Yolk-Sac-Fry Mortality (M74) In Baltic Sea Salmon (Salmo Salar L.): Analyses Of Its Relation To The Diet

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In recent years, Baltic Sea salmon (Salmo salar) have suffered reproduction difficulties, due to high mortality during the larval phase (the M74 disease). Analyses oflong term mortality records (1928-98) from two salmon hatcheries indicate that M74 did not occur or was infrequent before the 1970s. The disease can be cured by thiamine treatment and this indicates that the mortality results from a thiamine deficiency. The frequency of M74 is strongly correlated to the abundance of sprat (Sprattus sprattus (L.)) and it has been suggested that M74 results from changes in the food web. To study the possible role of diet changes in relation to M74, we have compared stomach content in salmon collected 1959-62 and 1994-97. Sprat, herring (Clupea harengus L.) and threespined stickleback (Gasterosteus aculeatus L.) dominated the diet in both periods. The average size of consumed sprat and herring was significant smaller in the 1994-97 compared to 1959-62. The results.also indicate that herring, and to some extent also threespined stickleback, has increased in the diet of salmon. This suggests that sprat constituted a smaller part of the salmon diet in 1994-97. It is thus difficult to explain the M74 with its positive correlation to the sprat population alone. The ultimate cause ofM74 and the thiamine deficiency is not known, but our hypothesis is still that it is related to foodweb changes. Possible causes includes: changes in thiamine or thiaminase content in forage fish (related to the decreased size of these fish in the diet or their decreased growth/condition), increased growth rate and winter-feeding of salmon and general changes in thiamine synthesis in the Baltic Sea (potentially influenced by eutrophication).