Maritimes Region Stock Status Report 2003/046 December 2003 Northern Shrimp on the Eastern Scotian Shelf (SFA 13-15) Background The northern or pink shrimp, Pandalus borealis, is the only shrimp species of commercial importance in the Maritimes Region. Shrimp are crustaceans, and have a hard outer shell which they must periodically shed (molt) in order to grow. The females produce eggs once a year in the late summer- fall and carry them, attached to their abdomen, through the winter until the spring, when they hatch. Consequently, shrimp bear eggs, or are "ovigerous" for about 8 months of the year. Newly hatched shrimp spend 3 to 4 months as pelagic larvae, feeding near the surface. At the end of this period they move to the bottom and take up the life style of the adults. On the Scotian Shelf, the northern shrimp first matures as a male, at 2 years of age, and at age 4 it changes sex, to spend another 1 to 2 years as a female. Shrimp live 5 to 8 years, depending on conditions. Shrimp concentrate in deep "holes" on the eastern Scotian Shelf, but nearshore concentrations along coastlines closest to the offshore populations have recently been discovered. They prefer temperatures of 2 to 6 ºC, and a soft, muddy bottom with a high organic content. The trawl fishery on the Scotian Shelf has concentrated during summer in the offshore holes, and on an inshore area near the Bad Neighbor Shoal. The main management tools are limits on the number of licenses and size of vessels used, minimum codend mesh size (40mm), use of a Nordmøre separator grate, and a Total Allowable Catch (TAC). The fleet is divided into two sectors, a midshore sector consisting of vessels 65-100' LOA based in New Brunswick on the Gulf of St. Lawrence side, and an inshore sector consisting of vessels <65' LOA based on the Atlantic coast of Nova Scotia. A trap fishery, currently consisting of 9 active licenses, started in Chedebucto Bay in 1994. trap fishery in Mahone Bay on the South Shore. Summary • The DFO-industry survey abundance index increased for the first time in 3 years, suggesting that the recent population decline in 3 of the 4 survey areas bottomed out in 2002. Abundance in the fourth area (Misaine) remains high. • Commercial catch rates (CPUEs) were the highest observed in the history of the fishery, but these have not been considered representative of abundance in most areas. • The year-class recruiting to the 2004 fishery (1999) is about average. However, the 2001 year-class appears to be very strong. These shrimp should start to recruit to the fishery in 2005. • The spawning stock (females) increased in 2003 to the second highest on record. • Total exploitation and female exploitation indices in 2003 were the lowest observed.
Transcript
Maritimes Region Stock Status Report 2003/046
December 2003
Northern Shrimp on theEastern Scotian Shelf
(SFA 13-15)Background
The northern or pink shrimp, Pandalus borealis, is theonly shrimp species of commercial importance in theMaritimes Region. Shrimp are crustaceans, and havea hard outer shell which they must periodically shed(molt) in order to grow. The females produce eggsonce a year in the late summer- fall and carry them,attached to their abdomen, through the winter until thespring, when they hatch. Consequently, shrimp beareggs, or are "ovigerous" for about 8 months of theyear. Newly hatched shrimp spend 3 to 4 months aspelagic larvae, feeding near the surface. At the endof this period they move to the bottom and take up thelife style of the adults. On the Scotian Shelf, thenorthern shrimp first matures as a male, at 2 years ofage, and at age 4 it changes sex, to spend another 1to 2 years as a female. Shrimp live 5 to 8 years,depending on conditions.
Shrimp concentrate in deep "holes" on the easternScotian Shelf, but nearshore concentrations alongcoastlines closest to the offshore populations haverecently been discovered. They prefer temperaturesof 2 to 6 ºC, and a soft, muddy bottom with a highorganic content.
The trawl fishery on the Scotian Shelf hasconcentrated during summer in the offshore holes,and on an inshore area near the Bad Neighbor Shoal.The main management tools are limits on the numberof licenses and size of vessels used, minimumcodend mesh size (40mm), use of a Nordmøreseparator grate, and a Total Allowable Catch (TAC).The fleet is divided into two sectors, a midshoresector consisting of vessels 65-100' LOA based inNew Brunswick on the Gulf of St. Lawrence side, andan inshore sector consisting of vessels <65' LOAbased on the Atlantic coast of Nova Scotia. A trapfishery, currently consisting of 9 active licenses,started in Chedebucto Bay in 1994. trap fishery inMahone Bay on the South Shore.
Summary
• The DFO-industry survey abundanceindex increased for the first time in 3years, suggesting that the recentpopulation decline in 3 of the 4survey areas bottomed out in 2002.Abundance in the fourth area(Misaine) remains high.
• Commercial catch rates (CPUEs)were the highest observed in thehistory of the fishery, but these havenot been considered representativeof abundance in most areas.
• The year-class recruiting to the 2004fishery (1999) is about average.However, the 2001 year-classappears to be very strong. Theseshrimp should start to recruit to thefishery in 2005.
• The spawning stock (females)increased in 2003 to the secondhighest on record.
• Total exploitation and femaleexploitation indices in 2003 were thelowest observed.
Maritimes Region Eastern Scotian Shelf Shrimp
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• The proportion of the total catchtaken during the ovigerous periodwas 32%, up from last year but lowerthan the peak in 2000 (37%).
• Bottom temperatures have declinedsince their peak in 2000. This maybe related to improved recruitmentas suggested by the strong 2001year-class, however, surfacetemperatures are also associatedwith recruitment and these haveremained relatively high.
• The distribution of the large 2001year-class is widespread and fishersmay have more difficulty avoidingsmall shrimp in 2004. This shouldbe reviewed during the fishery.
• Current recruitment should supportthe 2004 fishery at least at the 2003level. Most of the biomass is nowconcentrated in SFA 14 and thecatch will likely be removed primarilyfrom this area as fishers takeadvantage of the accumulatedbiomass of larger shrimp.
The Fishery
The introduction of the Nordmøre gratein 1991 reduced groundfish bycatchesto low levels (2-4%) and allowed theshrimp fishery to expand to its fullpotential. In 1996, the inshore (23vessels <65’ LOA) component of thetrawler fleet moved from individualquotas (IQs) to individual transferablequotas (ITQs), while the midshore (6vessels 65-100’ LOA) moved from acompetitive fishery to IQs. All vesselshave been under ITQs since 1998.Temporary mobile licenses were
introduced in 1998 as part of a co-management agreement to takeadvantage of increasing stock sizes andTACs, while facilitating effort reductionin the event of the rapid downturn oftenseen in shrimp fisheries. With the TACdecreased, temporary access wasremoved in 2002 and 2003.
The TAC has essentially been caughtevery year since individual SFA quotaswere combined into a single TAC in1994, although there have been minorshortfalls due to late quota reallocations.
The temporal pattern of the fisheryhas changed considerably in recentyears. The amount of the catch takenduring the egg-bearing period (August-April) increased significantly to 37% in2000 as fishers took longer to catchhigher quotas, but it decreased to 25%in 2002 as a result of the TAC decrease(long term average – 23%). In 2003 thisincreased to 32% but this was primarilydue to an unexpected summerslowdown to avoid “soft” shrimp, andmay not reflect a longer term change infishing patterns.
Maritimes Region Eastern Scotian Shelf Shrimp
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The spatial pattern of the fishery hasalso changed. Prior to 1999, most ofthe effort and catch was in the MisaineHole (SFA 14), while fishing in otherareas varied between years. In 1998,the N.S. trawler fleet (vessels <65' LOA)fished inshore in SFA 15 for the firsttime, taking 20% of the TAC in a smallarea near the Bad Neighbour Shoal.This rose to a maximum 44% of thecatch in 1999, but has decreased toabout 20% in 2003 as much of the effortshifted again to SFA 14 where most ofthe biomass now concentrates. TheGulf fleet is restricted to the offshoreholes and did not exhibit this shift ineffort. Spatial analysis of catch andeffort data shows an overall increase inthe area with very high (>450 kg\hr)catch rates, but a decrease in the areawith moderate catch rates, consistentwith increased aggregation of shrimp onthe fishing grounds. In 2003 there weresigns that the aggregation wasbeginning to reverse, consistent with theincrease in biomass observed in thesurvey. The widespread distribution ofthe large 2001 year-class suggests thatthis pattern may continue.
A REA WHERE C PUE WA S A BOV E GIV EN V A L UES
0
5 0
10 0
15 0
20 0
25 0
30 0
35 0
40 0
93 9 4 95 9 6 97 9 8 9 9 00 0 1 02 0 3
Are
a (N
o. 1
x1 m
in. u
nits
)
>450
>350
>250
>150
The length composition of the catchhas changed in recent years and showsa progressive narrowing of the shrimpsize distribution. A decrease in thenumber of smaller (<20mm) shrimp
caught can be attributed to theincreased use of square meshedcodends beginning in 1996, and todecreased recruitment. A decreasingtrend in the average size of females inthe catch is interpreted to be due in partto the removal of accumulated older andlarger animals in the population by thefishery.
A decreasing trend in the proportion offemales caught is due to the relativelygood recruitment of males to the fisherybefore 2000. This trend then reversedas males became less abundant and thestrong 1993-1995 year-classesdominated the population and catch asfemales. Average Count (numbers ofshrimp per pound) data provided byindustry indicate that fishers continue tohave no difficulty in staying below the 65count limit to obtain maximum prices.Counts actually decreased and averagefemale size increased considerably in2002-2003 as the biomass accumulatedin the larger sized shrimp due todecreased recruitment, and possiblydue to size-specific concentration ofthese sizes on the fishing grounds.
MEAN S IZE AND % FEMALES IN CATCH
25
25.2
25.4
25.6
25.8
26
26.2
26.4
26.6
94 95 96 97 98 99 00 01 02 03
Mea
n si
ze o
f fem
ales
(m
m)
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
Per
cen
t fe
mal
es i
n c
atch
m ean s izeproportion
The trap fishery off Canso (SFA 15)continued to have above average catchrates during the 2002-2003 season. Inaddition, fishers in the northern part ofChedabucto Bay experiencedexceptional catches. This may berelated to the increased aggregation ofshrimp within the inshore area and\orincreased onshore migrations.
Resource Status
Assessments are based on twocommercial catch rate (CPUE) indices(Gulf vessels only 1978-2003, and all
vessels 1993-2003) obtained fromtrawler logbooks, samples fromcommercial trawl and trap catches(since 1995), a DFO shrimp survey(1982-88), a DFO-industry shrimpsurvey (since 1995), and logs from thetrap fishery.
The two commercial CPUE indicescontinue to show an increasing trend,and were the highest in 2003 for bothseries. However, spatial analyses ofcommercial and survey data and thedecrease in the survey abundance index(see below) indicate that commercialCPUEs were probably notrepresentative of abundance inLouisbourg, Canso, and the inshore.
The overall DFO-industry surveyabundance index increased in 2003 forthe first time after a 3-year decliningtrend, suggesting that the decline, first
Maritimes Region Eastern Scotian Shelf Shrimp
5
observed in 2000, has bottomed out.The decline was observed in 3 of thefour survey areas while a fourth(Stratum\SFA 14\Misaine) hasincreased to the highest levelsobserved, which have been maintainedfor the last 3 years. The spawningstock biomass (female) increased in2003 to the second highest on recordand it has remained at a high levelduring the last 9 years relative to theperiod of low, but increasing biomasses,in the 1980s.
DFO-IN DU S TRY SU RV EY
0
50
100
150
200
250
300
350
1995 1996 1997 1998 1999 2000 2001 2002 2003
Cat
ch/s
tand
ard
tow
louisbourg misainecanso inshorestratified
The survey age composition during thelast few years has been dominated bythe strong group of year-classes (1993-1995), which were near the end of theirlife cycle in the early 2000s.Abundance of age 4 shrimp in 2003(i.e. 1999 year-class shrimp that will beentering the fishery as females in 2004)was about average. The abundance ofage 2 shrimp (2001 year-class) in thestandard survey is the highest observedin the series. Although this estimate isusually considered unreliable, thestrength of this year-class has beenconfirmed by two independent juvenilesurveys, which also found this year-class to be widely distributed.
Largely due to TAC decreases, theindices for total exploitation andfemale exploitation, based on catchweight and the survey biomass, havedecreased to about half of the maximumvalues (approximately 20%) observed in2001.
Decreases in average size at sexchange (Lt) and maximum size (Lmax)are associated with populationdownturns, possibly due to decreasedpopulation fecundity (smaller shrimpproduce fewer eggs). Size at sexchange and maximum size have showna slight decreasing trend during the1990s, possibly caused by warmertemperatures which increased growthrate and decreased size at sex changeand maximum size. However, size atsex change and maximum size remainsubstantially larger than the period oflow abundance in the 1980s.
Regarding ecosystem considerations,feeding studies have shown that shrimpare important prey for many groundfishspecies and significant negativecorrelations between shrimp andgroundfish (that eat shrimp) abundancehave been demonstrated from the Gulfof Maine to Greenland. Many groundfishstocks remain at low levels on theeastern Scotian Shelf and naturalmortality due to predation is probably
below the long-term average and wasprobably not a factor in the recentdecreases in the shrimp population.
Population fluctuations of northernshrimp stocks near the southern limits ofthe species range also show negativecorrelations with water temperatures.On the Scotian Shelf, the populationincrease since the late 1980s may beassociated with colder surface andbottom water temperatures. Warmertemperatures in the late 1990s mayhave contributed to decreasedrecruitment of some cold water indicatorspecies including shrimp, capelin andsnow crab, however, Greenland halibuthas not shown this trend. Bottomtemperatures appear to have cooled onthe shrimp grounds since 2000 and maybe related to the strong 2001 year-class.Continuing stable and high biomasses inthe Misaine area despite relativelyheavy fishing during the late 1990s,suggest that the cause of the recentdecline in the other areas is due more toenvironmental factors than to fishing.
0
1
2
3
4
5
6
82 84 86 88 90 92 94 96 98 01 02
Tem
pera
ture
(ºC
)
0
50
100
150
200
250
300
350
400
Gro
undf
ish
pred
ator
abu
ndan
ce (k
g/to
w)
BO T T O M T EM PE RAT U RES AN D PR EDAT OR ABU NDAN CE
GROUNDFISHPREDA TORS
GROUNDFISH SURV EYTEM PERA T URES
SHRIM P SURV EYTEM PERA T URES
The figure below provides a summary of23 indicators related to the health of theeastern Scotian Shelf shrimp stock.Each indicator was assigned a color forevery year there is data according to itspercentile value in the series i.e. >0.66
Maritimes Region Eastern Scotian Shelf Shrimp
7
percentile = green or good, 0.66-0.33= yellow or intermediate and <0.33 =red or bad. It should be noted thatthese boundaries, although consistentacross indicators, may not beappropriate for some and need to berefined. Indicators have been groupedinto stock characteristics of abundance,production, fishing effects andecosystem.
The overall improvement in the trafficlight table during the last two years (e.g.
13 green and 5 red lights in 2003 versus7 green and 10 red in 2001) isencouraging, but should be interpretedcautiously. For example, the twocommercial CPUE indicators are green,but are not currently representative ofoverall abundance. Fishing mortalityindicators show a marked improvementduring the last 2 years mainly due to thedecreased TAC in 2002-2003. Themajority of production indicators are nowgreen due to improved recruitment.
Note: Not all indicators in the Traffic Light table are discussed in the text. Please consult CSASResearch Document 2004/001 for further details.
Ecosystem indicators have beensomewhat ambivalent, with no strongsignals that could suggest a regimeshift. However, there has been a trendtoward more yellow and red lights since
the period of rapid shrimp biomassincreases in the early 1990s.
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Sources of Uncertainty
As with most research vessel surveyestimates, DFO-industry shrimp surveyresults are associated with highvariances. The spatial analyses whichdiscounted commercial catch\effortindices in favor of survey results ascorrectly depicting recent abundancechanges, are subject to interpretation.Age 2 recruitment estimates from thestandard survey trawl are notconsidered consistently reliable, andsuch estimates from juvenile surveysare based on short time series. There isconsiderable subjectivity associated withassigning modal groups to year-classesin the length frequency analyses,consequently estimates of year-classstrength and population numbers-at-ageusing these analysis must be interpretedcautiously. Although consistent, theboundaries between traffic lights arearbitrary and may not be appropriate forall indicators. The traffic light analysis isfor discussion/consensus buildingpurposes and is not intended to bedefinitive of stock health.
Outlook
The strong 1993-1995 year-classes,which supported the increasing TACsduring the late 1990s were followed byweaker year-classes, decreasingbiomasses, and lower TACs in the early2000s. Despite this, the fisherycontinued to enjoy high catch rates andgood counts as larger shrimpconcentrated into dense aggregations.
After a three year decline, biomasses inthree of the four main fishing areasappear to have bottomed out in 2002.The small increase in these areasobserved in 2003, together with signs ofimproved recruitment, may signal the
beginning of a biomass resurgence.However, biomasses in these areas arestill relatively low. Abundance in SFA14 (Misaine) remains near the highestvalues observed. The bulk of the catchin 2003 was again taken from this areaas fishers took advantage of thebiomass accumulated as larger andolder shrimp. A similar situation can beexpected in 2004.
Abundance of 4 year old shrimp in 2003was about average. These shrimp willrecruit to the 2004 fishery as femalesand should sustain it at least at the 2003level. Two independent juvenile surveysand the standard survey indicate thatthe 2001 year-class is very strong. It isnot possible to determine if this signalsthe beginning of another round of aboveaverage year-classes such as occurredin the early to mid 1990s, but thedecreased temperatures since 2000suggest that this may be the case. If so,biomasses can be expected to continueincreasing, especially when the 2001year-class begins to recruit significantlyto the fishery in 2005 as males. In 2004,the average carapace length of thisyear-class will probably be below<20mm, with an average count >100individuals per pound, consequently itmay be more difficult for fishers to avoidsmall shrimp.
For more Information
Contact:
Mr. Peter KoellerInvertebrate Fisheries DivisionBedford Institute of OceanographyP.O. Box 1006Dartmouth, N.S., B2Y 4A2
Koeller, P. 2000. Relative importance ofenvironmental and ecologicalfactors to the management of thenorthern shrimp (Pandalus borealis)fishery on the Scotian Shelf. J.Northwest Atl. Fish. Sci. 27: 21-33.
Koeller, P., M. Covey, and M. King.2004. An assessment of the easternScotian Shelf shrimp stock andfishery for 2003 and outlook to2004. DFO Can. Sci. Adv. Sec. Res.Doc. 2004/001.
Koeller, P., L. Savard, D. Parsons, andC. Fu. 2000. A precautionaryapproach to assessment andmanagement of shrimp stocks in theNorthwest Atlantic. J. Northw. Atl.Fish. Sci. 27:235-247.
This report is available from the:
Maritime ProvincesRegional Advisory ProcessFisheries and Oceans CanadaP.O. Box 1006, Stn. B203Dartmouth, Nova ScotiaCanada B2Y 4A2
