Yolk-Sac-Fry Mortality (M74) In Baltic Sea Salmon (Salmo Salar L.): Indications From Where And When They Feed

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Baltic Sea salmon have suffered reproduction difficulties in recent years, due to high larval mortality (the M74 syndrome). Records from two salmon hatcheries indicate that M74 did not occur or was infrequent before the 1970s. We examined whether differences in feeding areas and timing of the spawning migration could help in explaining the variation in M7 4 frequencies among salmon stocks from different rivers. The disease was found in salmon populations with a feeding migration limited to the Gulf of Bothnia, in others that mainly feed in the Gulf of Finland as well as in populations feeding almost exclusively in the Baltic Proper. This indicates that the agent(s) causing M74 is present in all these areas. However, the M74 frequency varied considerably among different populations, with the highest incidence among salmon from the northern Gulf of Bothnia. Latvian salmon from the River Daugava have not shown signs of M74, even though Baltic salmon populations from most other areas were severely affected. This population is primarily feeding in the eastern part of the Baltic proper and seems to enter the Gulf of Riga later during the spawning run. It has a later spawning run and feeds later in the summer than other populations examined. It is suggested that this may help in avoiding M74.