MlXlNG OF WILD AND HATCHERY-REARED SALMON DURING MIGRATION IN THE BALTIC SEA

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1 11<" I.' International Council for the Exploration of the Sea C.M. 1987/M: 10 Anadromous and Catadromous Fish Committee MlXlNG OF WILD AND HATCHERYREARED SALMON DURING MIGRATION IN THE BALTIC SEA bv Abstract Erkki Ikonen Finnish Game and Fisheries Research Institute Fisheries Division, P.O.Box 193, Sf00131 Helsinki. Finland In 1986 the total smolt production of salmon in the Baltic Sea was 4.7 million and the proportion originating from natural spawning was about 10 %. The Baltic Sea receives about rivers still supporting natural salmon stocks. The greater part of the natural smolt production takes piace in the rivers flowing to the Gulf of Bothnia (ICES Subdivisions 30 and 31). The feeding areas of salmon stocks originated from the rivers in Subdivision 31' are mainly situated in the Baltic Main Basin. River Neva stock in the Gulf of Finland (lces 32) feeds mainly in the Gulf and when released in the southern part of the Gulf of Bothnia, this stock also mainly stays in the southern part of this Gulf. The stocks originating from the Gulf of Riga area feed in the northern part of the Main Basin and partly in the Gulf of Finland. Salmon stocks originating from the Baltic Hain Basin feed in this,area. The salmon stocks. both wild and hatchery reared are weil mixed during feeding migration. In the offshore catches in the Main Basin and the Gulf of Bothnia the percentage of salmon of wild origin has been about 15. in the Gulf of Finland. the proportion of wild salmon in offshore catches is negligible. In the coastal pound net catches which mainly consist of salmon taken on their spawning run,the percentage of salmon of wild origin has varied between 50 and 70 in Subdivisions 2931 and in the Gulf of Finland the percentage has been atout 15. During spawning migration in the coastal area the stocks originating from the Gulf of Bothnia seem to have similar migration routes. The timing of spawning migration differs between wild and hatchery reared salmon. the salmon of wild origin migrating earlier than hatcherv reared ones. 1

2 1. Introduction The Salmon stock in the Baltic Sea mainly originates from hatchery reared smolts. Natural spawning contributes only 10 % of the total production. In the beginning of this century. when most of the salmon rivers still produced smolts, the majority of smolts originated from the rivers flowing to the Gulf of Bothnia and at present most of the wild production still comes from these rivers. Wild production also occurs in some rivers emptying into the Hain Basin, the Gulf of Riga and the Gulf of Finland. The main reason for to releasing hatcheryreared smolts at the river mouths is to compensate for the loss of smolt production caused by damming the rivers. The migration behaviour may differ between the separate salmon stocks but the feeding areas are common to most of the stocks. The aim ef this paper is to present observations on the mixing of wild and hatcheryreared salmon during migration in the Baltic sea. 2. Haterial and methods Altogether 9989 scale samples were collected from the catches of the offshore fisherv. carried out in the Baltic Hain Basin(lCES Subdivisions 28+29). the Gulf of Bothnia (ICES 30) and the Gulf of Finland (lces 32). The sampies were collected by fishermen from commercial salmon catches taken with drift nets and long lines, during the fishing season. SeptemberJuni. in (Table 1). These sampies do not cover the total catches, because the minimum permitted length of salmon in the Baltic Sea salmon fishery is cm, so that the age group AO+ is underestimated. However. in seme cases fishermen were specially licensed to sampie undersized salmon as weil. From the coastal fishery caught with. pound nets 15,4 scale samples were collected from Subdivisions 2932 (Table 1). The differentation of the wild and hatchery reared salmon was based on the structure of the freshwater zone in the scale (Antere and Ikonen 1984). 3. Results In the northern Hain Basin of the Baltic Sea (Subdivisions 28 and 29), age group A.2+ is the most common in the drift net and long line catches (Fig.l). Only in 1983 and 1985 was age group A.l+ the most common in the catches. The percentage of salmon of wild origin in the different age groups varies between zero and 29. Higher percentages also occur in A.3+ and older age groups, but the samples of these age groups are so small that they cannot be compared with age groups A.l+ and A.2+. The proportion of salmen cf wild origin can generally be seen to be slightly higher in age group A.1+ than in the others. In the southern part of the Gulf of Bothnia (Subdivision 30) the

3 , age group distribution is almost the same as in the Hain Basin (Fig.2). However the proportion of salmon of wild origin in age group 2+ is greater than in the Hain Basin samoles. In the Gulf of Finland (Subdivision 32) the age group distribution differs from that in other areas. The most common age group was A.l+, except in 1981, when 2+ was larger. The proportion of wild salmon in the catches was clearlv lower than in the other areas of the Baltic sea. In 1981, the proportion of wild salmon in age group 1+ was 4 70 but during later years the percentages were lower and in 1985 and 1986 no salmon of wild origin were detected. In the sampies taken from the pound net fishery, the most common age group is A.2+ (Fig.47). This gear mainly catches salmon during their spawning run along the coast. Age group A.l+, or grilse, are more numerous in the catches than in the sampies, but as most grilse are shorter than cm, which is the minimum legal size for salmon, most of them were released immediatly after capture. In all Subdivisions, the percentage of salmon of wild origin is clearly bigger in the coastal than in the offshore catchsamples. 4. Discussion Salmon smolts of wild origin migrate from the river to the sea in HavJune, when the water temperature increase to 10 0 C (Toivonen and Jutila 1982). Releases of hatcheryreared salmon smolts are also performed when water temperature in the river is about 10 0 C, so that the wild and hatcheryreared smolts start their migration to the sea at about the same time. Fish originating from Finnish releases or from wild production in the northern part of the Gulf of Bothnia (Subdivision 31) migrate along the Finnish coast to the Ouark and along the Swedish coast to the Hain Basin. Tag recoveries reveal no difference in migration pattern between wild and hatchery reared postsmolts (Ikonen and Auvinen 1985). Although the wild and hatcheryreared salmon migrate along the same route. the fish are not randomly distributed during their sea migration. Trap nets for Baltic herring also catch salmon postsmolts (Ikonen and Auvinen 1985). In these catches it has been observed on the basis of tagged fish that the postsmolts in a onedav catch mainly originate from one or two releases, although several releaseshave been made about the same time.this indicates that postsmolts migrate in the 'same shoal in which they started their smolt run. This kind of shoaling may still 'occur during their feeding migration. For example. the scale sampling in the Hain Basin area showed that the one night catch of one drift net vessel contained many fish of the same age group. During the next'night, the age comoosition might have been different. When the origin cf salmon is determined on the basis cf catch sampies, several sampies should be taken'. It has been observed that the proportion of wild salmon in the samples mav varv between 6 and 48 in the Hain Basin area (Anon. 1987). 3

4 The feeding areas of wild and hatchery reared salmon originating from northern stocks are very similar. The central and southern parts of the Hain Basin especiallv the areas of Gotland and Bornholm deeps, seem to be the most important feeding areas for the River Simojoki stock (wild) and the stocks of the Rivers Kemijoki, Iijoki and Oulujoki (Ikonen and Auvinen 1984). Although shoaling was observed, it was not possible to identify different feeding areas for salmon of wild and hatcheryreared origin in the Hain Basin of the Baltic Sea, In the Hain Basin and Gulf of Bothnia the proportion of wild salmon in the smolt production is about 10 %. However, the 300, do not contribute to the Hain Basin fisherv (Ikonen and Auvinen 1984, Anon.1987). According to the scale sampies, the percentage of wild salmon in the catches averaged about 15. If production of the Neva salmon and the better survival of wild smolts (Toivonen 1977, Anon.19) are taken into consideration, the percentage of wild salmon in the Hain Basin catches is roughly comparable to the smolt production. smolts produced by Neva salmon, In Subdivision 30, the percentages of wild salmon in the catches are slightly lower. One reason for this i5 releases of Neva salmon in this Subdivision.It also seems that salmon of wild orgn migrate in greater nunumbers than reared ones from Subdivision 31 to the Hain Basin. During feeding migration, 7% of salmon originating from the River Simojoki were caught in Subdivision 30 and 93 % in the Hain Basin. Of the hatcheryreared salmon originating from the Rivers Iijoki. Kemijoki and Oulujoki, 1217 % were. caught in Subdivision 30 and 8388 % in the Hain Basin (Ikonen and Auvinen 1984). In the scale sampies wild salmon belonging to age group A.1+ were slightly, in the majority in the Hain Basin, but in Subdivision 30 A.2+ was more common. In coastal pound net catches in Subdivisions 2931 salmen cf wild origin are more numerous than in offshore catches. In Subdivisions 2930 the percentage of wild salmon varied around 50 and in Subdivision 31 around 70. According to the smolt production. the proportion of wild salmon should be 15 %. KallioNyberg (pers.comm.) has reported'that the bigger the smolt is the faster is its growth rate during the first vear in the sea, The average size of reared smolts is greater than that ef wild smolts in the River Simojoki (Toivonen 1977). The most important gear in the salmon fishery in the Baltic Hain Basin is the drift net (Christensen and Larsson 1979). Because of this, fastgrowing fish will be caught bv drift nets more numerously than fish which reach a catchable size for drift nets in late winter. when poor weather conditions often prevent fishing. It is also known more grilse result from hatchery reared stocks than from wild ones (Ritter and Carev 19). Because the spawning run of grilse takes place in July and because of their small size. they are underestimated in the catch sameies. Ikonen (1986) has suggested thatowild,salmon migrate'earlier that hatchery reared ones. In the 4

5 beginning of the coastal pound net season strong winds are not so usual as later in summer. Accordingly, it is easier to catch salmon during the beginning of the run than later when storms prevent effective use of these gears or destroy them. For this reason, wild salmon mav be overrepresented in the coastal catches. These considerations do not, however, explain the whole difference between the coastal and offshore catches. Though no evidence of this is available, it is also believed, that not all hatchery fish migrate for spawning. It has also been sugested that their migration routes differ so much that the hatcherv fish are difficult to catch. However. the amount of hatcherv fish observed in the rivers in which they have been released has not been sufficientlv large tc support the idea. Nor it is confirmed by tag recoveries.on the basis of the data available, it is not yet possible to explain whv the amount of salmon of wild origin is larger in the eoastal than in the offshore catehes. In the Gulf of Finland the percentage of salmon of wild origin is almost zero. The coastal pound net catches contain salmon of wild origin (15 %), but the horne river or rivers of these wild fish are unknown. Taggings in the Gulf of Bothnia area show that very few fish of these northern stocks migrate to the Gulf of Finland (Ikonen and Auvinen 1984), and this wild salmon catch does not originate from River Neva stock (Kasakov pers.comm.). As they have been caught onlv during the spawning run and wild salmon are lacking in the offshore catches, it seems that these fish feed in other areas of the Baltic Sea. However, the river to which these salmon horne is not known. Postsmolts are caught with long lines in the Gulf of Finland. but these wild fish have not been detected among postsmolt samples. postsmolts inothe Gulf of Finland, but among postsmolt samples these wild fish have not been detected. REFERENCES Anon. 1987: Report of the Baltic Salmon and Trout Assessment Working Group. lces, Doe. C.M.1987/Assess:21 Ikonen E. and Auvinen H. 1984: Migration cf salmen in the Baltic Sea, based on Finnish tagging experiments. lces C.M.1984/M:4 Ikonen E. and Auvinen H. 1985: Migration of salmon postsmolts (Salmo salar L) in the Baltic Sea. ICES C.M. 1985/M:19 Toivonen. J. 1977: Differences in recapture cf wild and hatcheryreared salmon smolts. lees C.M.1977 / M:7. 8p. Toivonen. J. and Jutila E. 1982: Report on parr population densities, tagging experiments and river catches of salmon the River Simojoki in lces C.M.1982/M: 5

6 Ritter J.A. and Carev T.G. 19: Salmon ranching in the Atlantic Maritime Provinces of Canada.In: Salmon ranching (Ed. J.E. Thorpe), London 19. 6

7 Table 1 Percentage of salmon of wild origin in the Finnish catch samples from the offshore and coastal fisheries in lees Subdivisions 2832 Offshore fisherv Subdivisions Coastal fishery Subdivisions Year wild sample wild sample wild sample wild sample wild sample wild sampie wild sampie 'Tl,. size,no. % size,no. % size,no. % size,no. % size,no. % size,no. % size,no , , , ,0 16 1, , , , Table 2. Salmon smolt production in the Baltic Sea in in thousands of individuals Year Subdivisions 2531 Subdivision 32,üld hatcherv wild hatcheryreared total reared total ,7 3, , r") ,694 3, c r)... J.L ,841 3, ,237 3, ,870 4, ,425 4, ,045 5,

8 %, j 19 rf rf rf rf rf32l r li r r r 1985 rf245, rf147 r f l h 11 r,,... '...; f '. A3+ I % wild Figure 1. Age group distribution and percentages of salmon of wild origin in different age groups in the offshore catches by long line and drift net in Subdivisions 28 and 29 % 19 rf132 Al A1. r l\ rl.l 1982 n=558 + Al r,... Al+ A3+!G!hf!G! hf Pd Al+,...,... r Al Al r l Al:+,... Al r Al r r!g! A3+ A3+ A3+ A3+ I ' J 1984 rf rf299,a+ hfr, I h I I J u 1986 n=573 % wild Figure 2. Age group distribution and percentages of salmon of wild origin in different age groups in the offshore catches by long line and drift net in Subdivision 30

9 1981 n= n= n= n=1267 n= n='zn % n % wilci Figure 3. Age group distribution and percentages of salmon of wild origin in different age groups in the offshore catches by long line in Subdivision 32

10 1983 % n=694, n=692 r' n=1233 n=300 r" Alf r' Al±, A3+ A3+... Al+ l I Al+ I r" r L % wild ' Figure 4. Age group distribution and percentages of salmon of wild origin in different age groups in the pound net catches in the coastal area of Subdivision ,% n=1ce n= n=tio ;;.; 1985 n=1();l 1986 n=398 ""'"' r" A3+ Al,. Al+ Al ""'"' I r+ Al+... I Al+ A3+... r' Alf """' A3+ I ' I.. % wllc. I ' ' I... ' Figure 5. Age group distribution and percentages ef salmen of wild origin,in different age groups in the pound net catches in' the coastal area of Subdivision 30

11 , 19 % n= rf42l 1982 n= n= n=lcxll 1985 n= n=881 Jgt i lq! Al+ I ;...;;; Al,:!: Al Al A3+,. &r Al+ A3+ A3+ Al+,. Al+,... ",. Al Al J1± Al Al... I ' I r,... I r... f I % wild Figure 6. Age group distribution and percentages of wild origin in different age groups in the pound net the coastal area of Subdivision 31 % n=334 rf3/57 n=1023 n=826 n=6oo ' l. I. L. salmon of catches in Al,. A3+ I Al E*... I All Al+,. Al+ l I 'LL I.... %wild L Figure 7. Age group distribution and percentages of salmon of wild origin in different age groups in the pound net catches in the coastal area of Subdiviiion 32