This paper not to be cited without prior reference to the authors ANNUAL REPORT ON THE RESULTS OF THE BALTIC SEA TROUT TRANSFER EXPERIMENT

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1 This paper not to be cited without prior reference to the authors International Council for the Exploration of the Sea C.M. 1981/1'1:13 Anadromous and Catadromous Fish Cornrnittee Abstract ANNUAL REPORT ON THE RESULTS OF THE BALTIC SEA TROUT TRANSFER EXPERIMENT by E. Ikonen, J. Toivonen and H. Auvinen Finnish Garne and Fisheries Research Institute, Fisheries Division, P.O. Box 193, SF-00131, Helsinki 13, Finland A total of two-year-old hatchery-reared sea trout smolts originating from the Vistula River were tagged in Poland in March 1980, and the same number of two-year old hatchery-reared smolts originating from the Isojoki River were tagged in Finland in April 1geO. Half the Polish smolts were released in the rnouth of the Vistula and half were transferred to Finland and released off the Helsinki coast. Similarly, Finnish smolts were transferred to the mouth of the Vistula and were released off Helsinki. None of the Polish smolts transferred to the Gulf of Finland had been recaptured by May The Finnish smolts released in the Gulf of Finland had given 16 recaptures, and those transferred to the Vistula had given 39. Of the Polish smolts released in the Vistula, six had been recovered. Resurne Au cours de mars 1980, au total smolts de truite de mer eleves dans un etablissement d'eclore et transplantes a l'age de deux ans, originaires du fleuve Vistule, ont ete marques en Pologne, et un nombre pareil des smolts eleves dans un etablissement d'eclore et transplantes a l'age de deux ans, originaires du fleuve Isojoki, ont ete marques en Finlande au cours d'avril La moitie des smolts polonais a ete mis en liberte dans l'embouchure du fleuve Vistule et la moitie est transfere en Finlande et mis en liberte sur la cote de Helsinki. Egalement smolts originaires de Finlande ont ete transferes a l'ernbouchure du fleuve Vistule et ont ete mis en liberte a Helsinki. Pas un seul smolt polonais transfere au golfe de Finlande a ete retrouve avant mai Seize smolts finlandais, mis en liberte dans le golfe de Finlande, ont ete retrouves et egalerneht 39 de ceux qui ont ete transferes dans le fleuve Vistule. Six smolts polonais mis en liberte dans le fleuve Vistule ont ete decouverts.

2 2 Introduction During the 25th meeting of the Baltic Salmon Working Group, it was decided to perform a Finnish-Polish experiment in which sea trout smolts were tagged and transferred (Anon. 1979). The aim of this experiment is to search for sea trout strains especial 1y suitable for stocking and to find out whether the environment influences fish migratory behaviour and body growth. Tagging and transferring took place in Poland and Finland in spring This is a preliminary report on the recapturesmade during the year following tagging. Material and methods In Poland two-year-old sea trout smolts (Salmo trutta L.) originating from the Vistula River and reared at the Podkomorzyce hatchery were tagged in March Half the fish were tagged by Finnish taggers with light green plastic Finnish tags attached with a twofold polythene monofilament, and half were tagged by Polish taggers with yellow plastic Polish tags attached with single Monel metal wires (Anon. 1980). In Finland two-year-old sea.trout smolts originating from the Isojoki River and reared at the Hanka Taimen GY hatchery weretagged in April The tag types, taggers and tagging procedure were the same as in Poland (Anon. 1980). Half the Finnish sea trout smolts were transferred by ferry from Finland and released in the mouth of the Vistula on 25 April Half the Polish sea trout smolts had been released as a control group in the mouth of the Vistula on 29 March Similarly, half the Polish sea trout smolts were transferred to the Gulf of Finland and released off the Helsinki coast on 7 May 1980, and a Finnish control group was released at the same place on 8 May 1980 (Anon. 1980).

3 3 Results So far, none of the Vistula sea trout transferred to the Gulf of Finland have been recaptured. During and after transportation 38 dead smolts were observed. Of the control... group of lsojoki sea trout released in the Gulf of Finland, 16 have been recovered - (table 1). 10 with Finnish tags and 6 with Polish tags The Finnish smolts transferred to the Vistula (Swibno) have given 35 recoveries. The control group of Vistula sea trout released in the same area has given six recaptures (table 1). Most of the Finnish smolts released in the Vistula have.been caught near the releasing place. Three recaptures have been made in the upper part of the river (fig. 1). The control group recoveries have been made in the river mouth, except for one fish, which had migrated to the lces subdivision 30 off the Finnish coast. The control group of Vistula sea trout released in the Gulf of Finland has mainly been caught in the lces subdivision 32. One has been caught in subdivision 30 and one in 28 (fig. 2). Discussion Only one year has elapsed since this experiment was performed and this is probably the reason for the small number of recaptures. The results are still too meagre to allow any conclusions about differences in the sea trout strains, their.migration or growth. References Anon. 1979: The 25th meeting of the Baltic Salmon Working Group. lees, C.M. 1979/M: : Report on the Baltic Sea Trout Transfer Experiment. lces, C.M. 1980/M:12.

4 / Table 1. Returns from the sea trout transfer experiment reported by 31 May 1981 Code Origin Place of Place of release Tag type Tagged Recapture Other tags Total returns number tagging fish reported No. No. % No. % No. % 1 Vistula Podkomorzyce Finland, Santahamina Polythene ) Vistula Podkomortzyce Finl and, Santahamina r~one ) Vistula Podkomorzyce Poland, Swibno Polythene Vistula Podkomorzyce Poland, Swibno Monel O Isojoki Hanka-Taimen Oy Poland, Swibno Monel Isojoki Hanka-Taimen Oy Poland, Swibno Polythene O Isojoki Hanka-Taimen Oy Finland, Santahamina Monel Isojoki Hanka-Taimen Oy Finland, Santahamina Polythene ) lost tags or dead fish after tagging or transfer

5 ~ ~ 16 u G6() ---'T---"1----,-~--..., r---,.-- x Fig. 1. Recaptures af the sea traut released in the mouth of the Vistula, Swibno Isajaki sea traut, Finnish.Vistula sea traut, Polish

6 Fig. 2. Recaptures Qf the sea traut released in the Gulf af Finland, Santahamina Isojoki sea trout, Finnish Vistula sea trout; Polish