fin whale hotspotin the WAP (Herr et al. 2016). The results of our studypoint to a westerly extension of the high-densityarea, extending it from the South Shetland Islands tothe Orkney Islands across a sector from 63° W to43° W and from 60° S to 65° S. However, since the 2surveys took place in 2 different years, it cannot bediscerned from this study whether high fin whaledensities occur area-wide throughout the full rangeof the suggested high-density area simultaneously orif aggregations of fin whales shift within the area. Ashighly mobile oceanic predators, whales are knownto move dynamically with their prey (Santora et al.2014, Curtice et al. 2015), possibly leading to tempo-ral hot spot occurrences of fin whales throughout thesuggested high-density area. Further studies areneeded to characterise the spatio-temporal distribu-tion, movements and habitat use of aggregating finwhales within the area.
Future investigation should explore whether thedescribed range represents the full spatial extent ofthis apparently important feeding habitat for finwhales around the Antarctic Peninsula. Furthermore,investigations should focus on the temporal aspect offin whale presence in the area. Observations of highfin whale densities were reported from January toApril (Joiris & Dochy 2013, Herr et al. 2016, presentstudy). Acoustic recordings reported highest callingrates of fin whales in the area in May (Sirovic et al.2004, 2009). However, no information is availablefrom early summer, and it is unknown when finwhales start arriving in the area. Acoustic recordingas well as additional visual surveys could provideimportant information on the seasonal componentof fin whale aggregations in the area, and taggingstudies are needed to learn about fin whale move-ments in the area as well as migratory origins anddestinations.
Herr et al. (2016) suggested that fin whales aroundthe South Shetland Islands were feeding on aggre-gating Thysanoessa macrura around the shelf edgearea. However, from a single season observation, itcould not be discerned if they opportunistically feedon these organisms or if T. macrura plays an impor-tant ecological role in the area. Elephant Island inparticular is known for large patches of high krillconcentrations when ice-free (Hewitt & Demer 1993).There are strong indications that the main driver formost species encountered near the WAP is the avail-ability of krill (Friedlaender et al. 2006, Nowacek etal. 2011, Herr et al. 2016). Acoustic surveys for fin
520
Model Covariate AIC
fw1 No covariate 582fw2 Sea state 584fw3 Ice coverage 584fw4 Sighting conditions 584fw6 Group size 583
Table 3. Detection function modelling results. Model: nameof the model used as a substitute in the text; covariate: theenvironmental covariate used in the detection functionmodel; AIC: Akaike Information Criterion (a smaller valueindicates a more parsimonious model output). Model in bolditalics (fw1) indicates the final chosen model for all subse-
quent analyses
Fig. 3. Detection function for fin whales. The selected detec-tion function fw1 (solid line) for fin whales using no addi-tional covariates based on 38 records (after right truncationat 2500 m). Circles are the probability of detection for eachsighting given its perpendicular distance. The vertical lineindicates the estimated effective strip width (esw) at a width
of 1341 m
Viquerat & Herr: Fin whales around the South Orkneys and Elephant Island
whales indicate a decrease in calling activity afterMay, which can be attributed to the beginning of theformation of ice. This might result in a migration offin whales out of the area due to the marginal avail-ability of prey items (Sirovic et al. 2004).
Our study was conducted in mid-summer; hence,there was no ice in the study area. The low samplevariation across environmental parameters renderedany covariate insignificant except for the spatialsmoother s(x,y). The spatial smoother can be consid-ered as a general proxy for a combination of environ-mental parameters that could not be tested in themodelling step due to a limited number of samplesand lack of variation therein. While not immediatelyreferring to any specific range of environmentalparameters, we can hypothesise patterns within thedistribution. In this study, most fin whales wereencountered close to the shelf edge. This is also
reflected in the predicted distribu-tion. However, it would be careless todraw a general conclusion basedupon this observation alone becausethis survey did not cover much areabeyond the shelf edge. For the esti-mation of abundances, we thereforerestricted the prediction area to thevery area surrounding the track lineto avoid extrapolating be yond therange of environmental parametersactually covered. For a more de tailedecological model, the survey designhas to ensure wider coverage of envi-ronmental gradients. The opportunis-tic nature of our survey did not allowfor a more robust design.
Using a prediction area of 20 kmaround the track line, we estimatedabundances of fin whales at 528 ±362 around Elephant Island and at796 ± 516 fin whales for the SouthOrkney Islands. Due to the smallsample size and limited effort time,the CIs of the estimates include 0 ind.The low CI could only be remedied
by a more representative transect design and a pro-longed or repeated campaign, which we could notpursue in this opportunistic snap shot survey. Withthe most recent population estimate stating 5445 finwhales south of 60° S (Leaper & Miller 2011), basedon IDCR/SOWER surveys (Branch & Butterworth2001), a total of 1200 ind. within the spatially con-strained area around Elephant Island and the SouthOrkney Islands has to be considered a substantialnumber, suggesting that this area is, at least tem-porarily, a highly important habitat for SouthernHemisphere fin whales. The ecological significanceof this area is further underlined by a large numberof other baleen whales, such as humpback whalesMe gaptera nova e angliae and sei whales Balaeno -ptera borealis, and other marine vertebrates (marinebirds and seals) observed within multi-species feed-ing aggregations.
Table 5. Predicted fin whale density and abundance in 2 strata. Area: area of stratum; : animal density; : number of finwhales; SE: standard error; 95CI: 95% confidence interval
Table 4. Summary statistics of tested models in the additive modelling processof the segmented data. Covariate: covariate combination tested in the model(multiple covariates within brackets indicate interactions); θ: dispersion factorfor the Tweedie family; dev: deviance explained by the model; REML score:Restricted Maximum Likelihood score of respective model. The selected
model (m14) is given in bold italics
Endang Species Res 32: 515–524, 2017
As availability bias could not be accounted for inour study, the presented density and abundance esti-mates must be considered as minimum values. How-ever, the low survey speed in shipboard surveysallows a large amount of time for animals to surfaceand be detected (Dawson et al. 2008). Therefore, theimpact of availability is likely to be of minor concern,
especially compared to potential biases arising fromthe single observer setup. A single observer survey islikely to miss more of the animals available for detec-tion than a full survey team comprising a dedicateddata recorder and 2 observers. We thus chose to limitthe observer’s field of view in order to minimise theperception bias.
522
Fig. 4. Model prediction of fin whale density around (a) Elephant Island, (b) the South Orkney Islands and (c) the whole surveyarea. Fin whale sightings recorded on effort are marked as blue pentagons in (c). Unidentified large whale sightings aremarked as question marks. ACC: Antarctic Circumpolar Current; see Fig. 1 for more details and Fig. 2 for other abbreviations
Viquerat & Herr: Fin whales around the South Orkneys and Elephant Island 523
Despite these caveats, a single ob server can collectrobust data if adhering to line-transect distance-sam-pling standards, as conducted in this study. Our studyshows that straightforward setups using dedicatedline-transect methodology can yield robust snapshotsof local density and abundance from platforms ofopportunity and contribute valuable information onwhale populations in the Southern Ocean at low cost.
Acknowledgements. We thank the cruise leader PatricioArana, the scientific team of the Chilean finfish survey andthe head of our department, Ursula Siebert. We would alsolike to express our gratitude to the crew of the vessel ‘Cabode Hornos’ for welcoming us on board and helping us set upour equipment during the survey amidst their own duties.
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Editorial responsibility: Robert Harcourt, Sydney, New South Wales, Australia
Submitted: December 21, 2016; Accepted: April 9, 2017Proofs received from author(s): May 30, 2017
