STOCK ASSESSMENT OF SHRIMP Pandalus borealis (KRØYER 1838) IN SKJÁLFANDI BAY NORTHERN ICELAND

Transcription

1 P.O. Box 1390, Skulgt Reykjvik, Icelnd Finl Project 2008 STOCK ASSESSMENT OF SHRIMP Pndlus borelis (KRØYER 1838) IN SKJÁLFANDI BAY NORTHERN ICELAND Josephus Choe Junior Mmie Ministry of Fisheries nd Mrine Resources, Brooke fields Hotel Complex Jomo Kenytt Rod, Freetown Sierr Leone. jceemmie@yhoo.com Supervisors Hreiðr Þór Vltýsson University of Akureyri, Icelnd hreidr@unk.is nd Hlynur Ármnnsson Mrine Reserch Institute, Icelnd hlynur@hfro.is ABSTRACT The shrimp (Pndlus borelis) stock in Skjálfndi By northern Icelnd is ssessed in this report. Mngement nd ssessment methods of shrimp in Icelnd re reviewed nd suggestions for improvements re given for Sierr Leone. The nlyses re bsed on time-series dt of nnul utumn surveys between 1991 nd 2007 by the Mrine Reserch Institute in Icelnd. Length-frequency nlysis ws lso conducted. Five ge clsses were identified nd their proportions clculted using cohort slicing. Men lengths corresponding to reltive ges were estimted. Growth of P. borelis ws modelled using non-liner minimistion to find the best fit ccording to the von Bertlnffy growth curve. Growth prmeters estimted were L =24.28 mm (crpce length), growth k = 0.46 nd t 0 = The dynmic production model from the commercil ctches nd utumn surveys from1988 to 1999 gve n verge biomss of 2047 tons. Mximum Sustinble Yield per recruit ws 1.42 g which corrresponds to F mx = Mximum Sustinble Yield = 566 tons nd F 0.1 = The dynmic production model hs dvntges over the surplus production models generlly used in Sierr Leone. A review of this model showed tht it could be used to ssess peneid shrimps in Sierr Leone provided tht scientific surveys re done which give more relible estimte of the bundnce index thn ctch from commercil vessels. Dt should be collected monthly or sesonlly nd becuse it is very expensive to conduct surveys, sesonl surveys should be done. Periodic collection of dt, relting to selectivity, mturity, length nd weight re prerequisite for proper ssessment of the shrimps of Sierr Leone. The economy of Icelnd is to lrge extent dependent on cod nd the current mngement strtegy is to enhnce the cod stocks. This does not fvour shrimp. Perhps it is time to revise the current mngement strtegy. UNU Fisheries Trining Progrmme

2 TABLE OF CONTENTS 1 INTRODUCTION Objective Specific gols LITERATURE REVIEW The study re The ecology of Pndlus borelis Geogrphicl distribution Environmentl requirements Feeding nd growth Reproductive strtegy Mortlity Mngement of the shrimp fishery in Icelnd MATERIAL AND METHODS Dt Dt nlyses Length-frequency nlysis nd cohort slicing Estimtion of growth prmeters Length-weight reltionship Selectivity Dynmic production model The number model Recruitment Nturl mortlity Fitting the model Yield per recruit nd spwner stock per recruit RESULTS Length-frequency distributions nd men length t ge Length-weight reltionships Von Bertlnfy growth prmeters Yield nd spwner stock biomss DISCUSSION Mngement of the shrimp fishery in Sierr Leone Assessment nd mngement mesures to be incorported into the shrimp fisheries of Sierr Leone Assessment methods Mngement By-ctch reduction devices CONCLUSION ACKNOWLEDGEMENTS LIST OF REFERENCES UNU-Fisheries Trining Progrmme 2

3 LIST OF FIGURES Figure 1: Mp showing study re, Skjálfndi By northest of Icelnd. (Adpted from Jónsson 1996) Figure 2: Length-frequency distribution of P. borelis plotted from the utumn surveys dt for Figure 3: Length-frequency distribution of P. borelis in utumn surveys in Skjálfndi By Figure 4: Cumultive length-frequency distribution of P. borelis in utumn surveys in Skjálfndi By Figure 5: von Bertlnfy growth curve for P. borelis in Skjálfndi By. Light curve is fitted to initil growth prmeter vlues nd thick curve is the fitted growth prmeter estimtes Figure 6: Output from dynmic biomss model. Figures 6) nd 6b) show nnul ctch nd biomss index from surveys respectively (circles) nd lines fitting the models. 6 c, d, e, nd f re the fishing mortlities, recruitment, biomss nd spwner biomss estimted from the model. The sums of squres of errors re SSEI = 0.48, SSEY= 0.03, SSER= nd SSEF = Figure 7: Yield per recruit nd spwning stock per recruit curves for fishing mortlities rnging from 0 to 2. Arrow is showing fishing mortlity corresponding to F Figure 8: Annul shrimp recruitment estimted from the dynmic production model for ech yer regressed with the men number of cod (Gdus morhu) per hul in the previous yer from the MRI utumn surveys. The generl trend is tht s cod numbers decrese in yer, the recruitment of shrimp increses the following yer Figure 9: Regression of shrimp recruitment estimted from the dynmic production model in yer with the men number of hddock per hul in the previous yer from the MRI utumn surveys. When numbers of hddock were high in yer, the recruitment of shrimp ws low in the yer fter Figure 10: By-ctch levels in the shrimp fisheries of Sierr Leone in On verge the by-ctch level ws bout four times the mount of shrimp cught LIST OF TABLES Tble 1: Men crpce length (mm) of P. borelis estimted from the utumn survey dt nd estimted verge proportion of different yer clsses, using the cohort slicing method Tble 2: Comprison of growth prmeter estimtes from previous studies with tht obtined from this study using Phi prime (Puly 1979, Puly nd Munro 1984) Tble 3: Biomss estimtes of P. borelis in Skjálfndi By from Tble 4: Yield nd spwner stock biomss estimted from the yield per recruit models for different fishing mortlities of P. borelis in Skjálfndi By UNU-Fisheries Trining Progrmme 3

4 1 INTRODUCTION The pndlid shrimp of Icelnd hs been of considerble scientific interest becuse of its commercil vlue in the pst s well s its ecologicl interction with fish species including cod nd hddock. In 1998, the vlue of shrimp (Pndlus borelis) cught by vessels fishing in Icelndic wters ws bout USD 66 million, the second highest fisheries resource in terms of vlue fter cod (Hgstof Islnds 1999). Shrimp fishing in Icelnd ws experimented for the first time round 1924 in Ísfjrðrdjúp, fjord in the northwest peninsul of the country (Hllgrímsson 1993, cited by Grci 2007). Commercil fishery strted more thn ten yers lter in 1935, when lnd bsed processing plnts hd been built (Grci 2007). This probbly led to the extension to other inshore fishing grounds, including Arnrfjörður, three yers lter, nd to Húnflói in northern Icelnd (Hllgrímsson nd Skúldóttir 1981). The dvent of shrimp peeling mchines in 1959 s well s bigger nd better fishing vessels in the lte 1960s resulted in incresing fishing effort. Shrimp fishing ws profitble nd this resulted in further expnsion of these fisheries to other inshore res round Icelnd (Skúldóttir Ctches between 2500 nd 7800 tons were recorded during the period Lndings from the inshore fisheries were estimted t round 12,000 tons in 1996 but were on verge less thn 8,000 tons in most yers from 1973 to 2000 (Skúldóttir nd Sigurjónsson 2003). During the present 2008/2009 fishing seson only 122 tons of shrimp hve been lnded so fr (Fiskistof 2009) out of quot of 500 tons llocted to Arnrfjörður. The low TAC for inshore shrimp is becuse of collpse tht occurred in recent yers, nd ccording to the Icelndic Mrine Reserch Institute (MRI), is due to incresed predtion by cod (MRI 2008). Tht sid, it should be noted tht other shrimp fisheries in the world hve been severely ffected by fishing pressure including perturbtions cused by otter trwls on the bottom substrte in which shrimp minly burrow (Jennings nd Kiser 1998). Unlike Icelnd, the shrimps of Sierr Leone re of the fmily Peneide. Peneide re generlly lrger thn Pndlus. The genus Pneus occurs in tropicl nd subtropicl wters between the ltitudes 40 N nd 40 S. Adult shrimp rrely occur below 180 m (Biley-Brock nd Moss 1992). They re lterlly compressed, with the hed contining prominent rostrum. Shrimps belonging to fmily Peneide re dioecious, unlike Pndlus which re protndric hermphrodites. The mle Pneus is identified by the two pirs of modified ppendges on the first nd second bdominl segments (the petsm nd ppendix msculine) both of which re used in the trnsfer of sperm to the femle. In femles there is sperm receptcle (thelycum) locted between the bse of the fifth wlking legs (Biley-Brock nd Moss 1992). The most commonly exploited shrimp species re Peneus kerthurus (tiger shrimp), Prpeneus longirostris (deep wter rose shrimp), Peneus notilis (pink shrimp) nd Prpenepisis tlntic (white shrimp). Of the 80 commercilly exploited finfish, molluscs nd crustcens in Sierr Leone, shrimps hve on verge contributed lrge percentge to the totl income from the fisheries resources in recent yers, just s it did for Icelnd in the pst. The estimted vlue for UNU-Fisheries Trining Progrmme 4

5 shrimp exported from Sierr Leone in 2006 ws round USD 8,000,000 which ws 56.3% of the totl vlue of fisheries exports in tht yer (Anon. 2008). Shrimp stocks in Sierr Leone re minly exploited by Chinese owned vessels with the primry im of exporting the ctch. Recent estimtes by the Sttistics Unit of the Ministry of Fisheries nd Mrine Resources using the Fox version of surplus yield model, gve potentil yield of 1555 mt for nd F msy of 4950 fishing dys (Anon. 2008). Lndings during this period were on verge bout 1590 mt. The estimted potentil yield cn be misleding due to the fct tht the effort ws bsed on fishing dys rther thn stndrdised effort from the commercil fishing vessels which hve different gross registered tonnge (GRT) nd whose ctch sttistics were used for the estimte. It is worth noting tht there is sometimes misreporting of ctch sttistics. Around the region of West Afric including the wters of Sierr Leone, illegl unreported nd unregulted (IUU) fishing prctices re sid to be high. However, the extent of IUU fishing nd is unknown, posing problem for stock ssessment. Commercil fisheries for peneid shrimps hve long history long the costs of mny sub-tropicl nd tropicl countries, often leding to over-exploittion (Grci nd LeReste 1981). The dvent of specilised industril fishery in the 1950s, brought bout n increse in the exploittion of shrimp (Leite nd Petrere 2006) nd by the end of the 1970s, there ws n incresed demnd for shrimp prticulrly in high vlue mrkets like the USA, Europe nd Jpn (Leite nd Petrere 2006) resulting in n nnul production to the tune of 700 thousnd tons by the erly 1980s (Isc et l. 1992, cited by Leite nd Petrere 2006). The expnsion of cpture fisheries nd vilble (but often indequte) informtion on the biology of peneid shrimps, hve led to the conclusion tht most stocks re either fully exploited or over-exploited (Leite nd Petrere 2006). Icelnd hs experienced sitution where inshore pndlid shrimp stocks hve collpsed nd fishing in these res hs been stopped, including Skjálfndi By. The collpses hve been ssocited with cod predtion on shrimp rther thn the effect of fishing. However, shrimp fishing hs been llowed gin in the inshore re of Snæfellsnes nd in the inshore re of Arnfjörður, TAC of 150 tons ws llocted in the fishing yer nd 500 tons for the fishing yer. The MRI continues to monitor the stocks by bi-nnul independent surveys in spring nd utumn. The Mrine Reserch Institute hs lso encourged UNU-FTP fellows, for exmple Ceesy (2000) nd Lm Anh (2007), to use dt from the inshore stocks with view to estimte the biomss nd predict stock development. The effect of predtion of cod nd hddock on shrimp cnnot be overemphsised. The economy of Icelnd is to lrge extent dependent on cod nd the current mngement strtegy is to enhnce the cod stocks. This does not fvour shrimp. Perhps it is time to revise the current mngement strtegy. UNU-Fisheries Trining Progrmme 5

6 Sierr Leone hs never documented collpse in the shrimp fisheries but it is worth gining knowledge from Icelnd nd to see if the monitoring, stock ssessment nd mngement mesures might be dpted to the sitution in Sierr Leone. The Europen Union is funding three yer project on Institutionl Support for Fisheries Mngement, ISFM, nd prt of the project is to estimte the biomss of the fisheries resources nd exploittion ptterns. For rtionl exploittion of ny fisheries resources, the knowledge of its popultion dynmics is very importnt. This study ssesses the stock of Pndlus borelis in Skjálfndi By (Figure 1) by using dynmic production model. 1.1 Objective The min objective of this study is to ccess the sttus of the stock of Pndlus borelis in Skjálfndi By north of Icelnd, using dynmic production model Specific gols To estimte the trend in biomss of P. borelis To estimte growth prmeters of P. borelis To estimte equilibrium yield potentil nd spwner stock biomss To compre the shrimp stock ssessment nd mngement system of Icelnd with tht of Sierr Leone nd suggest corrective mesures where necessry. To evlute the ppropriteness of using dynmic production model for stock ssessment in the shrimp fisheries in Sierr Leone, considering the differences in stock identifiction, nd the life history, biology nd behviour of peneid shrimp to tht of inshore stocks of P. borelis in Icelnd, tking lso into ccount the vilbility nd resolution of dt. 2 LITERATURE REVIEW 2.1 The study re Skjálfndi By in the northest of Icelnd is 10 km wide t the inner prt nd bout 51 km between the cpes of Gjögutá nd Tjörnestá (Figure 1, Gíslson 2004). The mximum depth is bout 220 m nd the by is pproximtely 25 km long. Bottom sediments on the slope re minly snd or grvel, while the rest of the se bed is lmost entirely covered by silt. River Skjálfndfljót is glcil river which crries lot of fine cly on the western side nd River Lxá in Aðldl, which is minly spring fed river, is on the estern side, but crries lot of volcnic blck snd from the brren interior of the country. These two lrge rivers not only influence the substrte but lso result in low slinity in the by (Gíslson 2004). Wrm geotherml wter from the nerby lke Mývtn (Ólfsson 1999) regultes the wter temperture in Lxá in Aðldl resulting in incresed surfce temperture in the by, prticulrly t the river mouth. UNU-Fisheries Trining Progrmme 6

7 Figure 1: Mp showing study re, Skjálfndi By northest of Icelnd. (Adpted from Jónsson 1996). 2.2 The ecology of Pndlus borelis Geogrphicl distribution The shrimp Pndlus borelis, known in Icelndic s stóri kmplmpi or rækj, is lso clled the northern shrimp, northern red shrimp, cold-wter shrimp, deep-wter shrimp or pink shrimp. It hs wide geogrphicl distribution, spnning from the northern prts of the North Atlntic to the northern Pcific Ocen. P. borelis inhbits mny wters in these regions s reviewed by Grci (2007). It is, however, endemic to the sub-rctic region between 35 N nd 82 N (Hørsted nd Smidt 1956, Bergström 2000). Its distribution hs been recorded clockwise from the southwest nd on to the est cost of Icelnd t depths rnging between m but mostly in offshore res below 300 m (Skúldóttir nd Sigurjónsson 2003). P. borelis lso inhbits inshore bys nd fjords where they re cught in shllower wters. Lrve, juveniles nd dults show different distribution ptterns. Lrger individuls inhbit deeper wters whilst lrve nd juveniles re found in shllower wters or ner shore, trend tht hs lso been observed in severl fish species. This pttern could be relted to temperture, which is reltively more stble s depth increses, nd the bigger shrimps prefer such environment. Younger shrimps hve greter tolernce for temperture nd slinity fluctutions thn dults (Hørsted nd Smidt 1956, Aschn 2000). Sesonl vritions in depth distribution hve been reported nd this could be UNU-Fisheries Trining Progrmme 7

8 scribed to temperture chnges. Verticl migrtion hs lso been reported in shrimp nd this is ttributed to feeding, moving up into the wter column t night nd returning to the bottom substrtum in the morning (Shumwy et l. 1985, Bergström 2000). Shrimp exhibit this verticl migrtion probbly to void predtors. Depth t which the species is most bundnt vries with ltitude; being most bundnt in much deeper wters t higher ltitudes (Shumwy et l. 1985) Environmentl requirements Just like for mny other mrine species, the nture of the substrtum, temperture nd slinity re fctors tht hve n effect on the distribution of the northern shrimp (Shumwy et l. 1985). Northern shrimp cn tolerte tempertures between -1.6 ο C nd +8 ο C (Bergström 2000) but occur mostly t 0 C. Development of P. borelis t ll stges in the life cycle increses with temperture, cusing shrimp in wrmer wters to mture t younger ge nd of smller size thn in colder wters (Nilssen nd Hopkins 1991, Skúldóttir nd Pétursson 1999). Northern shrimp prefers slinity between lthough it hs been reported t slinity s low s 23.4 in some res (Grci 2007). Slinity nd temperture influence the circultion ptterns of wter msses in the ocens. The circultion ptterns over time influence primry productivity nd subsequently the biodiversity of mrine ecosystems (Hunt nd Drinkwter 2005). In the lte 1990s, wrm sline Atlntic wter mss north of Icelnd (Gudmundsson 1998) resulted in the upwelling of nutrient rich bottom wters nd increse in primry productivity. Incresed temperture lso led to the migrtion of cod to the northern Icelndic wters wy from the inshore res thus resulting in decresed predtion nd probbly to incresed growth nd recruitment of shrimp. The species is reported to inhbit res where the bottom sediments re minly soft, muddy or sndy (Hørsted nd Smidt 1956, Shumwy et t. 1985, Bergström 2000), nd ocen bottoms with sediments rich in prticulte orgnic crbon (POC), Bergström (2000) Feeding nd growth P. borelis is ctholic feeder. It filter-feeds s it moves through the wter column during night (Wollebæk 1903, Hørsted nd Smidt 1956, cited by Grci 2007) devouring zooplnkton. It is known to feed on smll benthic invertebrtes including polychetes, whilst forging or even scvenging on the bottom sediments (Shumwy et l. 1985). Offshore, mles of P. borelis grow throughout the yer (Grci 2007). However, slower growth rte is observed in winter nd this is ttributed to food limittion. Mles grow throughout the yer, lthough more slowly in winter. Mture femles grow during summer nd sometimes lso every second winter in offshore popultions. This is becuse egg bering occurs every second yer (Skúldóttir et l. 1991). In the inshore wters, P. borelis my grow from spring to utumn (Grci 2007), with no growth occurring in winter (Nilssen nd Hopkins 1991). Growth rte is therefore fster in wrmer wters nd UNU-Fisheries Trining Progrmme 8

9 slower in colder wters. It ws, however, noted by Nilssen nd Hopkins (1991) tht chnges in wter temperture, density of the popultion nd recruitment hve the potentil to trigger differences in growth rtes nd the length t which P. borelis chnge sex. These effects were observed off the northest cost of Icelnd where shrimp popultions found in the re contining north Icelndic winter wter with tempertures of 2-3 C nd Arctic bottom wter below 0 C lying under wrmer upper lyers, spwn for the first time t 6 yers nd rech L 50 (crpce length t which 50% of the femles re mture) t mm (Grci 2007). However, in Vestfirðir nd Snæfellsnes, west cost of Icelnd which received wrmer Atlntic wter t temperture of 7 C, shrimp spwn t ge 3 nd L 50 of mm. Although P. borelis is difficult to ge due to the fct tht shrimp hve no hrd prts such s otholits nd scles, ge nd growth estimtes hve been done by mny uthors (Skúldóttir 1981, Shumwy et l. 1985, Bergström 2000). It is lso known tht growth of the species cn be modelled using the von Bertlnffy growth eqution (Bergström 2000). Length t ge of P. borelis in Húnflói, n inshore fjord in Icelnd, vries from 12.1 mm crpce length for 2 yer old to 22.1 mm for the 6 yer plus group (Skúldóttir, pers. comm.) Reproductive strtegy In Icelndic wters, P. borelis re mles when they first reproduce (Berkeley 1930) but lter chnge sex nd become femle for the rest of their lives (Skúldottir nd Pétursson 1999). This is distinctive feture of pndlid shrimps, but there re reports from the Gulf of Mine nd Sweden tht some individuls develop into femles from birth (Bergström 2000). According to Rsmussen (1953), sex chnge in the northern shrimp is size dependent but does not depend on ge lone. Age t sex chnge vries from plce to plce. In Icelnd, the inshore shrimp chnge sex when they re bout 3ö5 yers old. Sex chnge vries with loction nd it is lso influenced by temperture (Appollonio et l. 1986). Fertilistion in Pndlus borelis is externl nd the ovigerous period in offshore popultions where tempertures ner the bottom re round o C is 10 months. The wrmer inshore popultions hve shorter egg crrying period of 5 months. The inshore stocks spwn once yer during summer or erly utumn in most plces but when temperture is low s in the offshore popultions, spwning tkes plce only once every 2 yers (Skúldóttir et l. 1991). Fecundity of the species correltes positively with size nd temperture, lrger femles crrying more eggs rnge from but mostly round 2000 nd regions with reltively low tempertures hve been reported to lower fecundity in P. borelis (Nilssen nd Hopkins 1991, Bergström 1997). The life cycle of P. borelis is reviewed in Bergström 2000 nd Grci (2007). After htching, the pelgic lrve go through six developmentl stges before reching the post-lrvl stge. Post-lrve moult six times nd finlly metmorphose into young UNU-Fisheries Trining Progrmme 9

10 shrimps. The species become sexully mture fter nother seven stges including the juvenile stge. On verge the species cn live from yers (Shumwy et l. 1985) Mortlity Mortlity of P. borelis is influenced to gret extent by cod which is one of its mny predtors. Other predtors include hddock, Greenlnd hlibut, redfish nd sebirds. Totl mortlity coefficient, Z, is estimted to be between (Grci 2007), highest in res where fishing is intense. Prsitic infesttion is lso contributing fctor to mortlity in P. borelis. The prsite Phryxus bdominlis, n isopod, is found on the til of smples from Greenlnd (Hørsted nd Smidt 1956) nd my lso ttck tissues of the muscle. 2.3 Mngement of the shrimp fishery in Icelnd Inshore shrimp fisheries re mde up of nine seprte popultions inhbiting six fjords nd ner-shore res in Icelndic wters (Skúldóttir nd Pétursson 1999, cited by Ceesy 2000). The inshore stocks in the vrious fjords of Icelnd re mnged s seprte units. Mngement regultions for inshore nd offshore shrimp stocks of Icelnd include obtining fishing license, minimum squre mesh size in cod end of 36 mm, minimum lnding size with crpce length, CL>15 mm in the inshore nd CL>13 mm for offshore stocks, nd quot system bsed on Totl Allowble Ctch (TAC). Less thn 30% of the ctch cn be below the stipulted minimum size. Keeping logbooks nd recording of ctches is mndtory by lw. The TAC ws first implemented in the period between 1962 nd 1967, but without specifying which vessel should fish in which re. A new TAC system ws introduced in 1974 which lloctes ctch to vessels, inshore res nd restrictions of lndings by res. No one ws permitted to lnd outside the fjord in which they were permitted to fish. TACs for ech fjord re set by the MRI, bsed on independent ssessment nd divided mong fishing vessels by the Fisheries Directorte of Icelnd. Bi-nnul inshore trwl surveys hve been crried out by the MRI since 1973, in lte winter (Februry), sometimes referred to s the spring survey nd gin in the utumn (September/October). The surveys were stndrdised in 1988 using fixed trwling sttions, ger nd towing speed. MRI recommends hrvesting strtegy which is bsed on the biomss indices from the two most recent surveys nd long term trend in ctches with view to secure long-term optimum ctch thereby sustinbly exploiting the resource. The mngement of shrimp stocks hs evolved over the yers. For exmple in 1962, the minimum mesh size ws 32 mm nd in mm Nordmøre sorting grid ws included in the regultions for the mngement of shrimps of inshore stocks t Eldey, Skjálfndi By, nd round the Snæfellsnes peninsul in western Icelnd (Skúldóttir nd Sigurjónsson 2003). This ws, however, not mndtory. Individul Trnsferble Quot (ITQ) cme into existence in 1990, with the limittion tht quots cn only be trnsferred to vessels registered to fish in the sme re. UNU-Fisheries Trining Progrmme 10

11 In ddition to the bove mngement strtegies, strict by-ctch regultion is implemented which limits the number of juvenile fish, prticulrly cod nd hddock which cn be cught s by-ctch. During utumn there is usully n influx of juvenile fish into the inshore to the extent tht fishing cn be closed until survey results show their disppernce from the fishery (Skúldóttir, pers. comm.). 3 MATERIAL AND METHODS 3.1 Dt The Mrine Reserch Institute inshore reserch survey dtbse ws used in this study. The dt covers Icelndic inshore wters from 1973 to the present, which hs been collected by reserchers t the MRI during binnul spring nd utumn surveys of the in inshore shrimp stocks. Ctch dt from commercil fisheries ws lso used. The inshore shrimp trwl dt include crpce length-frequency by sex, nnul ctches in weight nd numbers, ctch nd other informtion on durtion of tows, ger used, mesh size, verticl nd horizontl opening of the ger, bottom temperture nd depth of ech survey re. Crpce length ws mesured from the posterior mrgin of the orbit to the posterodorsl mrgin of the crpce (Bergström 2000, p.186). No direct ge determintion is crried out for P. borelis during the surveys since the species lck otoliths nd hrd prts. For the purpose of this study, utumn survey dt from in Skjálfndi By is used. 3.2 Dt nlyses Length-frequency nlysis nd cohort slicing Length-frequency distributions were plotted by yer nd the most distinct modes identified. These modes were ssumed to be men lengths corresponding to reltive ges. The dt were then pooled for the entire period of study nd the length-frequency distribution plotted. Cohorts in the combined length-frequency were sliced using the method of McDonld nd Pitcher (1979), which ge-disggregtes length distributions. This ws done by using functions in the sttisticl softwre, R (R 2.8.1). The men lengths observed from the length-frequency plotted by yer were then used in some functions written in R (Stefnsson nd Tylor 2008) to estimte the true men-length of the different cohorts together with their stndrd devitions. The ssumption is tht ech of the combintions of cohorts length distributions re in the form of Gussin distribution (Stefnsson nd Tylor 2008) with men length, µ nd vrince, σ 2 in the form: ( x ) /(2 ) f x e...(1) 2 nd the cumultive distribution ws x x F( x) ( t) dt ( )...(2) UNU-Fisheries Trining Progrmme 11

12 Now ssuming tht fixed ge group is distributed long the length xis ccording to the Gussin density, with men length, ( ) nd stndrd devition, ( ). The proportion of individuls in length ctegory l whose width = 1 is given by: 1 1 ( l ) ( l ) (3) 1 1 since this is probbility of shrimp hving length between ( l ) nd ( l ). 2 2 Suppose the true proportion of shrimp in ge group is then the proportion of shrimp in the length group l, cross the entire ges in the length distribution is:... (4) From the bove, we hve to estimte the proportions in ech ge group, the men length t ge, nd the stndrd devition From the length-frequency plotted, some men lengths t ge were estimted nd guess ws mde for fixed stndrd devition corresponding to the men lengths. Hving fixed these vlues, the proportions for ech men length were estimted using n R commnd (Stefnsson nd Tylor 2008). A number of runs were done in the R commnd first with fixed vlues, then by chnging the vlues or using the estimted proportions from the model runs, predicted proportions t length were obtined. The predicted length distribution is given by: 1 1 ( l ) ( l ) 2 2 yl...(5) nd to estimte the unknown prmeters,, nd tht gives the best fit, the sums of squres of the observed proportion t length nd the theoreticl proportions is clculted by: 2 ( yl yl )...(6) where yl is the mesured, (observed) proportion nd ŷl the modelled proportion. This minimises the discrepncy between the observed nd the theoreticl vlues Estimtion of growth prmeters Once the men length t ge of ech cohort nd the stndrd devitions hve been estimted, then the verge growth rte cn be modelled. The von Bertlnffy growth eqution (von Bertlnffy 1938) ws pplied to the estimted men length-t-ge nd the stndrd devition dt by the non-liner minimistion function in R. UNU-Fisheries Trining Progrmme 12

13 The von Bertlnffy model hs been used extensively to describe the verge growth of finfish (Dickie 1968) nd shrimp (Bergström 1992) nd is often used in fish popultion dynmics. The von Bertlnffy growth model ssumes tht growth is fster when the fish nd species is young, slows s it mtures nd levels off t lter ge. It is ssumed tht the growth of P. borelis in the Skjálfndi By is well described by the von Bertlnffy growth function: ( k ( t t0 ) Lt L (1 e )...(7) Where L t is the verge length-t-ge t, L is the symptotic length, k is the growth velocity prmeter nd t is hypotheticl ge t length equl to 0. 0 A function to estimte growth prmeters ws written in R nd by non-liner minimistion, estimtes of the lest sums of squres were obtined. Initil vlues of L, k nd t 0 were set nd the lest sums of squred error, SSE, ws minimised using n itertive process. The estimted growth prmeters which gve the minimum SSE were tken s tht of P. borelis in the study re nd for the period of study. To verify the gross ccurcy of the growth prmeters, the growth performnce index (Phi prime vlue, ø' = log 10 k + 2log 10 L (Puly 1979, Puly nd Munro 1984) ws used to compre the results with those from previous studies Length-weight reltionship The reltionship between length nd weight of fish nd shellfish including shrimps is expressed s: W=L b...(8) Where W, is the weight in grms, L is the crpce length in mm, nd nd b re prmeters. Eqution (8) bove ws log-trnsformed. Ln(W)=Ln()+ b*ln(l)...(9) nd simple liner regression pplied to get the (intercept) nd b (slope) vlues. The length-weight reltionship ws computed seprtely for shrimp without sternl spines (mture femles) nd those with sternl spines (immture femles, mles nd trnsitionl). The length-weight reltionship ws then used to convert length t ge to weight t ge Selectivity A selectivity ogive ws used to estimte the selectivity for different length clsses of P. borelis from Skjálfndi By. The selection pttern ws bsed on the result of 36 mm (cod-end) mesh size (Skúldóttir pers. comm.). The results were obtined from the MRI. 3.3 Dynmic production model UNU-Fisheries Trining Progrmme 13

14 The von Bertlnffy growth prmeters were used to convert length of shrimps to ge in order to run n ge-bsed dynmic model. The bsis for this ge-structured stock ssessment model, s it is sometimes clled, is to estblish popultion dynmics model nd then see how the model s predictions of nnul ctches in this cse, relte to the observed dt. The model incorportes recruitment, nturl mortlity nd weighting correction fctors which re explined below The number model The stock size of shrimp for ech yer ws estimted bsed on the number model: N N R C...(10) y 1 y y y Where R y represents the recruitment in yer y, N y is the number of shrimp tht survive in the yer y, nd C y is the ctch in the sme yer. The biomss is then: By ( Ny * w )...(11) When modelling popultion of numbers of shrimp with true ges 1,..., A my use ge A+1 s plus group. As result of mortlity, the plus group is reduced ech yer, but lso new ge group enters the plus group nd this is represented by eqution (12) ZA N ( N N ) e...(12) A 1, y 1 A, y A 1, y Recruitment The spwning stock per recruit in the fishery will be estimted by using the Beverton- Holt eqution: s R...(13) s (1 ) k Where R = recruitment S = spwning stock α = coefficient used s multiplier for prospective recruitment K = size of the spwning stock tht produces hlf mximum recruitment Nturl mortlity Nturl mortlity, M, is lso needed s one of the inputs in the model. Nturl mortlity ws not estimted becuse of lck of dt nd the populr Puly s empiricl formul (Puly 1979 pplies only to fish nd not crustcens. Nturl mortlity ws therefore ssumed to be 0.5. This vlue is usully used in the literture (Fu nd Quinn II 2000, Skúldóttir et l. 2004) Fitting the model UNU-Fisheries Trining Progrmme 14

15 The model is fitted by first using initil prmeters stted bove nd followed by defining the functions in R which utilises non-liner minimistion. The model is evluted by compring the model output to the dt nd this is done by computing the sums of squres between ech dt set nd the fitted vlues they correspond to (Stefnsson 2007). Coefficients of vrition, CVs for different prmeters re used s weighting fctors ssigned to the sums of squres which re then minimised in order to estimte the prmeters in the model. Assuming tht ll terms re log trnsformed dt. Ech term is then of the form: 2 (ln( x ) ln( x ˆ )) t t t Where x t s were nnul lndings, biomss index, recruitment fctor, fishing mortlity nd is the weighting fctor. Sttisticlly, the correct weighting fctor is the inverse of the vrince, 1 2 ln( x t ) But in the cse of low dispersion, the stndrd devitions of the log-trnsformed quntities nd the coefficient of vrition of the originl numbers re similr. Thus CV(x) is used s ln( x t ). The CV for ctch, CV(Y) ws ssumed to equl 0.1 since ctch dt re quite precise. The CV(Y) used here thus ssumes tht 95% of nnul ctch estimtes lie within 20% (two stndrd devitions) of their true vlues (we ssume here gin tht the errors re symmetricl). The coefficient of vrition for biomss index, CV(I), ws set to 0.1, coefficient of vrition of fishing mortlity, CV(F), ws set to 0.4 nd coefficient of vrition of recruitment, CV(R), ws set to During the itertion process, these vlues were llowed to devite freely until better CVs were obtined to fit the model. There is, however, no generl consensus s to how to fit these CVs (Stefnsson pers. comm.). The model is fitted by first using the initil prmeters stted bove, followed by defining the functions in R which utilises non-liner minimistion. The model is evluted by compring the model output to the dt nd this is done by computing the sums of squres between ech dt set nd the fitted vlues they correspond to. Coefficients of vrition, CVs for different prmeters re used s weighting fctors ssigned to the sums of squres which will then be minimised in order to estimte the prmeters in the model (Stefnsson 2007). UNU-Fisheries Trining Progrmme 15

16 3.4 Yield per recruit nd spwner stock per recruit Yield per recruit nd spwner stock per recruit were estimted for different fishing mortlities rnging from 0 to 2. The spwner stock biomss ws estimted for 3 yer old shrimps. This is when they chnge sex in Skjálfndi By (Skúldóttir nd Pétursson 1999). The estimtion of yield potentil from stock is done in two stges by first estimting the number of recruits nd then clculting the yield per recruit, which depends on the size (ge) t recruitment nd the fishing mortlity. These clcultions re used to estimte how much of prticulr yer clss could be cught for sustinble utilistion. The ctch in numbers of shrimp t ge ccording to Stefánsson (2008b) is given by: F Z Z C (1 e ) e R...(14) Z where C is ctch t ge, F is fishing mortlity, Z is totl mortlity nd R is recruitment nd the ctch in weight is given by: Y F Z w Z (1 e ) e R...(15) Z then totl yield of the yer-clss then sums up to: Z Y (1 e ) w e R Y F Z This gives yield per recruit s: Y F Z Z (1 e ) w e Z Z...(16)...(17) R The yield per recruit ws clculted for fishing mortlities, F, rnging from 0 to 2.0 to find vlue of F tht will provide high yield to the fishery, while t the sme time keep the stock size resonbly high. A yield per recruit curve ws then plotted nd the fishing mortlity F mx, which corresponds to mximum yield per recruit, ws noted. From the sme yield per recruit curve, the fishing mortlity t which the slope is one-tenth of the slope t the origin is F 0.1. This indictes fishing mortlity which is lower thn tht required to obtin the mximum yield nd will result in economic optimistion of the fisheries nd prevent recruitment overfishing (Gullnd 1983). The model though theoreticl, is used to give plusible indictions s to whether hevy or light effort gives rtionl exploittion or if it is better to ctch young or old shrimp. The ssumption in the yield per recruit model is tht recruitment is constnt, nd tht the ge structure of the popultion is the sme s if single cohort is followed through time. Hence mesure of yield per recruit (Beverton nd Holt 1957). The clculted yield per recruit for different fishing mortlities ws then multiplied by the ssumed virgin stock to give n estimte of yield. UNU-Fisheries Trining Progrmme 16

17 4 RESULTS 4.1 Length-frequency distributions nd men length t ge Crpce length-frequency distributions plotted for ech yer did give some modes but only few (Figures 2 nd 3). However, plot of the combined crpce length-frequency distributions plotted length-frequency distribution of P. borelis in utumn surveys in Skjálfndi By (Figure 4), reveled five distinct peks corresponding to discrete ge groups when cohort slicing ws done. The men length t ge estimtes rnged from mm (CL) for shrimps tht re 1 yer old, to mm (CL) for those t ge 5 (Tble 1). Proportions corresponding to reltive ges revel tht the 2 yer olds were bundnt nd proportions of 5 yer olds in the stock were smll. Tble 1: Men crpce length (mm) of P. borelis estimted from the utumn survey dt nd estimted verge proportion of different yer clsses, using the cohort slicing method. Age Men crpce length (mm) Stndrd devition Proportion Length-weight reltionships Length- weight reltionships obtined for P. borelis in Skjálfndi By re given below: Shrimps with sternl spine: W= *L Shrimps without sternl spine: W= *L Von Bertlnfy growth prmeters Men lengths were plotted ginst reltive ge together with Von Bertlnffy growth curve (Figure 5). The verge growth prmeter estimted for P. borelis in Skjálfndi re symptotic crpce length CL =24.28 mm, growth velocity prmeter, k = 0.46 nd t 0 = with SSE = The estimtes of the symptotic crpce length s well s the growth velocity prmeters estimted for this re re similr to those estimted from previous studies on P. borelis elsewhere (Tble 2). UNU-Fisheries Trining Progrmme 17

18 Tble 2: Comprison of growth prmeter estimtes from previous studies with tht obtined from this study using Phi prime (Puly 1979, Puly nd Munro 1984). Study re L (mm) K (yr -1 ) ø' Reference Skjálfndi By This study Húnflói Lm Anh (2007)** Flemish Cp Skúldóttir et l. (2007) Kchemk By Fu nd Quinn II (2000) Jn Myen Ashcn et l. (1996) ** Unpublished report UNU-Fisheries Trining Progrmme 18

19 Frequency Frequency Frequency Frequency Frequency Frequency Frequency 0 60 Frequency Mmie Crpce length(mm) Crpce length(mm) Crpce length(mm) Crpce Length(mm) Crpce length(mm) Crpce length(mm) Crpce length(mm) Crpce length(mm) Figure 2: Length-frequency distribution of P. borelis plotted from the utumn surveys dt for UNU-Fisheries Trining Progrmme 19

20 Frequency Frequency Frequency Frequency Frequency Frequency Frequency Frequency Mmie Crpce length(mm) Crpce length(mm) Crpce length(mm) Crpce length(mm) Crpce length(mm) Crpce length(mm) Crpce length(mm) Crpce length(mm) Figure 3: Length-frequency distribution of P. borelis in utumn surveys in Skjálfndi By UNU-Fisheries Trining Progrmme 20

21 Figure 4: Cumultive length-frequency distribution of P. borelis in utumn surveys in Skjálfndi By UNU-Fisheries Trining Progrmme 21

22 Crpce length(mm) Mmie Figure 5: von Bertlnfy growth curve for P. borelis in Skjálfndi By. Light curve is fitted to initil growth prmeter vlues nd thick curve is the fitted growth prmeter estimtes. ge UNU-Fisheries Trining Progrmme 22

23 Biomss(tons) Spwner biomss(tons) Fishing mortlity Annul recruitment(millions) Yield(tons) Biomss Index(tons) Mmie )Yers b)yers c)yers d)yers e)yers f)yers Figure 6: Output from dynmic biomss model. Figures 6) nd 6b) show nnul ctch nd biomss index from surveys respectively (circles) nd lines fitting the models. 6 c, d, e, nd f re the fishing mortlities, recruitment, biomss nd spwner biomss estimted from the model. The sums of squres of errors re SSEI = 0.48, SSEY= 0.03, SSER= nd SSEF = 1.60 The output gve better fit of the yield over the yers (Figure 6). The model, however, did not show good fit with the biomss index. This could be becuse of bises in the survey. Fishing mortlities rose shrply from 1991 to 1995 nd remined high, bove 2.0 until closure in 1999 (Figure 6c). Spwner biomss shows decresing trend since 1991 (Figure 6f). Stock biomss lso decresed from 1991 to 1994, incresed in 1995 nd finlly decresed gin (Figure 6e). Averge stock biomss ws 2047 tons from 1991 to 1998 (Tble 3). UNU-Fisheries Trining Progrmme 23

24 Tble 3: Biomss estimtes of P. borelis in Skjálfndi By from Yer Biomss estimte (tons) Yield nd spwner stock biomss The men recruitment (1 yer old shrimp) between ws estimted to be million individuls (dynmic production model) nd this ws ssumed for the virgin stock fter recovery. Yield nd spwner stock biomss were estimted for fishing mortlities strting from 0 to 2 (Tble 4). The estimted yield increses from tons t F =0.2 to tons t F = 2.0 wheres the virgin stock spwner biomss ws tons (t F = 0, no fishing) nd tons t F = 2. Tble 4: Yield nd spwner stock biomss estimted from the yield per recruit models for different fishing mortlities of P. borelis in Skjálfndi By. Fishing mortlity Yield (tons) Spwner stock biomss (tons) From the yield per recruits curves, the estimted Mximum Sustinble Yield per recruit ws 1.42 g which corrrespond to F mx = 1.97, Mximum Sustinble Yield = 566 tons nd F 0.1 = 0.72 (Figure 7). UNU-Fisheries Trining Progrmme 24

25 Y/R (g) S/R(g) Mmie Fishing mortlity Figure 7: Yield per recruit nd spwning stock per recruit curves for fishing mortlities rnging from 0 to 2. Arrow is showing fishing mortlity corresponding to F 0.1. UNU-Fisheries Trining Progrmme 25

26 5 DISCUSSION P. borelis cught in Skjálfndi By hd mximum crpce length of 28 mm. This ws is lrger thn the mximum length of shrimps observed in inner fjords such s Arnrfjörður (26.5 mm, Ceesy 2000) nd in Húnflói (25 mm, Lm Anh The stock of Skjálfndi By is geogrphiclly closer to the offshore (deeper wter) stocks which re lso reltively lrge sized. Lrger shrimps prefer deeper wters where temperture is reltively stble (Grci 2007). There re clims tht the shrimp stock in Skjálfndi sometimes mixes with tht of the offshore (Skúldóttir, pers. comm.). In fct one mixed re lies between Skjálfndi nd the offshore stocks (Skúldóttir nd Pétursson 1999). Even though these res re well defined, there is the possibility of some mixing of these stocks. The symptotic length L nd instntneous growth prmeter, K, obtined from this study (L =24.3, K= 0.46) re slightly different from those reported for the inshore stock in Húnflói, northwest Icelnd (Lm Anh 2008). The growth rte estimted in this study is lso fster thn tht observed in Húnflói (Lm Anh 2008). The slightly higher growth rte (K= 0.46) observed in Skjálfndi By my correlte with higher nturl mortlity nd shorter life spn of the stock (Kinne 1970). It is probble tht the predtion effects by cod nd hddock re greter in Skjálfndi By thn in Húnflói but this cnnot be conclusive. The inflow of wrm geotherml wter from the djcent lke Mývtn, (Ólfsson 1999) which regultes the wter temperture in Lxá in Aðldl resulting in incresed surfce temperture in the by, my influence the bottom temperture when mixing tkes plce. If this were substntil then one would expect higher temperture in Skjálfndi By thus leding to fster growth. Skúldóttir (2007) observed tht growth rte of shrimp in Gullmrsfjord, Sweden where temperture ws higher ws fster thn in Ísfjrðrdjúp, Icelnd nd in the Flemish Cp where bottom tempertures were lower. Estimting growth prmeters bsed on length-frequency dt is difficult. Problems rise becuse during the ovigerous period, moulting does not tke plce nd there is lso vribility in growth rtes of different sexes, ge clsses nd in different sesons. The growth performnce index, ø', ssesses the relibility of L nd K, obtined in this study. The index (ø') obtined in this study nd computed vlues obtined in previous studies for P. Borelis in other res remin firly similr (Tble 2). The mximum size of reltively fst growing popultions will be reltively low (high K, low L ), but when growth is reltively slow (low K) in popultions of the sme species, L will increse, resulting in the growth performnce index being reltively constnt for the species (Puly The sme ppers to be true for shrimp. Fishing mortlities were high but the stock ws ble to withstnd fishing nd predtion pressures for some yers until the collpse in It might be tht cod ws feeding on the juveniles nd yet the dults were ble to reproduce nd increse in biomss but only for some time until recruitment could not compenste for mortlity. Another hypothesis might be the mtch-mismtch hypothesis (Cushing 1975). If when the lrve re htched nd there is plenty of food then they hve the tendency to grow nd be ble to recruit into UNU-Fisheries Trining Progrmme 26

27 the fishery lter in life provided there is not much predtion. If, on the other hnd, food is limited t the time of htching then the survivl rte will be reduced. In relity both physicl nd biologicl processes interct nd cn both hve n effect on recruitment nd other life processes (Heth 1992). The nturl mortlity M = 0.5 which ws used in this model ws not clculted but represents best guess, considering high nturl mortlity due to predtion. The effect of fishing mortlity is reduced with increses in nturl mortlity, s the two types of mortlity compete for the sme individuls. It is dvisble to ctch the species tht suffer high nturl mortlity t reltively young ge, before they die of nturl cuses, s this is the only time tht exploittion cn be optimum (Sprre et l. 1989). P. borelis is preyed upon to lrge extent by cod nd hddock resulting in high nturl mortlity. Unless the shrimp stocks re to be mnged s food for cod, it might be dvisble to reduce the minimum size t cpture while cohort biomss is still reltively high. When nturl mortlity is low the increses in biomss from growth of individuls exceed losses due to nturl mortlity for long period in the life of the cohort. A high M will result in biomss losses due to nturl mortlity exceeding gin due to growth t reltively young ge. The yield in the inshore shrimp of Skjálfndi By shows n increse from low figure (310 tons) in 1991, reching pek of 1027 tons in 1995 nd 1996 (Figure 6). After 1996, there ws decline in yield to n ll time low of 191 tons in 1998 when the stock collpsed. In yers when biomss is high, recruitment ws lso high. The shrimp biomss estimte in 1999 ws bout third of the biomss in This shows tht there ws not much recruitment to compenste for the reduction in biomss since Annul recruitment is influenced by density of cod nd hddock in the previous yer. When the cod (Gdus morhu) nd/or hddock in the re re less bundnt in given yer, shrimp recruitment increses nd vice vers. In some yers reduced recruitment of shrimps cn be observed when there is n increse in cod. For exmple, when the presence of cod in Skjálfndi By ws less in 1997, recruitment of shrimp ws high in When regression ws done the reltionship ws however wek (Figure 8). This is probbly due to short time series dt tht were vilble. UNU-Fisheries Trining Progrmme 27

28 Figure 8: Annul shrimp recruitment estimted from the dynmic production model for ech yer regressed with the men number of cod (Gdus morhu) per hul in the previous yer from the MRI utumn surveys. The generl trend is tht s cod numbers decrese in yer, the recruitment of shrimp increses the following yer. A similr sitution ws observed by Lm Anh (2008) in the inshore stocks of Húnflói. According to Lm Anh (2008) reltively low cod density in 1989 resulted in the incresed recruitment of shrimp to the fishery in Stefánsson et l. (1994) lso observed the sme trend in other Icelndic wters, s the mount of cod increses, the biomss of Pndlus borelis is reduced nd so is recruitment. In July 1991, Prsson et l. (1991) noted tht when cod moved into the St Anthony Bsin in northestern Newfoundlnd there ws drstic reduction in shrimp ctches during tht time. Lilly et l. (2000) lso suggested tht n increse in shrimp stocks observed in Northest Newfoundlnd ws to some extent relted to decresed cod stocks. Shrimp fisheries off the est cost of Cnd re sid to hve incresed fter the decline in cod; nd the development of the West Greenlnd shrimp fisheries during the 1970s is believed to hve been relted to the mjor decline in the cod stock in tht re (Lilly et l. 2000). There is evidence from nlysis of stomch contents of young cod tht shrimp re mjor prey (Mgnússon nd Pálsson 1991). The predtion effect of hddock (Melnogrmmus eglefinus) on Pndlus cnnot be underestimted (Skúldóttir pers. comm.). The higher the numbers of hddock present in the by in prticulr yer, the smller the mount of shrimp recruitment the following yer (Figure 9). This trend is similr to the effects shown by cod on shrimp in Skjálfndi By (Figure 8). UNU-Fisheries Trining Progrmme 28