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.