ICES Marine Science Symposia, 215: 361-370. 2002

Fish migration studies in the Baltic Sea - a historical review

Eero Aro

Aro, E. 2002. Fish migration studies in the Baltic Sea: a historical review. - ICES Marine Science Symposia, 215: 361-370.

The distribution o f Baltic Sea fish species throughout their life history may be random or patterned, and these changes have been estimated by tagging programmes since the early years o f ICES. The adaptive value of fish migration is strongly coupled with the optimization of the surrounding environment. The hydrographic conditions in the Baltic form an uneven continuity from southwest to northeast, and the physiological selection o f fish species, their distribution, and their capability to migrate has taken place in varying environmental temperatures and salinities. In the life history o f fish species in the Baltic Sea, the eurythermie and euryhaline species have had more adap­tive value than those species which prefer constant salinities and temperatures. The euryhaline and eurythermie species are usually able to migrate more intensively than stenothermic and stenohaline species. Marine species have both rather local and more migratory stocks. Freshwater species are, in general, more stationary than marine species. The advantage o f migrations in a patchy environment is that the impact of environmental variability on reproduction, survival, foraging, and growth decreases. ICES, as a coordinating organization, has played an important role in migration stud­ies during the last 100 years and has been significant as a forum for developing new ideas and hypotheses on fish migration in the Baltic Sea.

Keywords: Baltic herring, Baltic salmon, cod. flounder, freshwater species, ICES, migration, plaice, sea trout, tagging.

Eero Aro: Finnish Game and Fisheries Research Institute, Pukinmäenaukio 4, PO Box 6, FIN-00721 Helsinki, Finland; tel: +358 205 751 253; fax: +358 205 751 201; e-mail: eero.aro@rktl.fi.

Introduction

If the environmental needs o f fish species were constant through time, there would be no reason for them to move from place to place. The distribution o f fish throughout their life history may be random or pat­terned, and the environmental needs of individuals may vary from day to night, summer to winter, and from the time o f hatching to their adulthood. The spatial and tem­poral distributions of many fish species change in more or less regular and cyclical patterns. The adaptive value of fish migration is strongly coupled with the optimiza­tion o f the surrounding environment. The Baltic Sea is the product of the last glacial period and has many unusual characteristics. It hosts an aquatic biota where both marine and freshwater organisms live side by side with a number of living relict species. The hydrograph­ic conditions in the Baltic Sea form an uneven continu­ity from southwest to northeast, so the physiological selection o f fish species, their distribution, and their capability to migrate has taken place in varying envi­ronmental temperatures and salinities.

Historical background o f ICES and fish migration research in the Baltic Sea

Early years at the beginning of the 20th century

Since the early years o f ICES, fish migration, distribu­tion, and interaction studies have formed an important part o f ICES research activities (Hoek and Hjort, 1903; Garstang, 1903; Hoek and Garstang, 1903). These activ­ities led to the development o f Petersen's famous princi­ple (Petersen button), attaching a tag button by a silver wire near the middle o f the dorsal edge of the body.

Finnish Fisheries Inspector Oscar Nordqvist, who was Convener o f ICES Committee C (Committee for the Baltic) at the very beginning was the first to propose migration studies and large-scale tagging research in the Baltic Sea. He drafted a research proposal in 1902 for the Baltic (Nordqvist, 1903) which included basic work on salmon, sea trout, eel, sprat, cod, and flounder and the effect o f seals on fish stocks. His first draft also included migration studies, but his work had to be com­

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FIN

NOR SalmonEel

SWE EST RUS

LAT

LIT

RUSIPOLGER

Figure 1. The first recaptures o f Baltic salmon and eel in early 1900 as reported to ICES in 1907.

pleted by Filip Trybom (Sweden) and C. G. J. Petersen (Denmark) after Nordqvist’s unfortunate departure caused by the political situation in Finland. The Czar o f Russia issued an edict on 26 September 1902 to the Finnish Senate to discharge Oscar Nordqvist for politi­cal reasons, effective 1 October 1902.

However, the outcome o f Nordqvist’s initiative was a draft report, produced in 1907 by P. P. C. Hoek and F. Trybom and assisted by Nordqvist’s successor J. A. Sandman, to the governments of Denmark, Finland, Russia, and Sweden regarding the Baltic salmon fish­eries (Hoek and Trybom, 1907). The report included a discussion about the reasons for the declining stock, the nature of the fishery and its distribution between coastal and open-sea areas, minimum landing size, and mini­mum mesh size (Nordqvist, 1907), all in relation to the timing and pattern o f salmon migrations. Trybom (1907a) also drafted a special "proposal regarding an international Treaty between the Baltic countries" which contained a number o f actions to be taken to "prevent any further diminution of the stock of salmon, maintain the salmon stock in the Baltic and possible increase of

catch by the erection o f hatcheries where the greatest possible quantities o f salmon might be bred". This pro­posal, considered a predecessor of the present Inter­national Baltic Sea Fishery Commission’s (IBSFC) Sal­mon Action Plan (Anon., 1995), was not supported by the Baltic countries, and the ICES Bureau was not yet prepared to bring forward a definite proposal on the matter in June 1907.

The special report by Trybom (1907b) gave detailed results of Baltic salmon and sea trout tagging and mi­gration studies conducted by various Member Countries in the Baltic area during 1902-1906. Among the recap­tures was one salmon originally tagged off Bornholm in April 1906 and recaptured four months later in Bothnian Bay. This is believed to be the first documented Baltic salmon homing record in the scientific literature. Another salmon tagged in September 1905 in the east­ern part o f the Gulf of Finland outside the mouth o f the Kymijoki River was recaptured in May 1907 in the neighbourhood of Memel (Klaipeda) showing, as a first record, the possible extent of the feeding migration of Baltic salmon ( Figure 1 ). These early taggings o f Baltic

Fish migration studies in the Baltic Sea - a historical review 363

A. Coastal zoneB. Small restricted "shells"C. Rocky areasD. Steep slopes

E. Open/deep sea areaF. Larger reefs in open seaG. Special areas (wrecks etc).

Figure 2. Typical fishing ground categories in the Baltic Sea.

salmon and sea trout were aimed mainly at estimating the distribution area and proportion removed by open- sea fisheries and coastal fisheries (Trybom, 1907b), and although the number o f tagged fish each year was rather low, tagging provided new information on migration pat­terns of Baltic salmon and sea trout.

The state o f the eel stock in the Baltic also received attention (Trybom and Schneider, 1907) in Committee C at the beginning of 1900. O. Nordqvist and J. A. Pal- mén started tagging eels in 1903 at the Tvärminne Zoo­logical Station founded by Palmén in 1902 on Hanko Peninsula, Finland, the westernmost part o f the Gulf of Finland (Nordqvist, 1903). Eel taggings were expanded further in 1904-1907. These first silver eel taggings and the subsequent recaptures in the Atlantic Ocean (Figure 1) contributed to the development o f Johannes Schmidt’s famous hypothesis on eel life history (Schmidt, 1906).

Priorities during the 1920s-1940s

Fish migration studies in the Baltic in the 1920s-1940s concentrated mainly on commercially important marine, anadromous, and catadromous species such as Baltic herring, sprat, cod, plaice, flounder, eel, and salmon. The freshwater species in the Baltic did not receive any

special attention from ICES, although many laboratories tagged freshwater fish in coastal areas (Ekman, 1916; Henking, 1923).

The main aims o f all tagging studies during this peri­od were to estimate the spatial and temporal distribution patterns and their annual changes. In the 1920s and 1930s, scientists began the practice of categorizing the recapture information according to distance travelled and to the typical fishing grounds in the Baltic Sea (Figure 2).

In the 1920s, special attention was paid to the most abundant species in the pelagic system, Baltic herring and sprat, and to their predator, Baltic cod (Heidrich, 1925; Hessle, 1923, 1925, 1927). Hessle (1925) showed that the slow-growing, long-lived, open-sea herring spawn mainly along the Swedish east coast from Hanö Bay to the Åland Sea in deeper water than the coastal spring-spawning herring in other locations. Hessle (1925) also showed that Baltic herring along the Swe­dish coast undertook long migrations extending from the Åland Sea to the southern part o f the Baltic.

In the 1920s, sprat were shown to inhabit the Baltic from the Belt Seas and Western Baltic (ICES Subdivi­sions 22 and 24), to the Quark area in the north (Sub­division 30), and to the northeastern part of the Gulf o f Finland (Subdivision 32) (Hessle, 1927). Three dif­ferent sprat stock components were considered at that

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time, with a low rate o f mixing with the Kattegat and Skagerrak stocks due to the salinity gradient and differ­ences in many abiotic factors between the Western Bal­tic and the Kattegat (Hessle, 1927).

Hessle ( 1923) and his colleagues reported on the dis­tribution, spawning, growth, and periodicity o f Baltic cod year classes along the Swedish coast. Although these studies included only areas in the vicinity o f the Swedish coast, they indicated great fluctuation caused by changes in the distribution patterns o f cod year class­es. Kändler ( 1944), in his classical work in which small numbers of cod were tagged, showed differences be­tween cod stocks in the Baltic in the Transition Area around Bornholm (14°E).

In the case o f Baltic salmon, Järvi (1938) reported that they nearly all fed in the Baltic Main Basin (Sub­divisions 24-29) and that the most important feeding grounds were the Gotland Deep, the waters around Bornholm, and the Gdansk Basin. This important work summarized 15 years of information (1921-1935) on the basic biology, the fisheries, and obvious reasons for fluctuations in the Baltic salmon.

Decades o f tagging and migration studies during 1950-1980

Fish migration studies using tagging were intensified considerably in ICES circles in the 1950s. An initial stimulus came when a Danish cutter accidentally dis­covered dense concentrations o f young herring on the Bløden Grounds in July 1950 about 60-100 miles west o f Esbjerg, Denmark, in the North Sea. This discovery and the fishery which subsequently developed stimulat­ed discussion about the role and effect o f the Bløden fishery on the North Sea herring fishery and led to an extensive tagging programme for herring in the North Sea and the Baltic Sea. The establishment of the Inter­national Baltic Sea Fishery Commission (IBSFC), and two ICES stock assessment working groups necessary for IBSFC activities served to elevate the need for tag­ging studies in the early 1970s.

Thus, the present knowledge o f fish migration in the Baltic is based mainly on taggings made from the early 1950s to the 1980s, except in the case of Baltic salmon and sea trout, large numbers o f which have been tagged each year up to the present. Between 1950 and 1980, several hundred thousand fish were tagged throughout the entire Baltic area (Aro, 1989) including Baltic her­ring, sprat, cod, plaice, flounder, Baltic salmon, sea trout, and freshwater species such as whitefish, pike, pikeperch, perch, bream, and burbot.

A common feature o f all the tagging work has been to estimate annual migration patterns and their changes, the home range of various species, site fidelity for spawning, distribution o f stock components and their mixing, and the effects o f fishing at stock level, and also to evaluate the timing of annual migration patterns.

Main results from tagging and m igra­tion studies during 1950-1980

Herring

Tagging and migration studies have shown that in some areas Baltic herring can be divided into two or three components by their different migration patterns, such as spring-spawning coastal herring, spring-spawning open-sea herring, and autumn-spawning herring. Baltic herring in the southwestern Baltic, Kattegat, and Skag­errak have both spring-spawning and autumn-spawning components (Anwand, 1963a, 1963b), with spring- spawning stocks having a very clear migration pattern. The majority o f the fish migrate from the feeding and wintering areas in the Skagerrak and Kattegat through the Öresund and the Belts into the Baltic in autumn and early winter, with a minority coming from other feeding areas around Bornholm, off the Polish coast, and Hanö Bay to the spawning grounds (Biester, 1979; Otterlind, 1985). The migration pattern o f adult herring from the Baltic to the Kattegat and Skagerrak has also been con­firmed by the occurrence o f Anisakis nematode larvae in the adult herring from the Southwestern Baltic (Strzyzewska and Popiel, 1974; Friess, 1977; Kiihlmor- gen-Hille, 1983). Herring in the Southwestern Baltic seem to have a clear homing ability (Jönsson and Biester, 1979) despite some controversy. In the Central Baltic, there are assumed to be three different stocks: spring-spawning coastal herring, spring-spawning open- sea herring, and autumn-spawning herring (Popiel,1984). These stock components have different migration patterns in the Southeastern and Central Baltic. In the Bothnian Sea, there are two spring-spawning coastal herring components (Hannerz, 1955, 1956; Otterlind, 1957; Parmanne and Sjöblom, 1982, 1986), one along the west coast and one on the east coast, both of which show high spawning-site fidelity.

Sprat

Sprat inhabit the Baltic Sea from the Belt Seas and west­ern Baltic (Subdivisions 22 and 24), to the Quark area in the north (Subdivision 30), and to the northeastern part o f the Gulf o f Finland (Subdivision 32) (Hessle, 1927; Veldre, 1986; Rechlin, 1986). There are three different sprat stocks in the Baltic, and mixing with the Kattegat and Skagerrak stocks is considered to be very low, although there is no significant difference in morpho- metric characters and vertebrae counts (Lindquist, 1968). Mixing is probably prevented by the salinity gra­dient and differences in many abiotic factors between the Western Baltic and the Kattegat (Hessle, 1927; Aps et al„ 1987). Boundaries between neighbouring stocks are unclear, and stock mixing during feeding and win­tering is apparent (Rechlin, 1986). During the feeding period in July-November in the south and central parts

Fish migration studies in the Baltic Sea - a historical review 365

of the Baltic (van Khan et al., 1972) and in August- December in the north, sprat migrate from the more coastal areas to the offshore parts o f the Baltic basins forming feeding shoals which contain mainly older age groups (Lindquist, 1971). The range and amplitude of the migration patterns of sprat stocks and their mixing in the Baltic is not yet very clear. Some observations indicate more locality (Aps and Lotman, 1984; Aps et at., 1987), but some, on the contrary, indicate more migratory behaviour and stock mixing (Grauman, 1976; Khoziosky et al., 1983).

Salmon

Salmon in the Baltic are distributed from the Belt Seas and Western Baltic to the northern parts of the Gulf of Bothnia and the Gulf of Finland and very seldom migrate outside the Baltic (ICES, 1980; Lindroth et al.,1982). There are naturally spawning stocks and artifi­cially reared stocks of various origin. During the marine phase, the Baltic salmon have very clear migration pat­terns in the coastal and pelagic area. In general, both the wild and hatchery-reared stocks have the same migra­tion patterns, with some exceptions. The migrations are divided into post-smolt, feeding, and spawning phases.

Salmon smolts enter the Baltic usually in April-June, with those from the southern Baltic running a few weeks earlier than those from the north. The wild and hatch­ery-reared smolts start their post-smolt migration at about the same time. During the first few weeks, post- smolts are relatively stationary, adapting to the new environment, and they remain close to the river mouth or releasing place (Bartel, 1976; Ikonen and Auvinen,1985). From releases on the Finnish side o f Bothnian Bay (Subdivision 31), post-smolts migrate southwards along the Finnish coast to the Quark area where they shift to the Swedish coast. From the Swedish releases, the post-smolts migrate south along the Swedish coast of Bothnian Bay to the Quark where some shift to the Finnish side following the main current northwards along the Finnish coast, and do not leave Bothnian Bay (Larsson and Atheskar, 1979). In the Bothnian Sea (Subdivision 30) stock, the post-smolts behave like the northern stocks, except for the River Neva stock (from Russian origin in the eastern Gulf o f Finland, Sub­division 32) releases in the Bothnian Sea (Ikonen and Auvinen, 1982), which seem to have more localized ten­dencies.

Nearly all Baltic salmon stocks feed in the Baltic Main Basin (Subdivisions 24-29), except those o f River Neva origin. From the releases in the Gulf of Finland, about 31% of the Neva-origin fish feed in the Main Basin, while the percentage from the Bothnian Sea releases is less. The most important feeding grounds are the Gotland Deep, the waters around Bornholm, and the Gdansk Basin (Järvi, 1938; Halme, 1964; Carlin, 1959; Thurow, 1973; Christensen and Larsson, 1979; Ikonen

and Auvinen, 1984a, 1985). On these feeding grounds, the stocks mix thoroughly, and there is no clear evidence on the preferred feeding areas of individual stocks, al­though there are some aggregations on the main fishing grounds (Carlin, 1959; Halme, 1964). Feeding salmon are more or less clustered, and changes in density on the feeding grounds reflect their active movement in search of food (Thurow, 1973).

All Baltic salmon stocks exhibit homing behaviour, but in some releases, homing has not developed because of environmental factors (Christensen, 1982).

Sea trout

Sea trout in the Baltic are distributed from the south­western part to the northern parts o f the Gulf o f Bothnia and eastern parts o f the Gulf of Finland. Their migration may be divided into two patterns: the long-distance migrating stocks and the more stationary stocks (Zarnecki and Duszynski, 1961; Ikonen and Auvinen, 1984b). In the Baltic, sea trout occupy an intermediate position between the pelagic and coastal fish communi­ties, and their migration pattern is dependent on stock origin and the dimensions of the archipelago.

During their feeding migration, the stocks in the Bothnian Sea and Bothnian Bay intermix when migrat­ing from the east coast to the west coast and vice versa. The migration southwards from Bothnian Bay to the Bothnian Sea and to the Main Basin has also been observed even if the northern stocks are considered to be more local and less migratory in nature (Carlin, 1965; Ikonen and Auvinen, 1984b). In the Gulf of Finland, the feeding migration encompasses the whole area from west to east, and there appears to be some migration to the Archipelago Sea, the Bothnian Sea, and the Main Basin (Ikonen and Auvinen, 1984b). In the northeastern Baltic near the island of Saaremaa, the stock seems to be quite local, but in some cases it extends into the Bothnian Sea, the Åland Sea, near Gotland Island, and southwards to the Bornholm region (Rannak et al., 1983). In the Southern Baltic, the feed­ing migrations are more intensive and extend to quite large areas. From the Pomeranian coast and the Vistula region, sea trout migrate mainly eastwards, but parts of the stock also move westward along the Polish coast. The feeding migration of these stocks may reach the Gulf of Riga, the Gulf o f Finland, and the Bothnian Sea (Backiel and Bartel, 1963; Chrzan, 1963). The stocks along the Swedish south and east coasts migrate north­wards and to the Bothnian Sea, and the Gulf o f Finland, the Gulf of Riga, as well as southeasterly to the Gulf of Gdansk (Svärdson and Fagerström, 1982).

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Cod

There are two cod stocks in the Baltic which have been shown to differ in many ways. The Western Baltic stock (Gadus morhua morhua L.; the Atlantic cod or Tran­sition Area cod) is distributed west of Bornholm, in the Western Baltic, the Belt Seas, and the Sound (Sub­divisions 22-24) and has regular connections to the Kattegat (Division Ilia) and to the southeastern Baltic (Bagge et a i , 1994). The Eastern Baltic stock (Gadus morhua callarias L.; the Baltic Sea cod) is distributed east o f Bornholm to the northern parts o f the Bothnian Sea and to the eastern parts of the Gulf of Finland (Bagge et al., 1994). The border between these two stocks is diffuse, and mixing is evident in the Arkona and Bornholm Basins.

Western Baltic cod migrate in all directions after spawning (Berner, 1981) from the deep waters to more shallow coastal areas to feed. The young, immature age groups usually remain in the coastal areas before joining the mature stock. From the Arkona region, feeding migrations o f adults may reach the Belt Seas in the west and extend eastwards to the Slupsk Furrow, the Bornholm region, the Gdansk Deep, and even to the southern Gotland Deep (Lamp andTiews, 1974; Berner,1981 ). Cod in the Arkona Basin are always a mixture of the two main stocks, and the spawning migration occurs in two directions. According to transplantation experi­ments, the homing of cod is not well developed (Bagge,1983).

Eastern cod migrate to the Bornholm Basin spawning grounds in December-February from the feeding grounds in Slupsk Furrow, Gdansk Bay, Hanö Bay, and the Gotland Deep (Netzel, 1968, 1974). The spawners in the Gdansk Deep originate from areas south o f the Bornholm Basin and from southern parts o f the Gotland Deep (Netzel, 1974; Otterlind, 1976). There is also a spawning migration from the Swedish east coast, the Åland Sea, the southern Bothnian Sea, and the Gulf of Finland to the Bornholm Basin, the Gdansk Basin, and the southern parts o f the Gotland Basin in December- March (Otterlind, 1976; Sjöblom et al., 1980; Aro and Sjöblom, 1983a). The homing behaviour o f cod in the Southeastern and Northern Baltic is also unclear, and cod may use different spawning grounds in successive years (Bagge, 1983; Otterlind, 1984, 1985). During the feeding period, cod are distributed over large areas and may undertake extensive migrations (Otterlind, 1985; Aro 1989) that exhibit no pattern.

Flounder

There are several rather distinct flounder stocks or pop­ulations in the Baltic (ICES, 1978). Flounder are regu­larly distributed in all parts o f the Baltic, except Both­nian Bay, the easternmost part of the Gulf of Finland, and the deepest areas o f the Gotland Deep (Anon.,

1978). There are at least three stocks in the Southwest­ern and Southeastern Baltic (Subdivisions 22-26), three in the Central and Northeastern Baltic (Subdivisions 27-28), one each in the Åland Sea, Archipelago Sea, and southern Bothnian Sea, and two in the Gulf of Finland (Anon., 1978). Migrations between the mature flounder stocks in the southern and central Baltic are quite rare, extending from the southern part o f Öland Island to the Rosewie on the Polish coast (Otterlind, 1965, 1967; Vitinsh, 1976, 1977). Migrations are effec­tively blocked in the Southeastern and Central Baltic by the Gdansk and Gotland Deeps and in the Northern Baltic by the Gotland Deep and the deep southwest of the Åland Islands (Otterltnd, 1965; Vitinsh 1972, 1976, 1977; Aro and Sjöblom, 1983b). The annual migration patterns of flounder stocks are quite well known and, because of their general locality, the homing behaviour o f flounder is obvious.

Plaice

Plaice are distributed in the Baltic Sea from the Belt eastwards to the Gdansk Bay area and northwards to the southern Gotland Deep (Bagge, 1981). Plaice are very rare in the Northern Baltic. Feeding migrations from the deeper spawning grounds to shallower waters are pri­marily west and to a lesser extent east from the Arkona Basin. The westward feeding migration may reach the Belt Sea, the Sound, the southern Kattegat, and even the Skagerrak (Otterlind, 1967). Those emigrating out of the Baltic Sea are assumed not to return (Otterlind,1967). The eastward migration is most intensive from the Arkona Region to the east and southeast of Born­holm during November-February. Plaice feeding in the Gdansk Deep have been shown to migrate to the Born­holm Basin to spawn (Cieglewicz, 1961).

Freshwater species

In the Southern Baltic where the freshwater species are almost totally absent, shallow coastal bottoms and pelagic areas serve as habitat for the young stages of several marine pelagic and demersal species o f econom­ic importance. These areas also serve as habitat for a number o f other littoral and coastal species o f substan­tial ecological but minimal economic importance. In the Central and Northern Baltic Sea, there is a clear domi­nance of freshwater species in the littoral and coastal areas (Neuman, 1982), but their abundance decreases towards the seaward limit o f their distribution (Lehtonen and Toivonen, 1981). The number o f freshwater species is greatest in the archipelagoes, bays, inlets, and river mouths. The migrations o f freshwater species are gener­ally local, although some species long distances. Some o f the freshwater species lack a migration pattern or their migrations are too short to identify. Two whitefish

Fish migration studies in the Baltic Sea - a historical review 367

species, river-spawning whitefish (Coregonus lavaretus L.) and sea-spawning whitefish (Coregonus widegreni Malmgren), have different migration patterns. They are distributed along the coastal areas in the Baltic and are rare in the Belt Seas and along the south coast of Sweden (Svärdson, 1979), but are more abundant in the Gotland Island area, along the west coast of Estonia, and in the Gulf o f Finland, and the Gulf of Bothnia (Lehtonen, 1981). The feeding migration patterns vary between species and populations. Sea-spawning white- fish do not migrate very much and feed near the spawn­ing grounds (Lehtonen, 1981).

Pike exhibit a local distribution pattern and territorial behaviour (Ekman, 1916; Segerstråle, 1951; Halme, 1957; Halme and Korhonen, 1960; Strandman, 1964; Lehtonen, 1973). Local migrations or movements dur­ing feeding are very limited since the pike is an ambush hunter or stalker (Neill and Cullen, 1974). The home range of an individual pike is usually several square kilometres and when displaced it attempts to return to its original home range (Halme and Korhonen, 1960).

Pike-perch spawn in inlets and shallow bays in April-M ay (Lehtonen and Toivonen, 1981). After spawning, they remain on the spawning grounds and feed (Lehtonen and Toivonen, 1987) or migrate inside the archipelagoes, to the open sea, or along the coasts (Henking, 1923). Annual migrations take place between spawning inlets or bays, feeding areas in the archipela­goes, and wintering areas in deeper waters (Toivonen,1968). The average dispersal area is smaller when the coast is open and larger when the archipelago is rich (Toivonen, 1968; Lehtonen and Toivonen, 1987). The mixing o f local neighbouring stocks occurs in the win­tering areas, but pike-perch exhibit a clear homing behaviour to their former spawning grounds (Lehtonen,1985). In the Southern Baltic, the feeding migration is directed to the open sea, and pike-perch overwinter in shallow inlets (Henking, 1923; Filuk, 1962).

The perch is one o f the most common freshwater species in the Baltic archipelagoes. Perch form more or less separate local but non-discrete stocks along the Baltic coasts (Böhling and Lehtonen, 1985). Spawning occurs all along the coast, and the mixing o f neighbour­ing stocks during spawning is unlikely (Ekman, 1916). After spawning, feeding migrations, usually short (Johnson, 1978), but sometimes long (Henking 1923), take place to deeper areas along the coasts and inside the archipelagoes. The range o f feeding migrations is affect­ed by the distribution o f food resources, optimum tem­perature, and the abundance of neighbouring stocks, as well as the morphology o f the archipelago (Böhling and Lehtonen, 1985).

Conclusions

The fish fauna in the Baltic Sea may be classified into three different and intermediate communities: a pelagic

community, a benthic community, and a littoral and coastal community. The borders between them are not sharp, and individuals from neighbouring communities cross them frequently, particularly the littoral and coastal regions which serve the pelagic community as spawning and nursery areas. In the Gulf of Bothnia, the littoral and coastal community is dominated by fresh­water species, which very seldom migrate outside this environment, and the Baltic herring is actually the only native pelagic species using that environment as a spawning and nursery area (Andreasson and Petersson, 1982). The migration and movements o f the Baltic fish species occur at micro- and macroscales inside and be­tween these communities in annual, diurnal, horizontal, and vertical patterns.

The migration, spatial, and temporal distribution pat­tern of Baltic fish exhibits an annual cycle between spawning, feeding, and nursery habitats. Homing in spawning migrations and site fidelity has been ob­served, but results suggest that this link is weak in the case of cod, and the ratio o f emigration to other spawn­ing grounds seems to be dependent on the conditions of the spawning grounds. This also applies to spring- spawning Baltic herring and freshwater species. Baltic fish may use different spawning grounds in successive years, and evidence indicates that spawning migrations are more strongly linked to prevailing hydrographical conditions than to homing.

The feeding migration of Baltic fish seems to have different patterns for Baltic herring, sprat, salmon, and sea trout, but not in the case of cod. The movements of Baltic fish from one place to another during the feeding period seem to be once-a-year events. However, they may be just local seasonal movements, dispersals, or "true" migrations. The advantage of movements and migration in a patchy environment is, in general, that the impact of environmental variability on reproduction, survival, foraging, and growth decreases. The migratory behaviour promotes more flexibility in the face o f un­certainty. With migration, the risk from predation and cannibalism is balanced against the advantage o f re­maining in one place to exploit resources.

The freshwater species in the Baltic have unfortunate­ly been more or less neglected in ICES activities even though they are an important component o f the ecosys­tem. The number of original freshwater species in the Baltic is 23 (Lehtonen and Toivonen, 1981). The fresh­water species are practically absent in the southwestern part o f the Baltic except in some estuaries, fjords, and bays (Henking, 1923; Hempel and Nellen, 1974). There are 18 species in the Central Baltic, and freshwater species dominate in the north and northeastern parts of the Baltic (Lehtonen and Toivonen, 1981).

The general problem in quantifying movements and migrations (feeding or spawning) on a robust and usable spatial and temporal scale is that the tagging data rely totally on commercial fishermen’s catch and effort and their willingness to return tags. Tagging of Baltic fish

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was very popular from the 1950s to the early 1980s. Most o f these tagging studies and experiments were planned to estimate movements and fishing mortality. The results o f these studies have only been partly pub­lished in the scientific literature.

The analytical tools for mark-release experiments were developed decades ago and are well described (Seber, 1973; Burnham e t a l., 1987). However, data analysis is hampered by a lack o f essential information on fishing effort, spatial and temporal distribution of fishing activities, systematic determination of tag return rates from the commercial fishery, and so on. Many tagging experiments have neglected these issues by al­locating resources mainly to catching and tagging of fish instead o f to the recovery and reporting rates o f tags.

Before the establishment of the IBSFC in the early 1970s, there were national fishing zones and a huge international fishing zone in the Baltic Sea where fish­ing activities were largely unregulated. Consequently, detailed information on fishing and fisheries was very scarce, and the systematic collection o f fisheries data did not exist on a large scale. The situation has changed considerably since the beginning o f the 1970s, and the parameters necessary for the design and analysis of tag­ging experiments are now accessible. However, Baltic fish tagging experiments are not very appealing because conducting an experiment takes at least 6-8 years before final results are available. Despite these shortcomings, new tagging experiments on Baltic cod and herring should be encouraged.

References

Andreasson, S., and Petersson, B. 1982. The fish fauna o f the Gulf o f Bothnia. In Coastal Research in the Gulf o f Bothnia, pp. 301-315. Ed. by K. Müller. Dr W. Junk Publishers, The Hague. 462 pp.

Anonymous. 1995. Proceedings of the twenty first session. International Baltic Sea Fishery Commission. Warsaw, 4-8 September 1995. IBSFC, Warsaw.

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