Stomach analyses of Baltic salmon from 1959-1962 and 1994-1997: possible relations between diet and yolk-sac-fry mortality (M74)

In recent years, Baltic Sea salmon Salmo salar, have suffered high larval m ortality (M74) which can be cured by thiamine treatment. Analyses of lung t erm mortality records (1928 1998) from two salmon hatcheries suggest that b efore the 1970s M74 did not occur, or was less frequent. This indicates tha t varying M74 did not cause the long-term fluctuations of Baltic salmon cat ches in this period. The frequency of M74 has been correlated positively to the abundance of the salmon's primary prey, sprat Sprattus sprattus. Sprat , herring Clupea harengus and three-spined stickleback Gasterosteus aculeat us dominated the diet in both the periods of study, 1959-1962 and 1994-1997 . The mean size of consumed sprat was significantly smaller in 1994 1997 co mpared with 1959-1962. Herring and, to a lesser extent, three-spined stickl eback; increased in the diet of salmon, while sprat appeared to constitute a smaller part of the diet in 1994-1997. The cause of M74 and the thiamine deficiency involved remains unknown, but is thought to be related to change s in thiamine or thiaminase content in forage fish, winter-feeding of salmo n or general changes in the pelagic food web. caused by overfishing or eutr ophication. (C) 2001 The Fisheries Society of the British Isles.