Rapp. P.-v. Réun. Cons. int. Explor. Mer, 190: 72-96. 1989

A review of fish migration patterns in the Baltic

Eero Aro

Aro, Eero. 1989. A review of fish migration patterns in the Baltic. - Rapp. P.-v. Réun. Cons. int. Explor. Mer, 190: 72-96.

The fish species in the Baltic have several different migration patterns, which are mainly dependent on the adaptability of the fish to changing environmental con­ ditions. A common pattern among all fish species in the Baltic is the annual micro- and macroscale movements and migrations between spawning, feeding, and wintering areas, and in some cases with the same areas serving several different purposes. The euryhaline and eurythermie species are usually able to migrate more intensively than stenothermic and stenohaline species. Marine species have both local and migratory stocks. The freshwater species are generally more stationary than the marine species. The advantage of migrations in a patchy environment is that the impact of environ­ mental variability on reproduction, survival, foraging, and growth decreases. The migratory behaviour promotes flexibility in the face of uncertainty. With migration, the risk from predation is balanced against the advantage of remaining in one place to exploit resources. The migratory behaviour and reproductive requirements of fish in the Baltic Sea are tightly linked. In the littoral and coastal fish communities migrations are fairly short, along the coast and from the coast to deeper waters inside the archipelagos and estuaries. Migration to the open sea does not usually occur. The station-keeping and re-distributive behaviour is preferred to migratory behaviour. Among pelagic and benthic fish species migrations can be extensive.

Eero Aro: Finnish Game and Fisheries Research Institute, Fisheries Division, P. O. Box 202, SF-00151 Helsinki, Finland.

1. Introduction and in the Bothnian Bay (Subdivision 31) it is 10 ( Andreasson and Petersson, 1982). The number of origi­ If the environmental needs of fish species were constant nal freshwater species in the Baltic is 23 (Lehtonen through time there would not be any reason for them and Toivonen, 1981). The freshwater species in the to move from place to place. The distribution of fish southwestern parts of the Baltic are practically absent, through their life history may be random or patterned except in some estuaries, fjords, and bays (Hempel and and the environmental needs of specimens may vary Nellen, 1974). In the central parts of the Baltic there from day to night, summer to winter, and from their are 18 species and in the north and northeastern parts time of hatching to adulthood. The spatial and temporal freshwater species dominate (Lehtonen and Toivonen, distributions of many fish species change in fairly regular 1981). The division in the Skagerrak and Kattegat and cyclical patterns. The adaptive value of fish and the subdivisions in the Baltic Sea are shown in migration is strongly coupled with optimization of the Figure 1. surrounding environment. The reduction of the impact The hydrographic conditions in the Baltic Sea form of environmental variance for example on reproductive an uneven continuity from southwest to northeast, so success, competition for food, growth and survival are the physiological selection of fish species, their distri­ the benefits of migration. bution and capability to migrate have taken place in In the Baltic Sea the fish species are of marine or varying environmental temperatures and salinities. The freshwater ancestry. When the number of marine difference between summer and winter surface tem­ species in the North Sea is 120, in the Western Baltic perature in the Baltic is approximately 14°C, which is Sea (Subdivisions 22-24) area it is 69, in the Southern two to three times that in the eastern Atlantic Ocean and Central Baltic Proper (Subdivisions 25-28) it is 41, (Defant, 1974). The high or low salinity regulates fish in the Åland Sea, Gulf of Finland, and Gulf of Bothnia species distribution area via osmoregulatory mech­ (Subdivisions 29-30 and 32) it is 20 (Remane, 1958), anisms (Oikari, 1978). The salinity in the open Baltic

72 2 0 * 26' 28* 30* 66'

60

56'

Figure 1. Division Ilia in the Skagerrak and Kattegat and Subdivisions 22-32 in the Baltic Sea.

Sea is locally fairly constant, especially compared with although the number of autumn-spawning species areas such as estuaries. The success with which species increases from south to north. The fish fauna in the can extend their range of distribution towards a salinity Baltic Sea may be classified within three different com­ extreme depends on their tolerance of altered internal munities, all of which are more or less intermediate: a conditions as well as on structural and functional modi­ pelagic community, a benthic community, and a littoral fications. Marine and freshwater species of the Baltic and coastal community. The borders between them are Sea are able to live in the brackish water either because not sharp and they are used frequently by specimens they can maintain the osmotic and ionic balance of the from neighbouring communities. The littoral and body or because they can tolerate changes in body fluids coastal communities, in particular, serve the pelagic (Prosser, 1973). The absence of species in such habitats, community as a spawning and nursery area. In the which are within their range of osmotic tolerance, is Gulf of Bothnia the littoral and coastal community is obviously caused by other ecological and behavioural dominated by freshwater species which very seldom factors. Locally rapid or long-term changes in environ­ migrate outside this environment; the Baltic herring mental conditions are the most important abiotic factors is actually the only native pelagic species using this in the regulation of migrations and movements. In the environment as a spawning and nursery area (Andreas- life history of fish species in the Baltic Sea the eury­ son and Petersson, 1982). The migration and move­ thermie and euryhaline species have had more adaptive ments of the Baltic Sea fish species occur in micro- value to those species which prefer fairly constant sal­ and macroscale inside and between these communities inities and temperatures. Also the number of spring- having annual and diurnal horizontal and vertical pat­ spawning species dominates in the whole Baltic, terns.

73 The separation of fish stocks in the Baltic Sea is not to the feeding areas. There are three feeding migration yet finally determined. Some experts tend to divide patterns: along the coastal waters of FRG, GDR, and populations into several sub-stocks while others tend to Poland to west and east, to the Bornholm Basin open unite them. Both expert groups seem to have evidence sea areas, and out from the Baltic through the Öresund to support their respective theories. and the Belts to the Kattegat and Skagerrak. Those older more migratory specimens, which spawned first, 2. The pelagic community migrate mainly out from the Baltic to the Kattegat and Skagerrak, where they mix during feeding with the three 2.1. The Baltic Sea herring Kattegat and Skagerrak stocks (Ackefors, 1978; Anon., 1983a; Sparholt, 1987). The migration pattern of adult The most abundant species in the pelagial area is Baltic herring out from the Baltic to the Kattegat and Ska­ herring, which consists of very abundant spring-spawn­ gerrak has also been confirmed by the occurrence of ing stocks and nowadays less abundant autumn-spawn­ Anisakis nematode larvae in the adult herring from the ing stocks (Ojaveer, 1981). The spring-spawning stocks Southwestern Baltic (Friess, 1977; Kühlmorgen-Hille, or populations are divided in some areas (Subdivisions 1983) and also in the Gdansk Bay (Strzyzewska and 25-27) into two components by their different mor­ Popiel, 1974). Some of the older herring, however, phological, meristic, and biological characters: the migrate to the south of Bornholm, to the Oder Bank coastal herring and the open sea herring (Popiel, 1958, off the coast of Poland, and to the Hanö Bay open sea 1964, 1984; Ojaveer and Simm, 1975; Ojaveer, 1981; areas near the Swedish coast (Biester, 1979a), where Otterlind, 1985b). Sometimes the same procedure is they feed and winter together with the specimens from applied to the autumn-spawning stocks, too (Ojaveer, other stocks. The young age groups are more local and 1981). The spring-spawning coastal herring, the spring- coastbound than the older specimens, migrating closer spawning open sea herring, and the autumn-spawning to the coastline to the west and south along the coast. herring have different migration patterns. Later in the autumn the young age groups leave the 2.1.1. Herring in the Southwestern Baltic, the coastal areas heading to the open sea to join the adult Kattegat, and Skagerrak (Subdivisions 22-24 and stock (Weber, 1970). There have been some obser­ Division Ilia) vations of the same migratory behaviour of younger The herring stocks which inhabit Division Ilia (Kattegat specimens, too (Jönsson and Biester, 1979). The herring and Skagerrak), Subdivisions 22 and 24 (the Belt Sea in the Southwestern Baltic seem to have a clear homing and the Western Baltic), and Subdivision 23 (the ability (Jönsson and Biester, 1979) despite the contro­ Sound), have both spring-spawning and autumn-spawn­ versy on this point. The summary of the migration ing components (Anwand, 1963b). The spring-spawning pattern is presented in Figure 2a and b. stocks have a very clear migration pattern. The migratory behaviour of the small autumn-spawn­ The main spawning areas of the spring-spawning ing stocks is not so well known. Their spawning areas stocks are in the coastal waters from the Belts in the are located in the coastal areas of Mecklenburg Bay, in west to the east region of Rügen in the Arkona in the the banks of the Arkona Basin, and at the coast of east. The main spawning grounds are the coastal waters Bornholm in deeper waters than the spring spawners of the Southwestern Baltic (Jensen, 1955; Weber, 1974, (Rechlin, 1967a). The spawning migration occurs during 1975), Greifswalder Bodden and the coastal areas of the summer and early autumn. They spawn in August- Rügen (Anwand, 1961, 1963a; Biester et al., 1975, November and then migrate to feed, especially to the 1976a, b; Krüger et al., 1976; Jönsson and Biester, 1977; areas around Bornholm, where they are caught together Biester et al., 1977; Biester, 1979a, b, c; Jönsson and with spring spawners from other stocks (Popiel, 1984). Biester, 1979; Biester et al., 1979; Jönsson and Biester, The autumn-spawning stock in the Western Baltic prob­ 1981a, b; Otterlind, 1985b), in Pomorska Bay and in ably consists of three separate units, which have not the Firth of Szczecin (Popiel, 1984). been confirmed so far. Sometimes the autumn spawners The majority of the specimens migrate from the Ska­ from the Bornholm Basin may migrate to Gdansk Bay gerrak and Kattegat feeding and wintering areas and the Klaipeda region (Popiel and Strzyzewska, through the Öresund and the Belts into the Baltic in the 1971). late autumn and early winter and the minority from the other feeding areas around Bornholm, off the Polish 2.1.2. Herring in the Southeastern and Central Baltic coast and Hanö Bay to spawning grounds (Biester, (Subdivisions 25-27) 1979a; Otterlind, 1985b). They all enter the coastal In these subdivisions there are considered to be three spawning areas in March-May, the older age groups different stocks: the spring-spawning coastal herring, first (Krüger et al., 1976) and the younger age groups the spring-spawning open sea herring, and the autumn- and new spawners later (Biester and Hering, 1977; spawning herring (Popiel, 1964; Kompowski, 1969; Groth, 1985). Soon after spawning in late spring and Popiel and Strzyzewska, 1971; Popiel, 1984; Grygiel, summer they leave the spawning grounds and migrate 1984; Anon., 1986; Grygiel, 1987). They have been

74 Feeding algratloo

20* 24* 2b’

Feeding ■ lg t■tIon

Figure 2. The migration of spring-spawning herring in the Southwestern Baltic, the Kattegat, and Skagerrak (Subdivisions 22- 24 and Division Ilia) (a, b) and spring-spawning coastal herring in the Southeastern Baltic (Subdivisions 25-27) (c, d).

75 separated in accordance with morphological and mer- ing with the spring-spawning coastal stock (Kompowski, istic differences, differences in the otolith structures, 1969). The main part of the spawning stock, however, and growth rates. In some cases the distinctions in returns to its former spawning grounds. The summary characters have proved to be of little value for the of the migration pattern is presented in Figure 3a and b. separation (Biester, 1979c; Anon., 1983a), but in this The autumn-spawning coastal herring is in the feeding case there does seem to be enough evidence for sep­ grounds mainly in March-June. The feeding grounds aration, although some controversy remains (Anon., are situated in the open sea in the Bornholm Basin, 1986). There are three different migration patterns in Slupsk Trench and in the Klaipeda region (Popiel, the Southeastern and Central Baltic. 1984), and in the Gdansk Basin (Kompowski, 1969). The fast-growing, short-lived coastal spring-spawning These areas are usually occupied by older specimens. herring stocks spawn in the coastal regions of Poland The spawning migration takes place in late June and from the area east of Rügen, in Pomorska Bay, in July to the coastal spawning grounds and to the open Gdansk Bay, and Vistula Lagoon (Popiel, 1964; Anon., sea banks. They spawn in August-November and in the 1987a). The spawning starts in March in the western outer coastal area in deeper waters than the spring areas and later in April in Gdansk Bay, when the adult spawners (Popiel, 1984). The feeding migration back to specimens migrate to the coast from the feeding grounds the feeding grounds occurs soon after spawning and (Popiel, 1964, 1984). The main feeding grounds are they reach the feeding grounds in October-December. around Bornholm and in the Gdansk Basin; sometimes they are found in the Arkona region according to multi­ modal length frequencies (Popiel and Strzyzewska, 2.1.3. Herring in the Northern Baltic Sea Proper, the 1971). The feeding migration of adults occurs soon after Archipelago Sea, and the Gulf of Riga (Subdivisions spawning and they are in feeding areas usually from 28 and 29) July to December, when the spring-spawning coastal In the Northern Baltic Sea Proper, in the Archipelago herring otolith types are dominant in catches in the Sea, and in the Gulf of Riga there are both spring- Bornholm region and in the Gdansk Basin (Popiel, spawning stocks and autumn-spawning stocks (Sjöblom, 1958; Rauck, 1965; Kompowski, 1969; Grygiel, 1984, 1961; Ojaveer and Rannak, 1980). In the Gulf of Riga 1987). The younger age groups (0- and 1-group) feed in the spring-spawning and autumn-spawning stocks are the areas between the open sea and the littoral zone separated into coastal and open sea stocks (Ojaveer, (Popiel, 1984) and only a small number of one-year-old 1981). In the Ventspils-Saaremaa and in the region of specimens join the adult population in the open sea. Hiumaa, and also off the eastern coast of Gotland, there The summary of the migrations is presented in Figure are considered to be local stocks by the differences in 2c and d. their otolith structure, year-class abundance, growth The slow-growing, long-living open sea herring pattern and fluctuation of fishing localities, catches and mainly spawn along the Swedish east coast from Hanö catch composition (Ojaveer and Rannak, 1980). In the Bay up to the Åland Sea (Otterlind, 1957,1961a, 1962b, Archipelago Sea and the Åland Sea there are both 1976; Anon., 1987a). These spring spawners spawn in spring-spawning coastal herring and autumn-spawning April-June; in the northern areas spawning starts later, herring (Sjöblom, 1961). The autumn-spawning stock usually in May and lasts until the beginning of July. is at a very low level (Sjöblom, 1978). They spawn in deeper water than the coastal spring- The annual migration of the various stocks in the Gulf spawning herring in other locations (Hessle, 1925). A of Riga, Ventspils-Saaremaa, Hiumaa and in the east very large proportion of the older herring migrate very coast of Gotland follow the general seasonal pattern soon after spawning to feeding grounds in the Bornholm observed in the neighbouring stocks and populations. Basin, sometimes to the regions south of the Scania and There may be migrations out of the Northeastern Baltic sometimes to the Gdansk Basin and offshore regions of to the south with different intensities from year to Klaipeda (Otterlind, 1976). They are in the feeding year depending on feeding conditions in these areas areas usually in July-October, but in warm summers (Ojaveer, 1974). Mixing of the stocks has been they may stay there until December (Popiel, 1984). observed, which indicates different migration patterns. The smaller part of the population migrates along the Because of a lack of taggings, the migration patterns Swedish coast and off the coast to the south and north, have not been very well documented. but the migration to the Bothnian Sea is quite insig­ The spring-spawning coastal herring in the Archi­ nificant (Otterlind, 1976). During the late autumn and pelago Sea and in the Åland Sea has a clear annual early winter the spawning migration starts from the migration pattern and homing behaviour (Sjöblom, feeding grounds back to the spawning grounds, where 1961; Parmanne and Sjöblom, 1982, 1986). The spawn­ they are to be found in November and later (Otterlind, ing migration of the adults from the feeding areas occurs 1976). There is also evidence that some of those herring during winter and early spring. The feeding areas of do not return to the Swedish east coast, because they adults are located in the outer parts of the archipelago have been found in the Southern Baltic in spring, spawn­ near to the open sea and in the open sea areas. The

76 JO* 2s“

gr*gn4a

Figure 3. The migration of spring-spawning open-sea herring in the Southeastern and Central Baltic (Subdivisions 25-27) (a, b), and spring-spawning herring in the Northern Baltic Proper, the Archipelago Sea, and the Gulf of Riga (Subdivisions 28—29) (c, d). 77 older specimens migrate more than the younger age western side of the Bothnian Sea seem to be more groups (Sjöblom, 1961). During autumn and early bound to the coast than the Southern Baltic stocks winter, the younger age groups from the neighbouring (Otterlind, 1976). The feeding migration of the speci­ stocks also gather inside the archipelago near to the mens occurs mainly along the coast to the south and coast (from the Bothnian Sea and the western Gulf of north and there seem to be some connections to the Finland) and they undertake both local migrations and Åland Sea and to the Quark (Otterlind, 1957, 1962b), from outer archipelago to the inner and vice versa which are transition areas for the southern and northern (Sjöblom, 1961). The spawning starts in the middle of stocks. The proportion of eastward migration to the May, first in shallow waters in the inner archipelago Finnish coast is very low. bays, inlets and river mouths, shifting gradually to The spring-spawning coastal herring on the Finnish deeper waters with a rising water temperature (Sjöblom, coast also spawn in May-July along the whole coastline 1961). The spawning is usually over by the middle of from the northern side of the Archipelago Sea up to the July. The immature specimens stay inside the archi­ Quark. The migration of the adult population to the pelago and the younger age groups migrate outside the spawning grounds from the feeding and wintering areas, archipelago for the summer, preferring the waters just which are in the open sea and partly in the Archipelago north of Åland and returning back inside the archi­ Sea and in the Quark Archipelago, occurs during the pelago during autumn and early winter (Sjöblom, 1961 ). winter and in late winter, when the first spawning shoals The feeding migration of older age groups may extend are quite near to the coast. Some of the younger mature to central parts of the Baltic, to the Swedish east coast age groups winter nearer to the coastline and in the and sometimes even to the Southern Baltic (Parmanne Archipelago Sea (Sjöblom, 1961). After spawning, the and Sjöblom, 1982,1986). The older age groups usually feeding migration is quite rapid to the open sea near to stay away from the archipelago during winter and return the slopes of the Bothnian Sea Basin and in the outer there in early spring (Sjöblom, 1961). Through the archipelago. They are usually in the feeding grounds migration from the Archipelago Sea there are con­ in July-December. The feeding migration extends to nections to the Bothnian Sea stocks, to the western southern parts of the Bothnian Sea and inside the Archi­ parts of the Gulf of Finland and to the Åland Sea and pelago Sea, in the Quark and sometimes inside the the Stockholm Archipelago (Parmanne and Sjöblom, Bothnian Bay, too. There is also some exchange 1986). The summary of the migration patterns of these between the Finnish and Swedish coasts (Parmanne stocks is presented in Figure 3c and d. and Sjöblom, 1982, 1986). The spring-spawning coastal herring in the east coast of the Bothnian Sea has a clear 2.1.4. Herring in the Bothnian Sea (Subdivision 30) homing behaviour and about 95% of recaptures have In the Bothnian Sea (Subdivision 30) there are two been obtained inside 150 km from the tagging place spring-spawning coastal herring populations (Hannerz, (Parmanne and Sjöblom, 1986). 1955, 1956; Otterlind, 1957, 1962b, 1976; Sjöblom, 1961; Parmanne and Sjöblom, 1982, 1986), one along 2.1.5. Herring in the Bothnian Bay (Subdivision 31) the west coast and one on the east coast. The autumn- The annual migration pattern of the coastal spring spawning stock is very sparse (Sjöblom, 1978). Sep­ spawning herring in the Bothnian Bay follows the gen­ aration into the coastal and open sea components does eral pattern observed in other areas. The autumn- not apply in these populations, although different spawning stock is at a very low level (Sjöblom, 1966). spawning groups have been presented (Ehnholm, 1951). Spawning of the spring-spawning herring starts in late The migration pattern on both sides of the Bothnian May and is over by late July. The feeding migration Sea is the same, one a mirror image of the other (Fig. occurs mainly inside the Bothnian Bay and there is a 4a and b). certain exchange between the Swedish and Finnish The spring-spawning coastal herring on the Swedish coasts, especially near the Quark (Otterlind, 1962b; coast spawn in May-July in the coastal areas, starting Parmanne and Sjöblom, 1986). The migration of in the shallowest areas and shifting with a rising water Bothnian Bay stocks is presented in Figure 4c and d. temperature to deeper waters (Neuman, 1982). Spawn­ ing occurs along the whole coastline, although there are 2.1.6. Herring in the Gulf of Finland (Subdivision 32) some areas which are more preferred. The migration of There are both spring-spawning and autumn-spawning the adult population near to the spawning grounds takes varieties of Baltic herring in the Gulf of Finland. The place in late autumn and early winter mainly from the autumn-spawning stock is, as in general in the northern open sea areas. The new spawners, like the immatures, areas, at a very low level (Sjöblom, 1978; Ojaveer et winter inside the archipelagos and near to the coast, al., 1985). The coastal spring-spawning herring spawns and join the spawning stock later during the spawning in May-July, occurring along the whole coastline on time. The feeding migration starts soon after spawning. both sides of the Gulf of Finland (Parmanne and Sjö­ The main feeding areas are the slopes of the Bothnian blom, 1984, 1985; Raid, 1985). Spawning begins in the Sea Basin and the outer archipelago. The herring in the shallow littoral zone when the first spawning shoals are

78 Spawalag «lgratloi

iägviftl

20*

- □

Figure 4. The migration of spring-spawning herring in the Bothnian Sea (Subdivision 30) (a, b) and spring-spawning herring in the Bothnian Bay (Subdivision 31) and in the Gulf of Finland (Subdivision 32) (c, d).

79 arriving at the spawning grounds. Later on, during the divisions 22-25), one the Gdansk Bay and the Gotland course of spawning, the new shoals stop further away Deep area (Subdivisions 26 and 28) and one the North­ from the coast in the outer archipelago with the rising western and Northern Baltic and the Gulf of Finland temperature (Sjöblom, 1961). The herring seems to (Subdivisions 27, 29, 30, and 32) (Anon., 1987a). The spawn in two groups: the first in May, the second a boundaries between the neighbouring stocks are not month later (Sjöblom, 1961). They migrate to feeding very clear and the mixing of stocks during feeding and areas after spawning in July and August. Feeding areas wintering is apparent (Rechlin, 1967b, 1975, 1986). are in the open sea deep water areas (Sjöblom, 1961). The main spawning grounds of the sprat stocks in the Herring stays in the feeding grounds until November- Baltic are in Kiel Bay, Mecklenburg Bay, Arkona Basin, December. The younger age groups remain nearer to Bornholm Basin, Gdansk Deep, Gotland Deep, and the coast, while shoals consisting of old specimens generally all the coastal slopes of the deeps and also remain far out in deep water, to which they have gone the deeps themselves (Hessle, 1927; Grauman, 1975; after spawning. Elwertowski, 1976; Krenkel and Hinrichs, 1979; Grau­ During the feeding period herring migrate both east man and Krenkel, 1986; Alheit, 1986). The coastal and west, depending on their location. In the western areas, fjords, and archipelagos are not so often used parts of the Gulf of Finland some herring are observed for spawning (Hessle, 1927; Veldre, 1974, 1986). The to migrate to the Archipelago Sea (Parmanne and Sjö­ spawning time is very prolonged and starts in the West­ blom, 1986), but the migration out from the Gulf of ern Baltic usually in March-April and lasts until July- Finland seems generally to be insignificant (Parmanne August in the Northern Baltic Proper. Spawning time and Sjöblom, 1986). From the eastern part of the Gulf in the Southern Baltic is longer than in the Northern of Finland on the north coast, the dispersion during Baltic (Grauman, 1975). Spawning starts near the bot­ feeding time extends westwards as far as the mouth of tom in spring and a shifting to pelagial spawning in the the Gulf of Finland (Parmanne and Sjöblom, 1982). No surface layers above the deeps occurs during summer mixing between the north and south coasts of the Gulf (Grauman, 1975; Elwertowski, 1976). The sprat is a of Finland has been observed so far and the fish seem serial spawner and it can spawn several batches per to spawn in the same area in successive years (Parmanne spawning season (Heidrich, 1925; Alheit, 1986). and Sjöblom, 1982). The western part of the Gulf of During the feeding period, in July-November in the Finland on its south coast operates as a transition area South and Central parts of the Baltic (van Khan et al., to the Subdivision 28 stocks (Ojaveer and Rannak, 1972) and in August-December in the north, sprat 1980; Ojaveer et al., 1985). The migration patterns of migrate from the more coastal areas to the offshore the Gulf of Finland stocks are presented in Figure 4c parts of the Baltic Basins forming feeding shoals which and d. contain mainly older age groups (Lindquist, 1971). The younger age groups stay nearer to the coast and inside the archipelagos. The migration of the Western Baltic 2.2. The Baltic Sea sprat stock is directed to the Bornholm Basin and sometimes The Baltic Sea sprat inhabits the Baltic Sea from the to the Gdansk Basin, where these stocks mix when Belt Seas and Western Baltic (Subdivisions 22 and 24) feeding. From the Gdansk Bay there is a northward up to the Quark area in the north (Subdivision 30) migration pattern to the Gotland Deep central parts and to the northeastern part of the Gulf of Finland even although the main part of the stock feeds and (Subdivision 32) (Hessle, 1927; Anon., 1974; Ojaveer winters in the vicinity of Gdansk Bay (Elwertowski, etal., 1985; Shvetsov er a/., 1986; Veldre, 1986; Rechlin, 1964). Very dense shoals and concentrations of sprat 1986). There are three different sprat stocks in the Baltic have been observed during the feeding and wintering and the mixing with the Kattegat and Skagerrak stocks period in the Gotland Deep area (Rechlin, 1967b, 1975; is considered to be very low, although there is no Rechlin and Groth, 1979) and sometimes even in spring significant difference in morphometric characters and (Shvetsov et al., 1986). The range and amplitude of the in the vertebrae counts (Lindquist, 1968). The mixing migration patterns of sprat stocks and their mixing in is probably prevented by the gradient and differences the Baltic are not yet very clear. Some observations in many abiotic factors between the Western Baltic and indicate more locality (Aps and Lotman, 1984; Aps et the Kattegat (Hessle, 1927; Aps et al., 1987). Identi­ al., 1987), but some on the contrary indicate more fication of the sprat stocks in the Baltic has been carried migratory behaviour and mixing of stocks (Grauman, out using the differences in otolith structure, meristic 1976; Khoziosky et al., 1983). The summary of sprat and morphometrical characters, growth patterns, and migration is presented in Figure 5a and b. also using the hydrological conditions in the Baltic Basins (Anon., 1970, 1974; Aps, 1977; Ojaveer, 1981; 2.3. The Baltic Sea salmon Aps and Lotman, 1984; Rechlin, 1986; Aps etal., 1981, 1987). One of the stocks inhabits the Belt Seas, the The Baltic Sea salmon is distributed from the Belt Seas Western Baltic, and the region of Bornholm (Sub­ and the Western Baltic up to the northern parts of the

80 Gulf of Bothnia and the Gulf of Finland, and it very north again along the main current (Lindroth et al., seldom migrates outside the Baltic (Anon., 1980a; Lind- 1982). In the Bothnian Sea (Subdivision 30) stocking, roth et al., 1982). The salmon has naturally spawning the post-smolts behave like the northern stocks except stocks and artificially reared stocks of various origin. the River Neva stock (from USSR origin in the eastern There are now about twenty rivers supporting the nat­ Gulf of Finland, Subdivision 32) releases in the ural spawning of salmon in the Baltic. Most of them are Bothnian Sea (Ikonen and Auvinen, 1984a). They seem situated in the Gulf of Bothnia (Subdivisions 30 and to have more of a tendency to locality. A proportion of 31), some in the Gulf of Finland, in the Gulf of Riga, post-smolts always stays in the Bothnian Bay and in the and in the Baltic Main Basin, but the share of natural Bothnian Sea, but it is usually low and may vary from spawning in the total production is only 10% (Ikonen, year to year and river to river. 1987). The hatchery-reared smolt production in the In the Gulf of Finland (Subdivision 32) after the Baltic Sea is about 4-3 million and the wild production adaptation period, some of the post-smolts migrate thus about 0-4 million (Anon., 1987b). During the mar­ mainly along the Finnish coast westwards with the main ine phase the Baltic Sea salmon has very clear migration current (Kazakov, 1985; Ikonen and Auvinen, 1985); patterns in the coastal and pelagial area. In general, the some of them enter the Baltic Main Basin, but in lower wild stocks and hatchery-reared stocks have the same amounts than in the case of the Bothnian Bay and migration patterns with some exceptions. The mi­ Bothnian Sea stocks. A proportion of the post-smolts grations are divided into post-smolt, feeding, and migrates to the eastern parts of the Gulf of Finland spawning migrations. (Ikonen et al., 1987). This is caused by using the River Neva salmon for stocking. The release of smolts of 2.3.1. The post-smolt migration Bothnian Bay stock introduced into the Gulf of Finland The smolts enter the Baltic usually in April-June. In has resulted in the usual migration pattern to the the Southern Baltic the Smolts run earlier and in the Southern Baltic (Toivonen, 1973). north a few weeks later. The wild and hatchery-reared In the Gulf of Riga the post-smolts migrate along the smolts start their post-smolt migration at about the coast northwards and enter the Northern Baltic Proper same time. During the first weeks post-smolts are quite in late summer. By September-October they have stationary, adapting to the new environment and keep­ dispersed into the Gulf of Finland, the Åland Sea, some ing close to the river mouth or to the releasing place into the Bothnian Sea, the Baltic Main Basin east of (Bartel, 1976; Mitans and Rims, 1980; Jokiel and Bartel, Gotland and even into the Bornholm region (Mitans 1983; Jutila and Alapassi, 1985; Ikonen and Auvinen, and Rims, 1980; Kazakov, 1985). 1985), and they feed mainly on insects on the surface In the Vistula and Drawa river releases (Subdivisions layer of the sea (Lindroth, 1961,1965; Christensen and 24 and 26), the behaviour of the post-smolts of wild Larsson, 1979; Jutila and Toivonen, 1985). (later extinct Vistula salmon) and hatchery-reared In the Bothnian Bay (Subdivision 31), from the stocks is equal to that observed in other areas during releases on the Finnish side, the post-smolts migrate the first weeks in the Baltic (Chrzan, 1964; Zarnecki, southwards along the Finnish coast to the Quark area, 1964a, b, 1966; Bartel, 1976; Jokiel and Bartel, 1983). shifting to the Swedish coast in the Quark. There seems In late autumn the post-smolts migrate mainly north­ not to be any difference between the wild and hatchery- wards to the Klaipeda and Gotland regions and to the reared specimens (Ikonen and Auvinen, 1985). From waters around Bornholm. the Swedish releases the post-smolts migrate on the Swedish coast in the Bothnian Bay down to the Quark, 2.3.2. The feeding migration some shifting to the Finnish side following the main Practically all Baltic salmon stocks feed in the Baltic current northwards along the Finnish coast and not Main Basin area (Subdivisions 24-29) except those of leaving the Bothnian Bay (Larsson and Atheskar, 1979). River Neva origin. From the releases in the Gulf of In the Bothnian Sea (Subdivision 30) the post-smolts Finland about 31% of the Neva origin specimens feed migrate mainly along the Swedish coast (Finnish in the Main Basin; from the Bothnian Sea releases this releases in the Bothnian Bay); the first post-smolts reach percentage is even less. The most important feeding the Baltic main basin in autumn (September-October) grounds are the Gotland Deep area, the waters around and the Southern Baltic in December-January (Ikonen Bornholm, and the Gdansk Basin (Järvi, 1938; Halme, and Auvinen, 1982a, 1984a, 1985; Eriksson, 1985; 1961, 1964, 1966; Carlin, 1959, 1962; Thurow, 1962, Ikonen, 1986). From the Swedish releases in the 1969, 1985; Christensen and Larsson, 1979; Larsson et Bothnian Bay some specimens migrate in the Bothnian al., 1979; Lindroth, 1981; Lindroth etal., 1982; Ikonen Sea on the Swedish coast to the South and some shift and Auvinen, 1982a, 1985). In these feeding grounds to the Finnish coast before entering the main basin the stocks mix effectively and no very clear evidence on feeding grounds; as in the Bothnian Bay, some post- the preferential feeding areas of diverse stocks has been smolts shift from the Swedish coast in the southern part shown, although there are some aggregations in the of the Bothnian Sea to the Finnish coast and migrate main fishing grounds (Carlin, 1959; Halme, 1964). Feed-

81 20*

- A

Feeding and wintering grandi Spawning grounds Spawning migration

2 0 *

□

W

Nala feeding Spawning Migration - greend a

feeding Blgretlon

Figure 5. The migration of the Baltic Sea sprat stocks (Subdivisions 22-30 and 32) (a, b) and salmon (Subdivisions 24-32) (c, d). ing salmon are fairly clustered and the changes in the stocks and the more stationary stocks (Zarnecki and density on the feeding grounds are their active move­ Duszynski, 1961; Zarnecki et al., 1962; Chrzan, 1963; ments in searching for food (Thurow, 1973). Backiel and Bartel, 1963; Svärdson and Anheden, 1963; Zarnecki, 1964b; Christensen and Johansson, 1975; 2.3.3. The spawning migration Toivonen and Tuhkunen, 1975; Toivonen and Ikonen, All Baltic Sea salmon stocks have a homing behaviour, 1978; Svärdson and Fagerström, 1982; Ikonen and Auvi­ but in some releases the homing has not developed nen, 1982b, 1984b; Anon., 1987b). In the Baltic Sea, because of the environment (Christensen, 1982). The sea trout have an intermediate position between the first stage in a spawning migration is the rapid return to pelagic community and the coastal community and the coastal areas near the home rivers. After about one migration pattern is dependent on the stock origin and year in the sea 10-20% of the age group A.1+ (grilse) the dimensions of the archipelago. start migrating towards the home river or the river where they were released. In the Baltic they are almost 2.4.1. The post-smolt migration exclusively males (Peterson, 1967). Fewer from the wild The wild post-smolts enter the Baltic Sea in May-June stock smolts are recaptured as grilse than from the (Svärdson. 1966; Kangur and Ling, 1985). The reared hatchery-reared ones (Toivonen, 1977). In the Gulf of smolts are released usually in May-June too, although Bothnia (Subdivisions 30 and 31) the first grilse from in some cases autumn releases have been used (Bartel, the northern stocks appear in the Bothnian Bay in 1977), with poorer results. During the first months in April-May (Ikonen, 1986), and in June the majority of the sea, the post-smolts spread slowly along the coasts the grilse have reached the Bothnian Bay coastal areas (Skrochowska, 1969; Toivonen and Tuhkunen, 1975; (Ikonen and Auvinen, 1984a). In August-September Svärdson and Fagerström, 1982; Ikonen and Auvinen, they have reached the area where they were released. 1984b). In the Bothnian Sea and Bothnian Bay (Sub­ However, the main part of these A.1+ specimens stay divisions 30 and 31) the post-smolt migration follows on the feeding grounds and start their spawning the coastline mainly northwards and northwestwards migration later at age A.2+ or A.3+. The older age according to the main current (Svärdson and Fager­ groups A.2+ and older enter the Gulf of Bothnia in ström, 1982; Ikonen and Auvinen, 1984b), but some­ April-June and the migration route seems to go through times also against the main current. In the Gulf of the Åland Sea; the migration continues along the Fin­ Finland the post-smolt migration is directed both west­ nish coast northwards to the home river or releasing wards and eastwards along and offshore the coast (Sor- place (Ikonen, 1986). There is not much difference in munen, 1975; Ikonen and Auvinen, 1984b; Ikonen et the migration pattern between the wild- and hatchery- al., 1987) and no clear trend of migration along or reared stocks (Ikonen, 1987), although the wild speci­ against the main current has been observed. In the mens seem to migrate earlier than the hatchery-reared Southern Baltic the post-smolts keep close to the river ones. The timing and the route of the spawning mouth or releasing place during the first months in the migration in the Gulf of Finland Neva stock is not very sea (Svärdson and Fagerström, 1982), or they have clear, because the feeding grounds are mainly in the migrated westwards and eastwards along the coast, as vicinity of spawning grounds (Ikonen and Auvinen, in the Pomeranian and Vistula releases (Zarnecki and 1984a). The homing is observed by the high percentage Duszynski, 1961; Zarnecki etal., 1962; Zarnecki, 1964b; of specimens near the releasing place in late autumn Skrochowska, 1969). (Ikonen et al., 1987). The amount of native wild salmon in the Gulf of Finland is negligible and practically all 2.4.2. The feeding migration are from hatchery origin. However, the coastal pound During the feeding migration the stocks in the Bothnian net catches contain about 15% salmon of wild origin. Sea and Bothnian Bay intermix by the migration from The origin of these is not known (Ikonen, 1987). The the eastern coast to the western coast and vice versa. spawning migration of salmon released in the mouth of The migration southwards from the Bothnian Bay to the Neva River seems to follow the southern coast of the Bothnian Sea and from the Bothnian Sea to the the Gulf of Finland (Kazakov, 1985). The summary of Main Basin has also been observed even although the the Baltic Sea salmon migration is presented in Figure northern stocks are considered to be more local and 5c and d. less migratory in nature (Carlin, 1965; Toivonen and Tuhkunen, 1975; Svärdson and Fagerström, 1982; Ikonen and Auvinen, 1982b). The migration northwards 2.4. The Baltic Sea sea trout from the Bothnian Sea to the Bothnian Bay is also The Baltic Sea sea trout is distributed from the south­ evident (Svärdson and Fagerström, 1982; Ikonen and western parts of the Baltic Sea up to the northern parts Auvinen, 1984b). In the Gulf of Finland the feeding of the Gulf of Bothnia and eastern parts of the Gulf of migration extends to the whole area from west to east Finland. The migration of Baltic Sea sea trout stocks and there seems to be some migration to the Archi­ may be divided into two patterns: the long-migrating pelago Sea, the Bothnian Sea, and the Main Basin

83 (Rannak et al., 1983; Ikonen and Auvinen, 1984b). In 2.4.3. The spawning migration the Northeastern Baltic near Saaremaa the stock seems The spawning migration of sea trout into waters near to be quite local, but in some cases the migration extends the home river or the releasing place occurs in February- up to the Bothnian Sea, the Åland Sea, near Gotland July, depending on the latitude. In the Southern Baltic and southwards to the Bornholm region (Rannak et al., they appear earlier than in the northern areas. The 1983). In the Southern Baltic the feeding migrations are spawning migration pattern is not very clear, because more intensive and extend to quite large areas. From the feeding areas are in the vicinity of the spawning the Pomeranian coast and the Vistula region sea trout rivers or releasing places in the case of local stocks and/ migrate mainly eastwards and some part of the stock or they are missing homing behaviour. Some of the westwards along the Polish coast. The feeding migration stocks have a clear homing behaviour (Zarnecki et al., of these stocks may reach the Gulf of Riga, the Gulf of 1962; Svärdson and Fagerström, 1982), but in some Finland, and the Bothnian Sea (Backiel and Bartel, cases it appears to be relatively weakly developed. At 1963; Chrzan, 1963). The stocks in the Swedish south spawning time adults have been found in rivers or areas and east coasts have a migration pattern northwards other than those in which they were released (Zarnecki, along the coast and they extend feeding migrations up 1964b; Toivonen and Tuhkunen, 1975). The post-smolt to the Bothnian Sea, the Gulf of Finland, the Gulf of and adult feeding migration of sea trout is presented in Riga, and in a southeastern direction to the Gulf of Figure 6. Gdansk (Svärdson and Fagerström, 1982).

SEA TROUT

Figure 6. The post-smolt and adult feeding migration of sea trout in the Baltic (Subdivisions 24-32).

84 3. The benthic community from the Arkona Basin westwards to Mecklenburg Bay, Kiel Bay, the Little Belt, the Great Belt, and to the 3.1. The Baltic Sea cod southern Kattegat, and some migration eastwards to In the Baltic Sea there are two cod stocks which have the Bornholm Basin (Berner, 1967, 1981; Bagge et al., been shown to differ from each other in certain mor- 1974). The westward migration ceases in April-May, phometric and meristic characters (Schmidt, 1930; Kand­ while the spawning migration eastwards continues ier, 1944; Biriukov, 1969; Berner and Vaske, 1985), (Berner, 1981). The cod stock in the Arkona Basin is by the haemoglobin types (Sick, 1965; Jamieson and always a mixture of the two main stocks and thus the Otterlind, 1971), by the otolith structure (Berner, 1968; spawning migration occurs in two directions (Berner, Bagge and Steffensen, 1980), and by the allele-fre- 1980a, b). quencies of the loci coding some enzymes (Moth-Poul- In late summer and early winter the Mecklenburg sen, 1982). The Western Baltic cod stock (Gadus Bay area cod migrate to the west and northwest regions morhua morhua L. ; the Atlantic cod or transition area during feeding (Berner, 1974), but there is no clear cod) is distributed west from Bornholm; in the Western tendency between the westward or eastward migrations Baltic, the Belt Seas, and the Sound (Subdivisions 22- (Berner, 1981). In winter, when the spawning migration 24) and it has regular connections to the Kattegat (Div­ starts, there is a westward migration to Kiel Bay, the ision Ilia) as well as to the Southeastern Baltic (Berner, Little Belt, and the Great Belt again (Berner, 1967, 1962, 1967, 1968, 1969, 1971a, b, 1973, 1974, 1980a, b, 1981). According to the transplantation experiments 1981; Anon., 1978; Bagge, 1961, 1969a, b, 1981). The the homing of cod is not well developed (Otterlind, Eastern Baltic cod stock (Gadus morhua callarias L.; 1965b, 1985a; Bagge, 1983). the Baltic Sea cod) is distributed east from the Bornholm area up to the northern parts of the Bothnian Sea and 3.1.2. Cod in the Southeastern and Northern Baltic to the eastern parts of the Gulf of Finland (Hessle, 1923; (Subdivisions 25-32) Otterlind, 1961b, 1962a, 1966a, 1976, 1984b; Bagge, The spawning period of the cod stock in the South­ 1969c, 1981; Netzei, 1974a; Sjöblom et al., 1980; Aro eastern and Northern Baltic is very extended. The and Sjöblom, 1983a, 1984; Neuman, 1984). The border spawning time is March-August (Bagge, 1981) and it between these two main stocks is diffuse and mixing of usually peaks in May-June, but the timing varies from stocks is evident in the Arkona basin and in the year to year (Grauman, 1974). The major spawning Bornholm Basin. The summary of the migrations of cod grounds are the Bornholm Basin, the Slupsk Furrow, stocks is presented in Figure 7a. the Gdansk Deep, and the Gotland Basin, and the highest density of eggs is found in the Bornholm Basin 3.1.1. Cod in the Southwestern Baltic and the Kattegat (Grauman, 1970,1974; Müller, 1974; Bagge and Müller, (Division Ilia and Subdivisions 22-24) 1977a, b; Krenkel and Hinrichs, 1979; Krenkel, 1981; The main spawning grounds of the cod stock in the Wieland, 1987). Spawning starts earlier in the Bornholm Western Baltic Sea are in the Arkona Deep north of Basin than in the Gotland Basin. In the Northern Baltic Rügen, in Mecklenburg Bay, in Kiel Bay, and in the Proper, the Åland Sea, the southern Bothnian Sea and Belt Sea (Tiews and Lamp, 1974; Anon., 1978). The the Gulf of Finland (Subdivisions 29, 30, and 32), there spawning time is February-March in the western areas is some spawning activity from time to time, but no and in March-April in the Arkona region. A part of surviving eggs or larvae originating from this spawn the stock has been shown to migrate to the southern have been observed so far (Aro, unpublished). The Kattegat to spawn and the ratio of emigration is depen­ spawners migrate to the Bornholm Basin spawning dent on the conditions of the spawning grounds in the grounds in December-February from the feeding Baltic (Bagge, 1961, 1969a, b; Berner, 1967, 1974). grounds in Slupsk Furrow, Gdansk Bay, Hanö Bay, and In the Western Baltic cod migrate after spawning the Gotland Deep (Netzel, 1968, 1974a). The spawners from the deep waters to more shallow coastal areas in the Gdansk Deep originate from the areas south of to feed. The feeding migration seems to occur to all the Bornholm Basin and from southern parts of the directions (Berner, 1981). The young immature age Gotland Deep (Netzel, 1974a; Otterlind, 1976). There groups usually stay in the coastal areas before joining is also a spawning migration from the Swedish east the mature stock. From the Arkona region feeding coast, the Åland Sea, the southern Bothnian Sea, and migrations of adults may reach the Belt Seas in the the Gulf of Finland to the Bornholm Basin, the Gdansk west and extend eastwards to the Slupsk Furrow, the Basin, and the southern parts of the Gotland Basin in Bornholm region, the Gdansk Deep, and even to the December-March (Otterlind, 1976, 1985a; Sjöblom et southern Gotland Deep area (Berner, 1971a, b; Lamp al., 1980; Aro and Sjöblom, 1983a). The homing behav­ and Tiews, 1974; Tiews and Lamp, 1969, 1974; Berner, iour of this Southeastern and Northern Baltic cod stock 1981). The passive or active immigration of 0-group to is not very clear and thus cod may use different spawning the eastern areas is also obvious (Otterlind, 1966a). In grounds in successive years (Bagge, 1983; Otterlind, winter and early spring there is a spawning migration 1984a, 1985a).

85 COO (0 1v 1 at on Ilia and sub division« 22-2«)

COO (Sub-divisions 25- 32)

greunds V : : Fsséiag graunds and ’ dlatrlkatlon araa M u l t Migration

2a*

- c

Spawning grounds reading grounds

reading migration a m ^ Spawning migration

Figure 7. Summary of the migration of Baltic Sea cod stocks (Subdivisions 22-24 and 25-32) (a), flounder stocks (Subdivisions 22-30 and 32), (b) and plaice stocks (Division Ilia and Subdivisions 22-26) (c, d).

86 During the feeding period cod is distributed to very der stocks or populations (Anon., 1978, 1987c). Floun­ large areas and may perform intensive migrations der is regularly distributed in all parts of the Baltic Sea, (Bagge, 1981; Otterlind, 1985a), which are not very except in the Bothnian Bay, the easternmost part of the patterned. In the Southeastern Baltic these feeding Gulf of Finland, and the deepest areas of the Gotland migrations are usually from the deeper waters to the Deep (Anon., 1978). In the Southwestern and South­ shallower areas from Bornholm Basin to Slupsk Furrow, eastern Baltic (Subdivisions 22-26) there are at least Gdansk Deep, and Gdansk Bay, and from the Gdansk three stocks: in the Central and Northeastern Baltic Deep area in the opposite direction (Netzel, 1963,1968, (Subdivisions 27-28) three, in the Åland Sea, the 1974a). Northward migrations along the USSR coast Archipelago Sea, and the southern Bothnian Sea one, off Klaipeda and Liepaja have been observed, too. The and in the Gulf of Finland two (Anon., 1978, 1983b, younger age groups (one-year-old and two-year-old) 1987c). The migrations between the mature flounder are usually found in coastal areas or near the open sea stocks in the Southern and Central Baltic are quite banks (Netzel, 1974b; Lablaika and Uzars, 1982; Bagge sparse and the boundary between them may be drawn and Steffensen, 1986). The feeding migrations of the from the southern part of Öland to the Rozewie on the adult stock are not very clearly directed and they seem Polish coast (Otterlind, 1965a, 1967a, b; Vitinsh, 1971, to be more or less random movements inside the distri­ 1976, 1977). In the Southeastern and Central Baltic the bution area (Bagge et al., 1974; Kondratovich, 1977, migrations are effectively blocked by the Gdansk Deep 1980). In the southern part of the Bornholm Basin the and Gotland Deep, in the Northern Baltic by the Got­ cod stock seems to be more stationary than in the land Deep and the deep to the southwest from the Gdansk Deep and Gdansk Bay (Netzel, 1974a; Bagge Åland Islands (Otterlind, 1965a; Vitinsh, 1971, 1972a, et al., 1974). The rate of the feeding migration west­ b, 1976, 1977; Aro and Sjöblom, 1983b). The annual wards from the Bornholm Basin has been observed to migration patterns of flounder stocks are quite well be very limited, because of the threshold nature of the known and because of their general locality the homing Bornholm Straits (Otterlind, 1985a). From the North­ behaviour of flounder is obvious. ern Baltic Sea Proper and the Gulf of Finland the direction of feeding migration is mainly to the south and 3.2.1. Flounder in the Southwestern Baltic, the Belt is associated with the maturing of young cod (Otterlind, Sea, and the Sound (Subdivisions 22-23) 1962a, b; Sjöblom et al., 1980). In the Southwestern Baltic the flounder stocks are char­ The immigration of larvae, fry, young and adult cod acterized by great locality. The spawning grounds are to the Northern Baltic Sea is connected with events in situated in the offshore areas and spawning occurs in the Main Basins. The successful spawning in more February-April (Saeger, 1974). The annual migrations northern areas during the absence of stagnation in the of adult specimens occur between the spawning grounds Main Basins leads to shorter transportation of larvae in deeper waters and the feeding grounds in the coastal and fry to the north (Neuman, 1984). In the case of areas (Bagge, 1966; Saeger, 1974). The feeding strong year classes in the Central and Northern Baltic migrations in spring in the Western Baltic occur along spawning grounds, certain parts of the larvae and fry the coasts mainly in westerly and northerly directions very soon reach the northern areas by passive drifting during the feeding period in May-November (Bagge, (Otterlind, 1983, 1985a). The long stagnation period in 1966). The migrations to the Belt Sea and to the sou­ the Main Basins may cause intensive migrations of thern Kattegat are insignificant. In late autumn and immatures and adults from the Gotland Deep area or early winter the spawning migration starts to the deeper even from the Southern Baltic to the Åland Sea, the areas (Bagge, 1966). The young immature age groups Bothnian Sea, and the Gulf of Finland (Otterlind, 1983, spend two to three years in the littoral and coastal areas 1984a). The migration of adults was observed in spring before joining the mature stock (Anon., 1978). In the 1987 in the Åland Sea and the northern Bothnian Sea Belt Sea flounder is also very local and the migration (Modin, 1987; Aro, unpublished), when very big spawn­ during feeding time to the Western Baltic (Subdivision ing shoals were detected in these areas. From the 24) is limited (Bagge, 1966; Saeger, 1974). In the Sound Bothnian Sea there is migration of adults to the area flounder has been shown to migrate southwards to Southern Baltic, but the main part of the stock remains the southern part of the Sound during feeding followed in the Bothnian Sea area and does not migrate (Otter- by migration back again for spawning and to some lind, 1983; Otterlind and Nordberg, 1988). From the extent to the southern Kattegat (Bagge, 1966). The Åland Sea and the Gulf of Finland the migration of migrations to the Western Baltic are insignificant. maturing and adult specimens southwards is evident (Aro and Sjöblom, 1983a; Otterlind, 1985a). 3.2.2. Flounder in the Southwestern and Central Southern Baltic (Subdivisions 24-25) In the Southwestern and Central Southern Baltic the 3.2. The Baltic Sea flounder migrations of the adult flounder stock are also fairly In the Baltic Sea there are several rather distinct floun­ limited (Otterlind, 1967a). Spawning takes place in

87 March-May at a depth of 40-80 m in the Arkona Deep, there is one stock in Subdivision 28, including the Gulf the Slupsk Furrow, and the Bornholm Deep (Bagge, of Riga, with some connections with the stocks in Sub­ 1981). After spawning, feeding migration occurs to the divisions 26 and 32 (Vitinsh, 1972b, 1976). The spawning very shallow coastal areas, females leaving the spawning time is March-June and spawning occurs mainly in the grounds first (Bagge, 1981). The feeding migrations offshore areas (Otterlind, 1966b), but part of the adult from the Bornholm Basin are directed to the Polish and stock also spawns on the shallow coastal areas, GDR coasts and occur along the coast to the west up especially in the northern parts of the subdivision to the areas of Rügen and to the east up to the areas of (Otterlind, 1966b; Vitinsh, 1976). After spawning the Rozewie. The migration to the south coast of Sweden feeding migration is directed to the shallow coastal areas results in the west, east, and northward migrations along and along the coasts in both southerly and northerly the Hanö Bay coast (Otterlind, 1967a). The migrations directions up to the Åland Sea on the west coast and in north from the threshold area from Öland to the the Gulf of Finland on the east coast (Otterlind, 1965c; Rozewie are insignificant. To some extent there seems Vitinsh, 1972b). Mixing of the east and west coast stocks to be feeding migration from the Bornholm Basin and is fairly limited (Otterlind, 1966b; Vitinsh, 1972b). Arkona Deep spawning grounds to the Sound and even to the Belt Sea (Otterlind, 1967a), but on an annual 3.2.5. Flounder in the Northern Baltic Proper, the basis the rate is insignificant. The west border of the Archipelago Sea, the southern Bothnian Sea, and the stock seems to be in the Darss (Friess, 1974; Vitinsh Gulf of Finland (Subdivisions 29-30 and 32) and Friess, 1980; Friess and Borrman, 1982). During The spawning time is May-July and, contrary to the late autumn and early winter there is a spawning southern stocks, it occurs in the littoral and coastal migration to the main spawning grounds and a part of zones, in depths of 2-20 m (Sandman, 1906; Suuronen, the mature stock feeding in the Arkona region migrates 1979). After spawning the feeding migration takes place to the Bornholm Basin to spawn (Friess and Borrman, to deeper areas and during the feeding period there is 1982), but this migration is not a regular one. a diurnal migration pattern between the deeper and shallower areas (Suuronen, 1979). 3.2.3. Flounder in the Southeastern Baltic In the Archipelago Sea, the Åland Sea, and the (Subdivision 26) southern Bothnian Sea the feeding migrations are quite This stock mainly inhabits the Gdansk Deep area, the local and a vast majority of the specimens stay inside coastal areas of Gdansk Bay, and the southern part of these areas (Aro and Sjöblom, 1982, 1983b). Some the Gotland Deep; it has some connections with the exchange between the stocks in the Åland Islands and Bornholm Basin spawning grounds and even with the the Stockholm archipelago occurs, however, but the Arkona region (Cieglewicz, 1961,1963; Draganik, 1978; more intensive migration westwards is blocked by the Kuczynski, 1984). In the Gdansk Deep and the southern 250 m deep on the west side of the Åland Islands (Aro part of the Gotland Deep the spawning time is March- and Sjöblom, 1983b). June in the depth range of 40-80 m and the spawning In the Gulf of Finland there are two stocks: one is migration takes place from the coastal areas to the distributed along the Finnish coast with some feeding spawning grounds during winter (Bagge, 1981). After migration from the Finnish coast to the USSR coast spawning there are feeding migrations to the coastal (Aro and Sjöblom, 1983b); the other is distributed along feeding grounds in the Gdansk Bay, along the Pomer­ the USSR coast, having some connections with the stock anian coast westwards, and to the feeding grounds south in the Northeastern Baltic (Vitinsh, 1976). The western of Bornholm. The migrations north along the Polish part of the stock on the Finnish coast has been shown coast are not very intensive (Cieglewicz, 1963; Kuczyn­ to migrate to the Archipelago Sea and to the Gotland ski, 1984). The spawning migration during winter is region, but on an annual basis it is insignificant (Halme, directed to the Bornholm Basin, the Gdansk Deep, and 1962). The spawning migration to the coastal spawning the southern Gotland Deep (Cieglewicz, 1963). grounds takes place during the winter. The summary of all flounder stocks is presented in Figure 7b. 3.2.4. Flounder in the Central and Northeastern Baltic (Subdivisions 27-28) 3.3. Plaice in the Baltic Sea (Subdivisions 22- The migration of these three stocks follows the general 26) annual migration pattern of flounder in the Baltic, but they seem to be more coastbound than the other stocks Plaice is distributed in the Baltic Sea regularly from the (Anon., 1987c). On the east coast of Sweden, including Belts eastwards to the Gdansk Bay area and northwards the Gotland area there are two stocks: one along the to the southern Gotland Deep area (Bagge, 1981). Its coast of the Swedish mainland and the coasts of Öland occurrence in the Northern Baltic is very rare. It spawns and one on the east coast of Gotland (Otterlind, 1965c, in Kiel Bay at a depth of 20 m or more, in the Arkona 1966b). These stocks mix during the feeding period, Deep, the Bornholm Basin, and the Slupsk Furrow in however (Otterlind, 1965c). Along the USSR coast a depth range from 50 to 90 m in November-May, but it does not spawn in the Gdansk Deep (Mulicki, 1959). 4.1. Whitefish Spawning peaks in February-March. The feeding migrations are quite extensive from the Arkona region, The two whitefish species, river spawning whitefish but in the east of Bornholm the stock is less migratory (Coregonus lavaretus L. s. str.) and the sea-spawning (Otterlind, 1967a, b). The feeding migrations from the whitefish (Coregonus widegreni Malmgren), have dif­ deeper spawning grounds to the shallower waters occur ferent migration patterns. They are distributed along west and eastwards from the Arkona Basin, but the the coastal areas in the Baltic. They are rare in the Belt main direction is to the west. The feeding migration Seas and south coast of Sweden (Svärdson, 1979), and westwards may reach the Belt Sea, the Sound, the more abundant in the Gotland area, the west coast of southern Kattegat, and even the Skagerrak (Otterlind, Estonia, the Gulf of Finland, and the Gulf of Bothnia 1967a). Those emigrating out of the Baltic Sea are (Lehtonen, 1981a). The spawning time is in October- considered not to immigrate back (Otterlind, 1967a). November and spawning migration takes place in July- The eastward migration is most intensive from the September (Lehtonen, 1981a). The feeding migration Arkona Region to the east and southeast of Bornholm patterns vary between species and populations. during November-February. Those specimens feeding After spawning the river spawning whitefish disperse in the Gdansk Deep area have been shown to migrate along the coasts. From the Bothnian Bay and the Quark to the Bornholm Basin to spawn (Cieglewicz, 1961). (Subdivision 31) the feeding migration regularly occurs The summary of plaice migration is shown in Figure 7c between the Åland Islands and the spawning grounds and d. (Wigren, 1962), and the specimens show a clear homing behaviour. In the Bothnian Sea (Subdivision 30) the feeding migrations are not so extensive (Olsson, 1978) and the specimens remain in the Bothnian Sea and the Archipelago Sea. In the Gulf of Finland (Subdivision 32) feeding migrations extend about 70 km from the 4. The littoral and coastal community river mouths (Ikonen, 1980, 1982). The feeding In the littoral and coastal community the true offshore migration starts in May and in June-July the whitefish fish fauna is usually lacking, but the habitat is used by are in their feeding areas, which are often situated fairly the offshore species as a spawning and nursery area far from the coast in the vicinity of small islands (Ikonen, and sometimes as a wintering area, too. The balance 1982). The spawning migration starts in August. In the between the marine and freshwater species varies from Southern Baltic whitefish leave the coastal shallows and the Southwestern Baltic Sea to the Northern and North­ return to the sea after spawning, dispersing along the eastern areas of the Baltic Sea and from the open sea coast (Gaygalas, 1972). towards land (Neuman, 1982; Hildén et al., 1988). In The sea-spawning whitefish do not migrate so much. the Southern Baltic, where the freshwater species are They feed near the spawning grounds and feeding almost totally lacking, shallow coastal bottoms and migrations are directed mainly towards the coastline coastal pelagial areas serve as a habitat for the young (Lehtonen, 1981a). In October the mature fish are to stages of several marine pelagic and demersal species be found near the shallow spawning grounds (Sormus, of major economic importance. These areas are also a 1976; Segerstråle, 1983). The short spawning migrations habitat for a number of other littoral and coastal species are directed to spawning reefs and shallows (Lindroth, which are important ecologically but of little economic 1957). The sea-spawning whitefish probably winter near significance. The information on the migration patterns the spawning places and also feed in the same areas of these species and the young stages of the offshore (Lehtonen, 1981a). species is usually quite limited. In the Central and Northern Baltic Sea there is a clear dominance of fresh­ 4.2. Pike water species in the littoral and coastal areas (Neuman, 1982), and their number decreases towards the seaward In its distribution area pike is very local and is territorial limit of the archipelago (Lehtonen and Toivonen, 1981). in behaviour (Ekman, 1915; Segerstråle, 1951, 1953; The number of freshwater species is largest in the archi­ Halme, 1957; Halme and Korhonen, 1960; Strandman, pelagos, bays, inlets, and river mouths. The migrations 1964; Lehtonen, 1973). It spawns in May and early June of freshwater species are usually fairly local although in the shallow bays, inlets, and river mouths with reeds long-migrating species also exist. Some of the freshwater (Phragmites sp.) and shallow sheltered areas in the inner species lack the migration pattern or the migrations are and outer archipelago (Lehtonen, 1986). The feeding so short that the pattern is very hard to verify. The areas are in the vicinity of the spawning grounds littoral and coastal communities of the Central and (Lehtonen et al., 1983). Local migrations or movements Northern Baltic Sea with information on the following during feeding are very limited, because the pike is an freshwater species are considered below: whitefish, ambush hunter or stalker (Neill and Cullen, 1974). pike, pike-perch, perch, bream, and burbot. The home range of pike is usually a couple of square kilometres and when displaced it attempts to return to mum temperature, the abundance of neighbouring its original home range (Halme and Korhonen, 1960). stocks, as well as the morphology of the archipelago There is also some evidence that the home range of pike (Böhling and Lehtonen, 1985). The perch winters in may be larger (Johnson and Müller, 1978) and a clear deeper waters and the spawning migration to shallow spawning migration pattern has also been observed waters occurs in April-May (Neuman, 1982). Homing (Hudd et al., 1984). In the pike populations, station- is sometimes evident (Koli et al., 1978; Johnson, 1978), keeping and re-distributive behaviour is preferred to and sometimes not (Böhling and Lehtonen, 1985). migrations.

4.5. Bream 4.3. Pike-perch The bream stocks, spawning in spring, have similar The pike-perch spawn in inlets and shallow bays in migration patterns to pike-perch, but they do not necess­ April-May (Lehtonen and Toivonen, 1981). After arily winter in deeper waters (Lehtonen, 1977). In gen­ spawning they remain in the spawning grounds and feed eral, bream form quite local stocks with only a few there (Lehtonen and Toivonen, 1987), or they may specimens migrating out from the population’s home perform feeding migrations inside the archipelagos, to range (Hildén, 1986). Spawning takes place in May- the open sea, or along the coasts (Henking, 1923). The June in shallow bays and inlets. Although bream is a annual migrations take place between the spawning stationary fish species, it usually has a distinct seasonal inlets or bays, feeding areas in the archipelagos, and migration pattern (Backiel and Zawisza, 1968). The wintering areas in deeper waters (Toivonen, 1968; spawning migrations are rather short, a few kilometres Lehtonen, 1977, 1981b). The average dispersal area is at most, from deeper waters to those shallow spawning shown to be smaller when the coast is open and larger grounds (Hildén, 1980; Hildén and Lehtonen, 1982). when the archipelago is rich (Toivonen, 1968; Lehtonen Some of the spawning bream evidently stay in the and Toivonen, 1987). During the winter part of the spawning grounds, which serve as feeding and wintering stock stays in shallow inlets but a major part spends the areas, too. Some of the bream migrate to deeper waters winter in deeper waters (Winkler and Thieme, 1978). and winter there (Hildén and Lehtonen, 1982). The rate The mixing of local neighbouring stocks occurs in the of migration out from the spawning grounds and the wintering areas, but pike-perch shows a clear homing extent of the feeding migration seems to be dependent behaviour to its former spawning grounds (Lehtonen, on food resources available, the size of the stock, and 1985). In the Southern Baltic the feeding migration is the morphology of the coastline and the archipelago directed to the open sea and pike-perch overwinter in (Hildén, 1986). Thus the homing ability between the shallow inlets (Henking, 1923; Filuk, 1962). stocks also varies.

4.4. Perch 4.6. Burbot The perch is one of the commonest freshwater species in the Baltic archipelagos. Perch form more or less The burbot stocks along the Baltic Sea coasts are fairly separate local stocks along the Baltic Sea coasts (Böh- local, but the mixing of neighbouring stocks during ling and Lehtonen, 1985); these populations are not feeding is obvious (Hudd et al., 1984). The migration discrete units, however, because perch spawn all along pattern of burbot differs from the other freshwater the coast and the mixing of neighbouring stocks is likely. species in that it migrates into the shallow waters in the Perch is a spring spawner and the main spawning archipelagos, bays, inlets, river mouths, and even into grounds are in shallow waters in the archipelagos, bays, the rivers during the autumn and early winter to spawn inlets, and river mouths. The spawning time is May- (Lehtonen and Toivonen, 1981; Johnson, 1982; Hudd June. After spawning, the feeding migration takes place et al., 1984; Hudd and Lehtonen, 1987). The spawning to deeper areas along the coasts, within the archi­ time is in January-February and after spawning burbot pelagos; long migrations are not common, but they have migrate to the deeper waters to feed (Hudd and been noted (Henking, 1923; Lind etal., 1975; Johnson, Lehtonen, 1987). The feeding migrations into the 1978; Koli et al., 1978). During the feeding migration, deeper waters are directed in the Quark area on the which does not have any special pattern, the distri­ Finnish coast mainly northwards (Hudd and Lehtonen, butions of different populations overlap (Böhling and 1987) and in the Bothnian Bay on the Swedish coast to Lehtonen, 1985). The feeding migration may be very the southeast (Hedin, 1983). This has been explained limited, however, in which case no mixing of neigh­ by the main current hypothesis. In general the home bouring stocks is evident (Ekman, 1915); the feeding range of burbot stocks is not very large; they usually do and wintering areas are near the spawning grounds not migrate outside the archipelagos and migrations (Johnson, 1978). The range of feeding migrations is seldom exceed 20 km (Hudd and Lehtonen, 1987). The affected by the distribution of food resources, the opti­ burbot seems to have clear homing behaviour.

90 Backiel, T., and Zawisza, J. 1968. Synopsis of biological data References on the bream, Abramis brama L. FAO Fisheries Synopsis Ackefors, H. 1977. On the winter-spring spawning herring in No. 36. pag. var. the Kattegat. Medd. fr. Havsfiskelab., Lysekil., 225: 14 pp. Bagge, O. 1961. Mærkning af torsk i indre danske farvande. Ackefors, H. 1978. Swedish herring tagging experiments 1968- Skr. Danm. Fisk. Havunders., 21: 1-12. 1970 in the Kattegat. Medd. Havsfiskelab. Lysekil, 241: 10 Bagge, O. 1966. Tagging of flounder in the western Baltic, the pp. Belt Sea and the Sound in 1960-66. ICES CM 1966/D: 1, Alheit, J. 1986. Reproductive biology of sprat, Sprattus sprattus 15 pp. L.: Factors determining annual egg production. ICES CM Bagge, O. 1969a. Mærkning af torsk i den vestlige Osterso. 1986/H: 58, 16 pp. Skr. Danm. Fisk. Havsunders., 30: 1-8. Andreasson, S., and Petersson, B. 1982. The fish fauna of the Bagge, O. 1969b. Preliminary results of the cod tagging experi­ Gulf of Bothnia. In Coastal research in the Gulf of Bothnia, ments in the western Baltic 1969. ICES CM 1969/F: 29, 7 pp. 301-315. Ed. by K. Müller. Dr. W. Junk Publishers, pp. The Hague-Boston-London. Bagge, O. 1969c. Preliminary results of cod tagging experi­ Anon. 1970. Discussion and general conclusions of the sprat ments in the Baltic proper 1968 and 1969. ICES CM 1969/ symposium, Lysekil, January 1968. Medd. Havsfiskelab. F: 30, 11 pp. Lysekil, 88: 13 pp. Bagge, O. 1981. Demersal fishes. In The Baltic Sea, pp. 311- Anon. 1974. Report of the working group on assessment of 333. Ed. by A. Voipio. Elsevier Oceanographic Series No. pelagic stocks in the Baltic. ICES CM 1974/H: 3: 37 pp. 30. Elsevier Scientific Publishing Company, Amsterdam- Anon. 1978. Report of the working group on assessment of Oxford-New York. demersal stocks in the Baltic. ICES CM 1978/J: 3: 31 pp. Bagge, O. 1983. Migration of transplanted cod. ICES CM Anon. 1980a. Baltic salmon assessment working group. ICES 1983/J: 16, 12 pp. CM 1980/M: 3, 35 pp. Bagge, O., and Müller, A. 1977a. The spawning of cod in the Anon. 1983a. Workshop on stock components in herring land­ Bornholm Basin and the size of spawning stock. Meeres- ings from division Ilia. ICES CM 1983/Assess: 5, 32 pp. forsch., 25: 172-185. Anon. 1983b. Report of the working group on assessment of Bagge, O.. and Müller, A. 1977b. The mortality of cod eggs, demersal stocks in the Baltic. ICES CM 1983/Assess: 15, 87 the spawning of cod and the biomass of the spawning stock pp. in the Bornholm Basin. ICES CM 1977/P: 14, 17 pp. Anon. 1986. Report of the workshop on herring age reading Bagge, O., and Steffensen, E. 1980. Growth pattern in cod and stock differentiation. Sopot, 12-16 May 1986. ICES CM otoliths as indicator for mixing of stocks. ICES CM 1980/J: 1986/J: 25, 9 pp. 12, 11 pp. Anon. 1987a. Report of the working group on assessment of Bagge, O., and Steffensen, E. 1986. The total mortality of pelagic stocks in the Baltic. ICES CM 1987/Assess: 20, Part cod in the Baltic 1982-85 as estimated from bottom-trawl I, 163 pp. surveys. ICES CM 1986/J: 17, 28 pp. Anon. 1987b. Report of the Baltic salmon and trout assessment Bagge, O., Tiews, K., Lamp, F., and Otterlind. G. 1974. working group. ICES CM 1987/Assess: 21, 51 pp. German, Swedish and Danish cod tagging experiments in Anon. 1987c. Report of the working group on assessment of the Baltic 1968-1969. Rapp. P.-v. Réun. Cons. int. Explor. demersal stocks in the Baltic. ICES CM 1987/Assess: 22, 93 Mer, 166: 22-39. pp. Bartel, R. 1976. The Drava river salmon in the light of some Anwand, K. 1961. Mitteilungen über die bisherigen Ergebnisse recent tagging experiments. ICES CM 1976/M: 6, 8 pp. von Hering markierungen inden Gewässern um Rügen. Dt. Bartel, R. 1977. Variability of sea trout returns as shown from Fischerei-Zeitung, 8: 10-12. many years tagging experiments with hatchery reared parr Anwand, K. 1963a. Markierungen am Rügenschen Frühjahrs­ and smolts. ICES CM 1977/M: 9, 7 pp. hering im Jahre 1961. Int. Rev. ges. Hydrobiologie, 48: 2- Berner, M. 1962. Ergebnisse der Markierungsexperiment an 8 . Ostseedorsch (Gadus morhua L.) in der Bornholm- und Anwand, K. 1963b. Vergleichende Untersuchungen an Frü- Arkonasee, 1959. Z. Fisch. N.F.. 11(516): 453-462. jahrs- und Herbst-heringe aus den Gewässern um Rügen. Berner, M. 1967. Results of cod tagging in the western and Z. Fisch., 11: 211-249. central Baltic in the period 1962-1965. ICES CM 1967/F: 5, Aps, R. 1977. Läänemere kilu ühikvarude piiritlemisest. Abiks 10 pp. Kalurile, 7: 2-8. Berner, M. 1968. Dorschmarkierungen in der Mecklenberger Aps, R., Benenson, I., Kaleis, M., Korytin, N., and Krya- Bucht in Jahre 1965. Fischerei-Forschung, 6(1): 73-76. zhimsky, F. 1987. Spatial model of the Baltic sprat popu­ Berner, M. 1969. Ergebnisse der Dorschmarkierung des Jahres lation dynamics. ICES CM 1987/J: 16, 18 pp. 1967 in der Arkonasee und der Mecklenburger Bucht. Aps, R., Falk, U., and Oeberst, R. 1981. Morphological Fischerei-Forschung, 7(1): 39-43. '■ ariability of otolith growth zones in the Baltic sprat. Fische­ Berner, M. 1971a. Ergebnisse der Dorschmarkierung des rei-Forschung 19(2): 27-30. Jahres 1968 in der Bornholm- und Arkonasee. Fischerei- Aps, R., and Lotman, K. 1984. Natural tags of sprat in the Forschung. 9(1): 15-20. Gulf of Finland. ICES CM 1984/J: 12, 11 pp. Berner, M. 1971b. Ergebnisse der Dorschmarkierung des Aro, E., and Sjöblom, V. 1982. Stock assessment of flounder Jahres 1969 in der Mecklenburger Bucht und der Arkonasee. off the coast of Finland in 1975-81. ICES CM 1982/J: 25, Fischrei-Forschung, 9(1): 21-27. 15 pp. Berner, M. 1973. Ergebnisse der Dorschmarkierung des Jahres Aro, E., and Sjöblom, V. 1983a. Cod off the coast of Finland 1970 in der Mecklenburger Bucht. Fischerei-Forschung, in 1974-82. ICES CM 1983/J: 25, 17 pp. 11(1): 19-25. Aro, E., and Sjöblom, V. 1983b. The migration of flounder in Berner, M. 1974. Some results of cod tagging experiments of the northern Baltic Sea. ICES CM 1983/J: 26, 12 pp. the GDR in the Baltic 1968-1971. ICES CM 1974/F: 32, 17 Aro, E., and Sjöblom, V. 1984. Fishing effort and catch per pp. unit of effort of cod in Finnish fisheries in 1976-82. ICES Berner, M. 1980a. Results of cod tagging experiments in the CM 1984/J: 24, 15 pp. Arkona Sea during 1972 and 1973. Fischerei-Forschung, Backiel, T., and Bartel, R. 1963. Preliminary results of sea- 18(2): 25-29. and lake-trout tagging. ICES CM 1963/No. 120, 6 pp. Berner, M. 1980b. Calculation and analysis of migration para-

91 meters for Baltic cod on the basis of tagging experiments Draganik, B. 1978. The state of the flounder stocks in the from 1959 to 1975 in the area from Mecklenburg Bay to southern Baltic. ICES CM 1978/J: 11, 20 pp. the Bornholm Sea (ICES SD 22-25). Fischerei-Forschung, Ehnholm, G. 1951. Studier över strömmingen i östra Kvarken. 18(2): 31-49. Havsforskningsinstitutets skrift., No. 149: 1-94. Berner, M. 1981. Dislocation parameters of tagging experi­ Ekman, T. 1915. Meddelanden rörande utförda märkningar ments on cod in the Baltic (Subdivisions 22-25) from 1959— af gädda m. fl. fiskar i Södermanslands och östergötlands 1975. ICES CM 1981/J: 15., 26 pp. skårgård. Sven. Fisk. Tidskr., 1914-1916: 62-68. Berner, M., and Vaske, B. 1985. Morphometrie and meristic Elwertowski, J. 1964. The influence of the 1956 and 1963 characters of cod stocks in the Baltic Sea. ICES CM 1985/ winter seasons of the life of the southern Baltic sprat. ICES J: 11, 13 pp. CM 1964/No. 75, 5 pp. Biester, E. 1979a. The distribution of the Rügen spring herring. Elwertowski, J. 1976. The recruitment of Baltic sprat stocks ICES CM 1979/J: 31, 6 pp. in relation to the environmental conditions. ICES CM 1976/ Biester, E. 1979b. Studies on Rügen herring 1979. ICES CM P: 17, 13 pp. 1979/J: 32, 9 pp. Eriksson, C. 1985. Migration of salmon postsmolt (Salmo salar Biester, E. 1979c. Der Früjahrshering Rügens - seine Rolle in L.) through the Baltic main basin. Medd. Laxforskningsinst., der Fischerei der Ostsee und in den Übergangsgebieten zur 4: 9 pp. Nordsee. Inaug.-diss. Rostock 1979. 238 pp. Filuk, J. 1962. Nachkriegsstudium über Biologie und Fang des Biester, E., and Hering, P. 1977. Changes in the populations Zanders des Frischen Haffs. Zeitschr. Fisch., 10: 705-709. structure of the herring in the Griefswalder Bodden during Friess, C. 1974. Plattfischmarkierung in der Arkonasee 1970. spawning time in spring. ICES CM 1977/P: 20, 12 pp. Fischerei-Forschung 12(1): 13-16. Biester, E., Jönsson, N., Hering, P., Thieme, Th., Brielmann, Friess, C. 1977. Investigations on the spring spawning Rügen N., and Lill, D. 1979. Studies on Rügen herring 1979. ICES herring infested by Anisakis in 1975-1977. ICES CM 1977/ CM 1979/J: 32, 8 pp. P: 16, 6 pp. Biester, E., Jönsson, N., and Krüger, G. 1975. Preliminary Friess, C., and Borrman, H. 1982. Stock assessment for floun­ report on marking of spring herring in the Rügen-Bodden der in the Arkona Basin for the years 1970-1981. ICES CM 1975. ICES CM 1975/P: 11, 8 pp. 1982/J: 15, 11 pp. Biester, E., Jönsson, N., and Krüger, G. 1976a. Ergebnisse Gaygalas, K. 1972. Morphological and ecological charac­ der Markierungen 1975 am Rügenschen Frühjahrshering. teristics, population structure and the state of the stocks of Fischerei-Forschung 14: 19-21. the European whitefish (Coregonus lavaretus lavaretus L.) Biester, E., Jönsson, N., and Krüger, G. 1976b. The results in the Kurshuy-Mares (Kurisches Haff) and some aspects of marking experiments in 1975 on the Rügen spring herring. of fishery regulation. J. Ichthyol. 12(6): 953-961. ICES CM 1976/P: 15, 8 pp. Grauman, G. 1970. Peculiarities of cod spawning in the Baltic Biester, E., Jönsson, N., and Krüger, G. 1977. The results Sea in the spring-summer period of 1969. Annls. Biol., of tagging experiments in 1976 on Rügen spring spawning Copenh., 26: 136-137. herring. ICES CM 1977/P: 18, 10 pp. Grauman, G. 1974. Investigations of the spawning of Baltic Biriukov, N. P. 1969. Spawning communities of Baltic cod and cod in 1968-1970. Rapp. P.-v. Réun. Cons. int. Explor. the extent of their mixing. ICES CM 1969/F: 7, 17 pp. Mer, 166: 7-9. Böhling, P., and Lehtonen, H. 1985. Effect of environmental Grauman, G. 1975. Ecological peculiarities of reproduction of factors on migrations of perch (Perea fluviatilis L.) tagged fishes with pelagic eggs. ICES CM 1975/P: 12, 11 pp. in the coastal waters of Finland. Finn. Fish. Res. 5: Grauman, G. 1976. On the distribution and passive migrations 31-40. of Baltic sprat eggs and larvae in Bornholm-Slupsk spawning Carlin, B. 1959. Results of salmon smolt tagging in the Baltic ground. ICES CM 1976/P: 8, 11 pp. area. Rapp. P.-v. Réun. Cons. int. Explor. Mer, 147: 89- Grauman, G., and Krenkel, K. 1986. Investigations about the 96. dynamic of spawning processes of the Baltic sprat in 1984. Carlin, B. 1962. Synpunkter på frågan om Östersjöns lax- ICES CM 1986/J: 17, 16 pp. bestånd i belysning av de svenska märkningsförsöken. Medd. Groth, B. 1985. Relations of maturing and spent herring in the Laxforskningsinst., 5: 1-10. catches of spring fishery season in the Greifswalder Bodden. Carlin, B. 1965. Märkning av odlade havsöringsungar i utvan- ICES CM 1985/J: 8, 10 pp. dringsfärdig storlek. Sven. Fisk. Tidskr., 74(7/8): 97-101. Grygiel, W. 1984. Differences in morphology and growth of Christensen, O. 1982. Danish experiments with salmon smolt otoliths of juvenile herring taken on various fishing grounds released into the Baltic Sea from the Island of Bornholm. in the southern Baltic. ICES CM 1984/J: 25, 21 pp. ICES CM 1982/M: 9, 7 pp. Grygiel, W. 1987. Southern Baltic herring: Some remarks on Christensen, O., and Johansson, N. 1975. Reference report on morphological structure of its otoliths. ICES CM 1987/J: 4, Baltic salmon with additional information on Baltic sea 16 pp. trout. Coop. Res. Rep. Cons. int. Explor. Mer, 45: 1-160. Halme, E. 1957. Gäddmärkningar utförda av Timmer Spinn- Christensen, O., and Larsson, P. O., 1979. Review of Baltic fiskeklubb r.f., Fiskeritidskrift for Finland, Ny serie, 1: 8-15. salmon research. Coop. Res. Rep. Cons. int. Explor. Mer, Halme, E. 1961. Report of salmon tagged in Finland. ICES 89: 1-124. CM 1961/No. 17, 4 pp. Chrzan, F. 1963. Preliminary report on tagging experiment of Halme, E. 1962. Tvärminnessä suoritettujen kampelamer- sea trout in the region of Vistula Firth. ICES CM 1963/No. kintöjen tuloksia. Kalamies, 1: 4-8. 95, 7 pp. Halme, E. 1964. Report on salmon tagged in Finland in 1959- Chrzan, F. 1964. The movement and growth rate of tagged 1964. ICES CM 1964/Salmon and Trout Committee No. 61, Drawa salmon. ICES CM 1964/No. 77, 10 pp. 7 pp. Cieglewicz, W. 1961. Tagging experiments with flatfish in the Halme, E. 1966. Report on Salmon tagged in Finland in 1959— southern Baltic. ICES CM 1961/No. 95, 2 pp. 1966. ICES CM 1966. 3 pp. Cieglewicz, W. 1963. Flounder migrations and mortality rates Halme, E., and Korhonen, M. 1960. Haukien vaelluksista in the southern Baltic. ICES CM 1963/No. 78, 7 pp. rannikoillamme. Kalamies, 4: 1-12. Defant, F. 1974. Klima und Wetter. In Meereskunde der Hannerz, L. 1955. Strömmingsmärkningen i Bottenhavet 1954. Ostsee, pp. 19-32. Ed. by L. Magaard and G. Rheinheimer. 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92 tigations in the Bothnian Sea 1954. Annls. biol., Copenh., smolts (Salmo salar L.) in the Baltic Sea. ICES CM 1985/ 11: 156-158. M: 19, 11 pp. Hedin, J. 1983. Seasonal spawning migrations of the burbot Jamieson, A., and Otterlind, G. 1971. The use of cod blood (Lota lota L.) in a coastal stream of the northern Bothnian protein polymorphisms in the Belt Sea. the Sound and the Sea. Fauna Norrlandica, 6: 1-9. Baltic Sea. Rapp. P.-v. Réun. Cons. int. Explor. Mer, 161: Heidrich, H. 1925. Über die Fortpflanzung von Clapea sprattus 55-59. in der Kieler Bucht. Wiss. Meeresunters., 20: 1-47. Jensen, A. J. C. 1955. Danish herring tagging inside the Scaw, Hempel, G., and Nellen, W. 1974. Fische der Ostsee. In 1949-1952. Rapp. P.-v. Réun. Cons. int. Explor. Mer, Meereskunde der Ostsee, pp. 215-232. Ed. by L. Magaard 140(2): 30-32. and G. Rheinheimer. Springer Verlag. Berlin. Johnson, T. 1978. Dispersal area of perch, Perea fluviatilis, Henking, H. 1923. Die Fischwanderungen zwischen Stettiner tagged in a stream flowing into the Bothnian Sea. Aquilo Haff und Ostsee. Zeitschr. Fisch., 22: 1-92. Ser. Zool., 18: 62-64. Hessle, C. 1923. Undersökningar rörande torsken (Gadus cal- Johnson, T. 1982. Seasonal migrations of anadromous fish larias L.) i mellersta Östersjön och Bottenhavet. Medd. från species in a northern Swedish coastal stream. In Coastal Kungl. Lantbruksstyrelsen, 243: 19-75. research in the Gulf of Bothnia, pp. 353-362. Ed. by K. Hessle, C. 1925. The herrings along the Baltic coast of Sweden. Müller. Dr. W. Junk Publishers, The Hague-Boston-Lon- Publ. Circ. Cons. int. Explor. Mer, 89: 1-55. don. Hessle, C. 1927. Sprat and sprat-fishery on the Baltic coast Johnson, T., and Müller, K. 1978. Migration of juvenile pike, of Sweden. Medd. från Kungl. Lantbruksstyrelsen, 262(1): Esox lucius L., from a coastal stream to the northern part 1-29. of the Bothnian Sea. Aquilo Ser. Zool., 18: 57-61. Hildén, M. 1980. Braxen i de finska kustvattnen. Fiskeri- Jokiel, J., and Bartel, R. 1983. A historical record based on tidskrift for Finland, 24(2): 28-31. tagging the Vistula salmon which has already been extinct. Hildén, M. 1986. Braxens, Abramis brama L., vandringar och ICES CM 1983/M: 17, 19 pp. dödlighet i finska kustvatten enligt märkningsresultat. Pro Jutila, E., and Alapassi, T. 1985. Recaptures of salmon post- gradu avhandling. Helsingfors Universitet, Zoloogiska Insti­ smolts (Salmo salar L.) during the first summer after release tutionen. 63 pp. in Finnish tagging experiments. ICES CM 1985/M: 28, 22 Hildén, M., Kuikka, S., Roto, M., and Lehtonen, H. 1988. pp. Differences in fish community structure along the Finnish Jutila, E., and Toivonen, J. 1985. Food composition of salmon coast in the Baltic Sea. ICES Symposium, ICES 1988 BAL/ post-smolts, (Salmo salar L.), in the northern part of the No. 15, 16 pp. Gulf of Bothnia. ICES CM 1985/M: 21, 12 pp. Hildén, M., and Lehtonen, H. 1982. Management of the Järvi, T. H. 1938. Fluctuations in the Baltic stock of salmon bream, Abramis brama L., stock in the Helsinki sea area. 1921-1935. Rapp. P.-v. Réun. Cons. int. Explor. Mer, 106: Finn. Fish. Res., 4: 46-61. 1-114. Hudd, R., Hildén, M., Urho, L., Axell, M.-B., and Jåfs, L.-A. Jönsson, N., and Biester, E. 1977. Preliminary results of 1984. Kyrönjoen suisto- ja vaikutusalueen kalatalousselvitys herring taggings in the Greifswalder Bodden 1977. ICES 1980-82. (Summary: Fishery investigations (in 1980-82) of CM 1977/P: 17, 6 pp. the Kyrönjoki River estuary and its influence area in the Jönsson, N., and Biester, E. 1979. Results of tagging experi­ northern Quark of the Baltic Sea.). Vesihallitus-National ments on the Rügen spring herring 1977/78. ICES CM 1979/ Board of Waters, Finland, Tiedotus-Report, No. 242 A: J: 29, 13 pp. 1-275. Jönsson, N., and Biester, E. 1981a. Herring tagging experi­ Hudd, R., and Lehtonen, H. 1987. Migration and home ranges ments 1980/81 along the coast of GDR. ICES CM 1981/J: of natural and transplanted burbot (Lota lota L.) off the 29, 10 pp. coast of Finland. Proc. V Congr. Europ. Ichtyol., Stock­ Jönsson, N., and Biester, E. 1981b. Migrations of the spring holm, 1985: 201-205. herring in the coastal and Bodden waters of GDR near Ikonen, E. 1980. Migration of river-spawning whitefish in the Rügen. Fischerei-Forschung, 19(1): 47-51. Gulf of Finland. ICES CM 1980/M: 3, 11 pp. Kangur, M., and Ling, P. 1985. Reproduction of sea trout Ikonen, E. 1982. Migration of river-spawning whitefish in the along the southern coast of the Gulf of Finland. Finn. Fish. Gulf of Finland. Finn. Fish. Res., 4: 40-45. Res., 6: 55-60. Ikonen, E. 1986. Spawning migration of salmon (Salmo salar Kazakov, R. 1985. Condition of fish stock, yield to fishery and L.) in the coastal waters of the Gulf of Bothnia. ICES CM migrations of Atlantic salmon from rivers of the USSR to 1986/M: 24, 12 pp. the Baltic Sea. Finn. Fish. Res., 6: 43-54. Ikonen, E. 1987. Mixing of wild and hatchery-reared salmon Khoziosky, S., Shvetsov, F., and Uzars, D. 1983. The dynamics during migration in the Baltic Sea. ICES CM 1987/M: 10, of mortality components in Baltic sprat in subdivisions 26 11 pp. and 28. ICES CM 1983/J: 7, 6 pp. Ikonen, E., Ahlfors, P., Mikkola, J., and Saura, A. 1987. Koli, L., Aro, E., and Rask, M. 1978. Tvärminnen ahvenen Meritaimenen ja lohen elvyttäminen Vantaanjoen ves- populaatiotutkimus. Loppuraportti luonnontieteelliselle istössä. RKTL/KTO Monistettuja julkaisuja, 62: 1-106. toimikunnalle. 12 pp. Ikonen, E., and Auvinen, H. 1982a. Results of stocking with Kompowski, A. 1969. Types of otoliths of southern Baltic Baltic salmon in Finland in 1969-1980. ICES CM 1982/M: herring. ICES CM 1969/H: 12, 17 pp. 38, 19 pp. Kondratovich, E. Y. 1977. 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93 (Sprattus sprattus L.) and cod (Gadus morhua L.) in the Lindroth, A. 1981. Anadromous and catadromous fishes. In Baltic Sea in 1978. ICES CM 1979/J: 24, 16 pp. The Baltic Sea. Elsevier Oceanographic Series No. 30, pp. Krüger, G., Biester, E., and Jönsson, N. 1976. Preliminary 292-311. Ed. by A. Voipio. Elsevier Scientific Publishing results of the herring marking action in the Griefswalder Company, Amsterdam-Oxford-New York. Bodden in 1976. ICES CM 1976/P: 16, 8 pp. Lindroth, A., Larsson, P. O., and Bertmar, G. 1982. Where Kuczynski, J. 1984. Observations on the results and effec­ does the Baltic salmon go? In Coastal research in the Gulf tiveness of tagging of flounder, Platichthys flesus L., in 1981— of Bothnia, pp. 387-414. Ed. by K. Müller. Dr. W. Junk 1982. Bull. Sea Fish. Inst. Gdynia, 1-2: 22-26. Publishers, The Hague-Boston-London. Kühlmorgen-Hille, G. 1983. Infestation with larvae of Anisakis Mitans, A., and Rims. E. 1980. Promyslovyj vozvrat pokat- spec, as a biological tag of herring in Subdivision 22, Western nikov baltijskogo lososja. Rybnoe hozjajstvo, (6): 33-36. Baltic Sea. ICES CM 1983/J: 11,7 pp. Modin, J. 1987. A note on large catches of cod, Gadus morhua, Kändler, R. 1944. Untersuchungen über den Ostseedorsch in the Bothnian Sea during spring 1987. ICES CM 1987/J: während der Forschungsfahrten mit dem R.F.D. “Poseidon” 26, 7 pp. in den Jahren 1925-1938. Ber. Dtsch. Wiss. Komm. Meeres­ Moth-Poulsen, T. 1982. Genetic variation of cod from the forsch., 11(2): 137-245. Danish Sound: Interrelations of stocks from adjacent waters. Lablaika, I., and Uzars, D. 1982. Young cod abundance survey ICES CM 1982/G: 46, 19 pp. in the eastern Baltic. ICES CM 1982/J: 10, 21 pp. Mulicki, Z. 1959. The state of the South Baltic flatfish stock. Lamp, F., and Tiews, K. 1974. Progress report of comparative Rapp. P.-v. Réun. Cons. int. Explor. Mer, 147: 39-47. tagging experiments on the Baltic cod. Rapp. P.-v. Réun. 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Kuhan (Stizostedion lucioperca L.), hauen of Slupsk Furrow in the years 1957/1963. ICES CM 1968/F: (Esox lucius L.) ja lahnan (Abramis brama L.) vaelluksista, 7, 13 pp. kuolevuudesta ja kasvusta Helsingin edustan merialueella. Netzel, J. 1974a. Polish cod tagging experiments in the Baltic Manuscript, University of Helsinki, Department of Zoology, in 1969 and 1970. Rapp. P.-v. Réun. Cons. int. Explor. Mer, 73 pp. 166: 40-46. Lehtonen, H. 1981a. Biology and stock assessments of core- Netzel, J. 1974b. Polish investigations on juvenile cod in gonids by the Baltic coast of Finland. Finn. Fish. Res., 3: Gdansk Bay and in the southern part of the Bornholm Basin. 31-83. Rapp. P.-v. Réun. Cons. int. Explor. Mer, 166: 62-65. Lehtonen, H. 1981b. Stock assessment of the pike-perch Neuman, E. 1982. Species composition and seasonal (Stizostedion lucioperca L.) in the Helsinki sea area. Finn. migrations of the coastal fish fauna in the southern Bothnian Fish. Res., 3: 1-12. Sea. 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Långvandrande skånsk Flundernmarkierung in der Östlichen Ostsee. Fischerei-For­ havsöring. Sven. Fisk. Tidskr., 72: 109—113. schung, 18(2): 73-76. Svärdson, G., and Fagerström, A. 1982. Adaptive differences Weber, W. 1970. Mortality and relative strength of the herring in the long distance migration of some trout, Salmo trutta, stocks in the western Baltic. ICES CM 1970/H: 14, 6 pp. stocks. Inst. Freshwater Res. Drottningholm Rep., 60: 51- Weber, W. 1974. A tagging experiment on spring-spawning 80. herring of the Kiel Bay. ICES CM 1974/H: 7, 5 pp. Thurow, F. 1962. Results of German salmon tagging in the Weber, W. 1975. A tagging experiment on spring spawning sea. ICES CM/1962 No. 57, 2 pp. herring of the Kiel Bay. Ber. dt. w'iss. Komm. Meeresforsch., Thurow, F. 1969. Investigations on the sea life of Baltic salmon 24: 184-188. in 1968. Annls. biol. Copenh., 25: 235-236. Wieland, K. 1987. Distribution and mortality of cod eggs in Thurow, F. 1973. Local movements of salmon in the Baltic the Bornholm Basin (Baltic Sea). ICES CM 1987/G: 56, 14 Sea. ICES CM 1973/M: 7, 5 pp. PP- , . Thurow, F. 1985. The migrations of Baltic cod. Inf. Fisch- Wigren, B. J. P. 1962. Resultaten av sikmarkmngar mom wirtsch., 32(1): 9-14. Aland och vid Luvia. Hus Biol. Stat. Medd., 3: 1-34. Tiews, K., and Lamp, F. 1969. Further comparative tagging Winkler, H., and Thieme, T. 1978. Untersuchungen an den experiments on Baltic cod. ICES CM 1969/F: 19, 6 pp. Zanderbeständen der Kustengewässern der DDR. Wiss. Tiews, K., and Lamp, F. 1974. Preliminary results of cod Zeitschr. Wilhelm-Pieck-Univ. Rostock 27(4): 439-445. tagging experiments in the Baltic 1968—1971. Rapp. P.-v. Zarnecki, S. 1964a. First experiment with introduction of the Réun. Cons. int. Explor. Mer, 166: 51-61. Swedish salmon into the Vistula River in 1962 and 1963. Toivonen, J. 1968. Kuhan (Lucioperca lucioperca L.) vael- ICES CM 1964/No. 78, 6 pp. luksista, kasvusta ja kuolleisuudesta Suomenlahden saa- Zarnecki, S. 1964b. Experiment with introduction of the smolts ristossa, Saaristomeressä ja Ahvenanmaalla. Manuscript, of the Pomeranian sea trout into the Dunajec River (Upper University of Helsinki, Department of Zoology, 203 pp Vistula) in 1961. ICES CM 1961/No. 79, 7 pp. (mimeogr.). Zarnecki, S. 1966. Introduction of Atlantic salmon (Salmo Toivonen, J. 1973. The stock of salmon in the Gulf of Finland. salar) from the River Indals into the Vistula Basin. ICES ICES CM 1973/M: 17, 5 pp. CM 1966/L: 8, 14 pp. Toivonen, J. 1977. Differences in recaptures of wild and hatch­ Zarnecki, S., and Duszynski, J. 1961. Migrations of sea trout ery reared salmon smolts. ICES CM 1977/M: 7, 5 pp. from Pomeranian rivers (Results of tagging in 1960). ICES Toivonen, J., and Ikonen, E. 1978. Havsöringen i Finland. CM 1961/No. 53, 5 pp. Fiskeritidskrift for Finland, 5: 104-109. Zarnecki, S., Duszynski, J., and Gordziejczyk, J. 1962. A Toivonen, J., and Tuhkunen, A. 1975. Migration of sea trout further communication concerning migration of sea trout along the coastal waters of Finland on the basis of tagging from Pomeranian rivers. ICES CM 1962/No. 73, 11 pp. experiments. ICES CM 1975/M: 3, 13 pp.

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