Population Structure and Migration Patterns of Atlantic Cod (Gadus morhua) in West Greenland Waters Based on Tagging Experiments from 1946 to 1964

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1 NAFO Sci. Cun. Studies, : - Ppulatin Structure and Migratin Patterns f Atlantic Cd (Gadus mrhua) in West Greenland Waters Based n Tagging Experiments frm t Hlger Hvgard Greenland Fisheries Research Institute Tagensvej, Cpenhagen N, Denmark and Steen Christensen Danish Institute f Fisheries Ecnmics Research, University f Suth Jutland Glentevej, Esbjerg rj), Denmark Abstract Ppulatin structure and migratin patterns f Atlantic cd (Gadus mrhua) in West Greenland waters is eval uated n the basis f tagging experi ments frm t. It is suggested that the stck is cmpsed f well segregated ppulatins, where the inshre ppulatins largely are statinary whereas the ffshre ppulatins perfrm regular seasnal migratins with hmings t the areas where they were tagged. Intrductin In West Greenland waters perids f high abundance f cd have alternated with perids f scarcity t the extent that smetimes they were thught t be absent (Hansen, ). Arund, high abundance f cd gave rise t a large fishery. Tagging experiments carried ut since the s have shwn large scale migratins f cd frm West Greenland t Iceland (Hansen et a/., ; Hansen, ; Ann., MS ) and this has been cnfirmed by studies f the pattern f parasitic infectins in the areas (Platt, ). The mre lcal migratins within the West Greenland area have nt attracted much attentin and have nly been superficially analyzed befre. In this paper these migratins are described n the basis f tag returns frm experiments carried ut ff West Greenland between and. Materials and Methds Cd have been tagged regularly at West Greenland (NAFO Subarea, Fig. ) since the late-s. The annual number tagged was highest in the perid - and this perid is als characterized by the widest gegraphical cverage and the highest rates f tag returns. Therefre, taggi ng experiments frm t were selected fr analysis. During this perid, cd were tagged f which, recaptures have been reprted. The numbers tagged by NAFO Divisin in relatin t the general site habitat is given in Table, and the numbers f annual taggings are shwn in Fig.. " ~-~-:nec--=--' '-----' " " " " " " A B OA, I) --:-~~~~ I I, I i C I I-----~ I OB I I I E I ',I " ' F...,, G ", GREENLAND " " " " " " Fig.. Map f Greenland shwing the NAFO Divisins and the six mst imprtant tagging areas (*). Cd used fr tagging were caught by varius gear, with jigs being the mst imprtant (% f the ttal). Only thse classified as being in excellent cnditin

2 Sci. Cuncil Studies, N., "f " Q) OJ OJ.s.r:. <n.;:: ' Q;. E ::J Z """''''' VI Year Fig.. Annual numbers f cd tagged ff West Greenland, -., ~, E ::J Z -,, r TABLE. Numbers f cd tagged and recaptured frm t and numbers f recaptures used in the analysis by divisin and habitat. N. N. N. f returns Div. Habitat tagged returned used in analysis fjrd A cast bank fjrd B cast, bank,,, fjrd C cast, bank,, fjrd,, D cast,, bank,,, fjrd E cast bank fjrd, if cast,, bank Ttal,,, were tagged. Cd larger than cm were mstly tagged with Petersen discs and smaller nes were mst ften tagged with Ncathene tags. Almst all taggings (%) were made during summer (May t September, Fig. ). Tag returns were recrded by date and site f catch. Otliths were returned with tags in, cases r % f the recaptures. Ages f returns withut tliths were derived frm the verall age-length relatinship f the returned tliths. Only cd returned at age r lder were included in the analysis. Three subsets f data were excluded frm the analysis: () returns frm utside West Greenland; () same year returns, which usually cntained little infrmatin n migratin; and () age r yunger Fig.. ~D n Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nv Dec Mnth Numbers f cd tagged ff West Greenland, -, by mnth. cd, as these were nly tagged inshre and the returns were few. In ttal,, returns were included in the analysis (Table ). Althugh, recaptures were available, the numbers were insufficient fr a full analysis f the migratin behaviur which included variatins between years and fish age. Fr example, if nine yearclasses were cnsidered ver the years studied in gegraphic areas fr duratins f mnths, a ttal f abut, recapture cmbinatins exist. Cnsequently a cnsiderable aggregatin f data was necessary. The data were aggregated ver years and age in rder t allw an analysis f the verall seasnal and gegraphic pattern f returns dependent n tagging lcatins, Even with this aggregatin, high numbers f recveries were nly available fr six tagging areas, namely the banks f Div. S, C and D, the castal areas f Div. ' D and F and the fjrds f Div. (Table ). Results In all divisins except ne, the majrity f recaptures were returned frm the same divisin where the fish were tagged (Table ). The nly exceptin was in Div. A where the cd fishery was relatively small and few recaptures were therefre expected. The seasnal distributin f recaptures were cmpiled fr the six mst imprtant tagging habitats (Table ). Returns reprted mstly during summer reflect the seasnality f the fisheries in the - perid, The gegraphic distributin f returns frm successive summers after the tagging year shws that mst

3 HOVGARD and CHRISTENSEN: Ppulatin Structure and Migratin Patterns f Atlantic Cd TABLE. Return divisin A B C E a Ttal % returns in tagging divisin a Divisin nt knwn Distributin f tag returns by tagging divisin. A Tagging divisin B C,,,, E returns were frm within the tagging divisin and that the gegraphic distributin changed nly marginally ver the years (Fig. ). The distributin f returns frm adjacent habitats within the same divisin was assessed fr Div. D (Table ), where largest numbers f cd were tagged in the three habitats labelled as the banks, the casts and the fjrds. The majrity (%) f the returns frm the fjrd taggings were taken within the fjrds and nly % f the returns were taken utside Div. D. Similarly, frm the bank taggings few returns ccurred in the adjacent cast and fjrd areas (%), hwever, a relatively large prprtin (%) was taken utside Div. D. Returns frm the castal taggings, n the ther hand, were relatively evenly distributed in all three habitats as well as areas utside Div. D. In general, mre tags were returned t the suth f the tagging area during the first half f the year and mre t the nrth in the secnd half f the year (Table ). This trend is shwn in mre detail in Fig., where the midpints f the recapture distributin in every mnth perid are pltted against time after tagging (midpints were calculated by weighting recaptures frm Div. A t F by values f t, respectively). A nrthward mvement f the return midpints were seen during late spring fllwed by a suthward mvement during the late autumn, and this pattern was repeated in successive years. There were, hwever, differences seen between tagging habitats with larger amplitudes in the nrth-suth scillatins fr ffshre bank tagged cd cmpared t castal and fjrd tagged cd. The return midpints f the ffshre taggings were all centered arund Div. D in winter, whereas in the summer the midpints differed with nrthern tagged cd being returned further nrth. TABLE. Mnth MaApr Nv-Dec MaApr Nv-Dec MaApr Nv-Dec MaApr Nv-Dec MaApr Nv-Dec MaApr Nv-Dec Distributin f returns by -mnth perids and return divisin fr selected areas. Values are given in percent. (N = ttal number, = area nt knwn.) Return divisin A B C E = Diy. (bank) = Diy. C (bank) = Diy. (bank) = Diy. (fjrd) = Diy. (cast) = Diy. F (cast) N Discussin Fish migratins as inferred frm the return pattern f tags is influenced by the distributin f fishing effrt and the reprting rates f different fleets. Effrt distri- butin and reprting efficiency by area and seasn were nt available fr West Greenland during the - perid and the return patterns culd therefre nt be crrected. In spite f this, sme basic aspects f stck structure and migratin pattern culd be

4 Sci. Cuncil Studies, N., Div.B-bank Div. C-bank Div. O-bank _ Year ~ Q; a. N = N = N = _ Year ~ ~ N = N = N = N = N = N = E Year ~ ~ A B C E Return divisin A B C E Return divisin A B C D E Return divisin Div. D-fird Div. D-cast Div.-cast _ ~ Year ~ N =.- r N = ~II /""""I N = n,..., Year _ ~ Q; a. N = N = - N =,...,,..., lin n,...,,...,nn _ Year ~ ~ Fig.. N = - II,..., r< A B C E Return divisin N = n II,..., A B C E Return divisin N =,..., /""""I,..-, A B C E Return divisin Distributin f tag returns (N) reprted during summer (May-September) in year t yearafterthetagging year. deduced frm the bserved return distributin. It was nted that the distributin f returns depended n where the cd were tagged and this indicated that different cd ppulatins did nt mix randmly. If randm mixing had ccurred, similar return distributins shuld be expected fr all tagging experi- ments irrespective f where the cd were tagged, as the return patterns shuld simply reflect the spatial distributin f the fisheries. Mst returns were taken within the area f tagging (Tables -) and mst returns during successive summers (the perid f tagging and als highest

5 HOVGARD and CHRISTENSEN: Ppulatin Structure and Migratin Patterns f Atlantic Cd Div. B-bank C TABLE. Distributin f returns frm taggings in Div. D during -. Div.D Outside Fjrd Cast Bank a Div.D Ttal fjrd D cast bank, a Area within Div. D nt reprted. Div. D-bank C TABLE. Distributin f returns frm taggings in Div. D during the perid - (data frm Hansen, ). Return area D fjrd D cast D bank Outside D Div. D-fird D fjrd D cast bank Div. F-cast Fig.. E Year Year Year Year Year Seasnal average pattern f returns pltted ver successive years, calculated as mean nrthern latitude frm Table. Pints (x) shw the bserved gegraphic midpints f -mnth perids. Only pints based n at least returns are pltted. TABLE. Mnth May Jun Jul Aug. Sep Oct Distributin f tag returns frm Nrwegian ffshre taggings in Div. B during - (data frm Rasmussen, ). Return psitin (deg. nrthern latitude) numbers f returns) als shwed they remained within the tagging area (Fig. ). This suggests that the cd ppulatins, t a great extent, are seg regated and either remain in the tagging areas r return t these areas in summer time. When cmparing the return pattern frm adjacent fjrd, cast and bank taggings in Div. D, nly a small prprtin f the cd tagged in the fjrds were returned frm utside that area. Als nly a few tagged n the banks were fund in the fjrds, while thse tagged in the intermediate castal area were fund distributed in all these areas (Table ). Similar results can be extracted frm cd taggings in the same areas frm the perid - (Table ). These findings suggest the existence f well segregated fjrd and bank ppulatins f cd in Div. D and that the castal area is a mixing zne fr these ppulatins. Other evidence als supprt the existence f segregated inshre ppulatins: () spawning in several fjrds are knwn (Hansen, ); () inshre cd ppulatins have been distinguished frm nearby ffshre ppulatins n the basis f parasitic infectins (Bje, MS ); and () tlith types, which are distinct frm the typical West Greenland Bank type, are ften seen in samples frm inshre areas. The experiments further suggest that the inshre cd d nt mve much, whereas the ffshre ppulatins undertake seasnal nrth-suth migratins (Fig. ). Similar migratins fr ffshre cd is reprted by Rasmussen () n basis f Nrwegian taggings frm the banks in Div. B (Table ). The apparent suthward mvement f the stck during winter can als be a result f an artifact f the winter fishery, fr example if fr sme reasn the fishery has been hindered in the nrthern areas. The ice chart infrmatin (Ann., ), hwever, des nt indicate that the ice cnditins in general restrict the access t the mre nrthern areas during winter. On the ther hand, a suthward winter migratin makes sense because extensive spawning takes place ff the banks f Div. D and E during spring. The reverse nrthward summer migratin culd be a feeding migratin, as assumed by Meyer (). If this is the case, the different distributins f returns frm the summer perids (Fig. -) suggest that different ffshre ppulatins hme t different feeding areas during summers. References ANON. MS. Reprt f the Nrth Western Wrking Grup,

6 Sci. Cuncil Studies, N.,. ICES C.M. Dc., N. F:, p. ANON.. West Ice Investigatins, Part. Danish Meterlgical Institute, Nautical Department. BOJE, J. MS. Parasites as natural tags n cd in Greenland waters. ICES C.M. Dc., N. G:, p. HANSEN, P. M.. Studies n the bilgy f the cd in Greenland waters. ICES Rapp. ~rc.~verb., : -. HANSEN, P. M., A. S. JENSEN, and A. T. TANING.. Cd marking experiments in the waters f Greenland, -. Medd. Kmn. Dan. Havunders., Ser. Fisk., (): -. MEYER, A.. German investigatins n the Greenland cd stck,. ICES Ann. Bii., : -. PLATT, N. E.. Cdwrm - a pssible bilgical indicatr f the degree f mixing f Greenland and Iceland cd stcks. ICES J. Cns., (): -. RASMUSSEN, B.. On the migratin pattern f the West Greenland stck f cd. ICES Ann. Bii., : -.

Holger H. Hansen Greenland Fisheries and Environmental Research Institute Tagensvej 135, Copenhagen N, DK-2200, Denmark. Abstract

Holger H. Hansen Greenland Fisheries and Environmental Research Institute Tagensvej 135, Copenhagen N, DK-2200, Denmark. Abstract NAFO Sci. Cun. Studies, : 7- han Hlger H. Hansen Greenland Fisheries and Envirnmental Research Institute Tagensvej, Cpenhagen N, DK-00, Denmark Abstract Analysis f sampling data fr Atlantic cd (Gadus mrhua)