This report not to be cited without prior reference to authors

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1 I 1 This report not to be cited without prior reference to uthors Interntionl Council for Explortion of the Se C.M I N: 11 (Ref. G) Mrine Mmmls Comrnittee THE EFFECT OF INVADING HARP SEALS (PHOCA GROENLANPICA) ON LOCAL COASTAL FISH STOCKS OF NORTH NORWAY 1 Norwegin College of Fisheries Science, University of,troms", by KJELL T. NILSSEN 1,' PER E. GROTNES 1 nd TORE HAUG 2 Breivik, N-9000 Troms", Norwy 2 Institute of Mrine Reserch, c/o Norwegin College of Fisheries Seienee, University of Troms", Breivik, N-9000 Troms", Norwy ABSTRACT Annul invsions of hrp sels (Phoc groen-indie) hve oceurred for more thn decde into costl res nd fjords of northern Norwy, nd these hve creted problems for fisheries in the re. Anlyses of bottom trwl survey dt collected during in Ullsfjord re presented in n ttempt to evlute possible interctions between invding sels nd loel fish stocks. Stomch contents dt from sels smpled in UIIsfjord re lso presented. The presenee in sel stomchs of sithe (Pollehius yirens) nd smll eod (Gdus morhu), whieh were not frequent in trwl ctches, seems to indicte tht sels my pursue prey in wters other thn the trwl res. Mny of the speeies oceurring in the sel stomchs were, however, lso encountered quite frequently in the trwl etehes. From the trwl survey dt it ppers tht in the presence of sels, eomrnercil fish speeies such s cod chnge behviour nd dispper from the trditionl fishing grounds. Sels my cuse considerble dmge to gill net etehes by eting prts (usully the ventrl soft prts) of fish entngled in the nets.

2 2 INTRODUCTION In their generl migrtion pttern, the hrp sels (Phoc groenlndic) in the Brents Se (see Smirnov 1924, Wiig 1988) leve the White Se re fter breeding nd moult nd follow the pck-ice belt northwrds into the Brents Se. Usully, they re distributed in open wters long the pck-ice belt in the Brents Se during summer nd utumn, nd in October-December they return to costl wters in the southestern Brents Se. Contrry to this hbitul migrtion pttern hrp sels strted to pper in ~ lrge numbers long the' cost of Finnmrk, North Norwy, in winter nd spring from 1978 onwrds (Bj0rge et le 1981). This berrnt migrtory pttern hs previled throughout the 1980's with drmtic increse in mgnitude nd extent in 1987 nd 1988 (Hug et le 1990). The invding sels hve cused lrge problems for Norwegin fisheries. This pplies, in prticulr, to fisheries in costl res nd fjords where substntil numbers of sels hve been cptured nd drowned in gill-nets every winter (Hug et le 1990). In ddition to the dmge to gill-nets, fishermen clim tht the presence of the sels leds to chngesin fish behviour 4t such tht fish become less vilble on the trditionl fishing grounds. It is lso sid'tht. the sels ttck nd et fish entngled in the gill-nets. During the yers trwl surveys hve been crried out in severl fjords in Troms, North Norwy.. These surveys were crried out twice yer in severl typicl "sel' invsion ers", first in Jnury (i.e., before the sels rrived in these costl res), nd then in Februry/Mrch (fter the rrivl of sels). Furthermore, the survey yers were either chrcterized by very hevy invsions (1987 nd 1988) or invsions on much more moderte scle (1986, 1989 nd 1990, see Hug et le 1990). These trwl survey dt hve been nlysed nd stndrdized in

3 order to investgte whether the clims bout interctions between the invding sels nd locl fish stocks, in prticulr cod (Gdus morhu) my be substntil. 3 MATERIAL AND METHODS Hrp sel stomch smples Stomch smples were collected frm hrp sels tken in gill-net fisheries in Ullsfjord, North Norwy (Fig. 1) in Jnury/Februry 1986 (9 sels) nd in Februry 1988 (50 sels). The mteril ws collected s prt of more thorough study of the diet of hrp sels invding costl res of North Norwy during winter (Hug et le 1990).' A detiled description of both the smpling procedures nd nlyticl methods cn be found in Hug et le (1990). Records of sel dmge upon cod in gill nets Records were kept of possible sel dmge upon cod entngled in gill nets during the course of two-week cr~ise onbord two cornrnercil vessels. The study ws crried out on the costl bnks off Troms during the period Jnury Dmge ws ssessed by estimting the reltive proportions (by weight) of fish tht hd been preyed upon by sels in'reltion to the totl ctch (both dmged nd undmged fish). Dmged fish were weighed with no correction being mde for the pieces removed by sels, wheres eviscerted weights were recorded for undmged fish. Trwl suryeys in Ullsfjord/S0rfjord in Bottom trwling using shrimp trwl (REFA Gisund Super 1280 mesh (in 44 rnrn) shrimp trwl with rubber/steel bobbins, sweep wires

4 4 40 m, fishing line length 53.2 m, nd the following mesh sizes: wings nd squre 65 rnrn,. belly 44 rnrn, codend 10 mm) ws crried out R/V John ~ in UIIsfjorden/S0rfjorden, North Norwy. (Fig. 1), during Jnury nd Februry/Mrch in (Tble 1). Towing speed ws 2 knots. Trwl surveys were not specificilly conducted for purposes of the present investigtion, nd the smpling ws not uniform with respect to hul durtion (from 9 to 74 minutes), time of the dy of huls, nd res nd depths. Also, informtion bout the quntities of the vrious species tken in ech hul is sometimes incomplete nd, in some cses, includes only nurnber of individuls. Thus, dt ws renlyzed, trnsformed nd extrpolted to stndrd hul of 30 minutes durtion. Informtion ws then used to provide crude estimte of the reltive contribution, by weight, of ech fish species to the totl ctch in ech such stndrd hul. The finl results given per sttion re the clculted men vlues per stndrd hul (CPUE = ctch per unit effort). Cod ws min. trget species during the trwl.surveys, nd considerble informtion bout the size composition of this species.in the ctches is vilble. The size composition of cod in sel stomchs wsclculted from otolith sizes (see Härkönen 1986) nd results from trwl ctches nd stornch nlyses compred in order to ssess size selectivity.in the predtion by sels upon fish. Hrp sel occurrence in UIIsfjord Detiled records of the extent nd sesonl vrition of hrp sel ctches in UIIsfjord for the period in question re.not vilble. Such informtion must, therefore, be drwn from the more generl ptterns of the hrp sel invsions in North Norwy (see Hug et l. 1990) nd from personl communiction with locl fishermen nd mngers of fish processing plnts in the re.

5 RESULTS Species composition in sel stomchs nd bottom trwl ctches In 1986 the number of prey species found in the nine sel stomchs exmined ws 12, wheres t lest 16 tx were tken in the trwl ctches (ll 1986 sttions pooled) (Tble 2). In 1988, when 50 sel stomchs were exmined, 14 different prey specieswere identified, nd 19 tx were cptured in bottoffi trwl ctches (ll 1988 sttions pooled). Gf the species tht contributed most tothe totl biomss in the bottom trwl ctches, cod, hddock (Melnogrmmus eglefinus) nd Norwy pout (Trisopterus ismrckii) were mjor prey of the hrp sels tken in the re (Fig. 2, Tble 2). Sithe (pollchi(s virens), species whichcontributed little to the trwl ctches, ws present in lrge quntities in the sel stomchs. Size composition of cod from sel stomchs nd trwl huls The 1986 smpies of cod identified in sel stomchs (25 fish tken from two stomchs) comprised only smll fish, nd these were lrgely within size groups tht were bsent from the 'trwl smpies (Fig. 3). The 1988 smpies of cod from sel stomchs (99 fish from 25 stomchs) we're generlly chrcterized by fish which were smller in size thn those tken in bottom trwl, lthough otoliths from some lrger cod were lso found in the sel stomchs (Fig. 3). Hrp sel dmge upon cod in gill nets During invsions of hrp sel to Norwegin costl fishing grounds fishermen hve climed tht sels ttck nd et prts of fish entngled in gill nets. Usully, the ventrl soft prts

6 6 including the liver nd intestines re tken. Registrtions mde during two weeks of fisheries on costl bnks off Troms, North Norwy, in Jnury 1986 suggest tht % of the dily ctches of cod (often lrge 3-4 kg fish)might hve been dmged by sels (Tble 3). Sottom trwl suryeys in Ullsfjorden In S0rfjorden in 1986 nd 1989 it ppers tht the men totl ctch weight per stndrd hul ws lrger in the Februry/Mrch surveys thn in thejnury surveys (Fig.4). In 1987 nd 1988 the sitution in this inner prt of Ullsfjorden ws reversed in tht4t the Jnury surveys gve the lrgest men ctch9s, whilst only smll ctches were obtined in Februry/Mrch. Trwl survey dt from the outer prts of Ullsfjorden re incomplete (Fig.4). This pplies in prticulr to Jnury dt which re not vilble for 1987, 1989 nd The reltively lrge ctches obtined in S0rfjorden in Jnury nd Februry/Mrchin 1986 nd 1989, nd in Jnury in 1987 nd 1988 were minly comprised of cod nd hddock (Fig. 5). In the Februry/Mrch ctches in 1987 nd 1988, cod ndhddock were. ",.' lmost bsent. These ctches contined minly herring (Clupe hrrengus), fltfish nd other miscellneous species. 4t Hrp sel occurrence in Ullsfjorden in Hug et l. (1990) describe the sel invsion in North Norwy s being moderte in.1986, very extensive in 1987 nd 1988, nd t very low level in The level in 1990 lso ppers to be very low (K. Pettersen, Norges Räfisklg, Troms0, Norwy, pers. comm.). The ptterns of hrp sel invsions in Ullsfjord pper to hve been similr to the more generl ptterns (B. Pedersen, fish processing plnt mnger, Ullsfjord, Norwy, pers. comm.): Byctches in the re were estimted to be sels in 1986,

7 7 in excess of 800 sels in 1987, pproximtely 500 sels in 1988 nd less thn 10 sels in ech of the yers 1989 nd In the yers with the most extensive invsions single nimls were observed s erly s round New Yer, but the min byctches, which were, in some instnces, s mny s 35 sels per bot (40 gillnets) perdy, were lwys mde'towrds the end of Jnury nd in Februry/Mrch (A. Hnsen, fishermn, Ullsfjord, Norwy, pers. comm.). DISCUSSION There ppers to be considerble degree of overlp between prey species occurring in the stomchs of the oportunistic (see Bowen 1985, Kpel & Angntyr 1989, Hug et l. 1990) hrp sels., nd orgnisms cptured in shrimp trwl withinthe sme mjor re nd period of time. This pplies in prticulr to species such s cod, hddock nd Norwy pout. There re, however, some pprent differences between the contents of sel stomchs nd trwl ctches, nd some of these differences my give some indiction of the hrp sel feeding hbitts in the re. The sithe is sholing species which occurs minly" in the,littorl zone s O-group (GOd0' et l. 1989), in upper nd midwter lyers when smll nd of medium size, nd in~deeper wter when lrge (Pethon 1985). Thus, the rre occurrence of sithe in the Ullsfjord bottom trwl ctches is not surprising. The species ws, however, frequently encountered in.sel stomchs. This my suggest tht the hrp sels feed minly in shllow nd/or pelgic wters. Feeding in more shllow res is lso inferred from the observed size compositions of cod in the sel stomchs. Cod in sel stomchs were generlly smller thn those in the trwl ctches. The smllest cod re known to occur in shllow sheltered res nd they generlly void more exposed loctions nd deeper wters where lrger ~od re usully found

8 8 (Hwkins et l. 1985, GOd0 et l. 1989).,Additionl fctors contributing to the observed difference in size composition of cod found in sel stomehs nd trwl ctches my be tht ll lrger cod either leve the re ( suggestion '1' clled into question by recent tgging. dt which seems to indicte thtcod re rther sttionry in the re, T. Pedersen, Norwegin College of Fisheries Science, Troms0, Norwy, pers. cornrn.), or move to deeper strt or strt unfit for surveying with trwl ger in the presence of the sels. The results from the trwl,surveys my support this suggestion. In S0rfjorden (the shll~west innermost prt of, Ullsfjorden) the trwl ctches tken in the "sel-yers" 1987 nd 1988 in Februry/Mrch (i.e., fter the sesonl rrivl of the sels) were very smll compred with those obtined in,jnury before'the rrivl of the sels. In the yers with only moderte invsion (1986 nd 1989, see Hug et.l. 1990) no suchehnges occurred. It ppers tht it ws the cod, nd, to certinextent, lso the hddock, tht disppered from the ctches when sels were present in there. The dt, presented do not, llow,ny firm conclusions s to emmigrtions. of the cod upon the rrivl of the sels. The survey dt from the outer prts of Ullsfjord re scrce nd inconclusive, but it my be noted tht the ctches obtined in the deepest strt were. prticulrly lrge in both 1987 nd1988, nd included considerble mounts of cod in the ltter yer. 'Whtever the ernmigrtion mechnisms my be, it ppers quite sfe to conclude tht in the pr~sence of sels, commercil fish.' species such, s cod chnge behviour nd dispper from the trditionl fishing grounds. Registrtions mde in cornmercil gill net fisheries,revel tht the hrpsels my cuse considerble dmge to the ctches (2 26% by weight). by tering, nd eting pieces, (usully the ventrl soft prts) of lrge (3-4 kg) fish entngled in the nets. Whether displys this feeding hbit on free-swimming lrge fish is, however, not known. If it did occur, such predtion would go

9 unregistered in studies where otoliths were used s the prlmry id for identifiction of fish prey in stomch contents. ACKNOWLEDGEMENTS Thnks re due to J.-E. Elissen, field ssistnts nd the crew on R/V John Ruud who surveyed Ullsfjord during nd gve us ccess to the dt. K.A. Fgerheim, C. Lydersen nd J. dos Sntos collected the hrp sel stomchs, nd M. Jobling provided vluble Comments upon the mnuscript. The ecologicl studies of hrp sels re supported by funding from the Norwegin Council of Fisheries Reserch (NFFR), project nos I nd I REFERENCES Bj0rge, A., Christensen, 1. & 0ritslnd, T Current problems nd reserch relted to interctions between mrlne mmmls nd fisheries in Norwegin costl nd djcent wters. ICES CM 1981/N:18: 10 pp (Mimeo). Bowen, W.D Hrp sel feeding nd interctions with commercil fisheries in the north-west Atlntic. Pp in: Beddington, J.R., Beverton, R.J.H. & Lvigne, D.M. (eds). Mrine Mmmls nd Fisheries. George Allen nd Unwl.l1, London. 354 pp. GOd0, O.R., Gj0sceter, J., Sunnnä, K. & Drgesund, O Sptil distribution of O-group gdoids off mid-norwy. Rpp. P.-v. Reun. Cons. int. Explor. Mer 191: '. -~~., ';.~~~:'

10 Härkönen, T Guide to otoliths of the bony fishes of the nortest Atlntic. Dnibu ApS, Hellerup, Denmrk. 256 pp. 10 Hug, T., Kr0yer, A.B., Nilssen, K.T. & Uglnd, K.l Hrp sel (Phoc groenlndic) invsions in North Norwegin costl wters: A preliminry report on their ge composition nd feeding hbits. lces CM 1990/N:6: 24 pp (Mirneo). Hwkins, A.D., Soufini, N.M. & Smith, G.W Growth nd feeding of cod, Gdus morhu L. J. Cons. int. Explor. Mer 42: Kpel, H. & Angntyr, L.A.' Feeding ptterns of hrp sels (Phoc groenlndic) in costl wters of West Greenlnd, with note on offshore feeding. Coun. Meet. int. Coun. Explor. Se 1989/N:6: 28 pp (Mirneo). Pethon, P Aschehougs store fiskebok. H. Aschehoug & Co. (W. Nygrd) A/S. 447 pp. Smirnov, N On the estern hrp sel Phoc groenlndic vr. ocenic Lepechin. Troms0 47(2): (Pgophoc) Mus. Ärsh. Wiig, Gr0nlndssel og selinvsjon Hv vet vi - vi? Nturen 1988(2): hv tror I :

11 11 Tble 1. Smpling loctions, depths nd number of shrimp trwl huls performed in inner (S0rfjorden) nd outer prts of Ullsfjorden, North Norwy, in Jnury nd Februry/Mrch in NUMBER OF HAULS IN LOCATION DEPTH (m) J F J F J M J 1989 F/M 1990 J F S0rfjorden: S0rstrumen 44 1 Stordl Njosken Skognes Olderbkken Outer Ullsfjord: Kuberget Ullsnes Gropelv Oldervik Eidstrnddjupet West of Arn0y

12 12 Tble 2. Reltive composition (in percent) of stornch contents in hrp sels (bsed on clculted fresh weight) nd of bottom trwl ctches (bsed on estimted round welghts) s observed in Ullsfjord, North Norwy, in Jnury-Mrch in 1986 nd = bundnt with contribution of less thn 1% to the totl biomss PREY ITEM TRAWL SEAL TRAWL SEAL Mollusc: Cephlopod: Todrodes sgitttus Crustce: Decpod: Pndlus borelis Euphusice: Euphusice indet. Pisces :' Rjide: RU sp. Clupeide: Clupe hrengus 2 13 Gonostomtide: Murolicus muelleri Osmeride: Mllotus yillosus Gdide: Gdus morhu Melnogrmmus eglefinus Merlngius merlngus Micromesistius poutssou Pollchius pollchius Pollchius yirens Triso'Pterus ismrckii Brosme brosme Moly moly e Anrh1chd1de: Anrhichs lupus Scorpenide: Sebstes sp. Pleuronectide: Pleuronectes pltess 38 Glyptocephlus cynglossus Hippoglossoides pltessoides Hippoglossus hippoglossus Microstomus k1ll Pooled fltfish Cyclopteride: Cyclopterus lurnpus

13 13 Tble 3. Reltive proportions of fish dmged by sels in totl gill-net ctches mde on costl bnks off Troms in Jnury Undmged ctch is given s totl eviscerted weight, nd dmged ctch is round weight with no corrections being mde for the pieces removed by sels. Totl ctch is the sum of dmged nd undmged ctch. Dt re drwn from ctches of two commercil vessels, with nets being huled every second dy. DATE UNDAMAGED CATCH DAMAGED CATCH RELATIVE PROPORTION (kg) (kg) DAMAGED (%) Jn Jn Jn Jn Jn Jn Jn Jn Jn Jn Men 8.4

14 'O ---"?'m ~GLOYA Fig. 1. Mp showing the survey re Ullsfjorden with its innerrnost prt S0rfjorden. The loclities of the trwl smpling sttions (with pproximte depths) re given.

15 ?ft 100 f- I (!) w ~ >- ce z 0 f- ::::> ce CI: f- z 0 u w > 50 ~...J W CI: 0 TRAWL SEAL TRAWL SEAL.. Herring ~ Cod ~ Hddock ~ Norwy pout rnrrn Sithe ~ Plice ~ Fltfish CJ Vrious Fig. 2. Reltive contribution (by clculted fresh weight) of vrious prey orgnisms to the totl biomss of vrious prey orgnisms in hrp sel stomehs nd of orgnisms cptured in stndrdized bottom trwl huls in Ullsfjorden in 1986 nd 1988.

16 50r , r I- I Cf) u.. u.. 10 f- 0 '-----r--i.--i~ I~ o ffi 100r------' , CO ~ ::J Z OL IIIIIIJ-~ Fig. 3. Size cornposition of cod found in hrp sel stomehs (blck colurnns) nd tken in trwl ctches (white colurnns) in Ullsfjorden in 1986 nd 1988.

17 ." i o 1200} O~--L-_ Ci J ~ 400 :I: CI: «~ 300 ~ Ul 1987 ~ :I: ~ ü:i 100 ~ :I: Ü ~ 0 '-----'------"r:z::zzll I--_~LLc.L r--...ll.ll.."' Ü...J 300 ạ ~ ~ 200 «CI: UJ ;:( ol.--...l---ii!".ll..ul_--il.-=="""" ---,--J::=:C.o 1989 o '-----' I!:LL..~--i S0RFJORD OUTER ULLSFJORD Fig. 4. Averge totl ctch weight per stndrd (30 minutes) trwl hul tken in in S0rfjorden nd outer Ullsfjord (bove nd below 200 m depth) in Jnury (white columns) nd in Februry/Mrch (htched columns).

18 ". 18 -Herring ~ Norwy pout ~ Cod lilliil Sithe o Vrious ~ Hddock ~ Fltfish J F J J F F F '0'< - I- :::c ~ 50 W ~ >- ce z 0 0 l- M ::l 100!!:! 1988 CI: I- Z 0 ü w > ~...J. W CI: 50 0.' SORFJORD >200m OUTER ULLSFJORD Fig. 5. Reltive contribution (by clculted weight) of the vrious orgnisms to the totlbiomss obiin~d in verge stndrd huls mde in in S0rfjord nd outer Ullsfjord (bove nd below 200 m depth) in Jnury (J), Februry (F) nd Mrch (M).