Keinänen, M., Uddström, A., Mikkonen, J., Casini, M., Pönni, J., Myllylä, T., Aro, E., and Vuorinen, P. J. 2012. The thiamine deficiency syndrome M74, a reproductive disorder of Atlantic salmon (Salmo salar) feeding in the Baltic Sea, is related to the fat and thiamine content of prey fish. – ICES Journal of Marine Science, 69: 516–528. This study clarifies how the diet of Baltic salmon leads to thiamine deficiency in eggs, and consequently to M74 mortality of yolk-sac fry. The main prey species, sprat (Sprattus sprattus) and herring (Clupea harengus membras), and their biomass in the Baltic Proper (BPr) and the Bothnian Sea, the two feeding grounds of salmon originating from the northern Gulf of Bothnia rivers, are compared. The thiamine concentration of both prey species is lowest in the youngest age groups. Because average fat content and energy density are greater in sprat than in herring, and greatest in youngest sprat, the supply of thiamine per unit energy is least in a diet containing many young sprat. Also, the greater is the supply of thiamine and fat from sprat in the southern BPr in the preceding year, the lower the concentration of thiamine in salmon eggs. Thiamine deficiency in eggs results from an unbalanced diet abundant in fatty prey fish, such as young sprat, from which the supply of thiamine is insufficient in proportion to the supply of energy and unsaturated fatty acids for salmon, which must undergo a long prespawning fasting period.

Mikkonen, J., Keinänen, M., Casini, M., Pönni, J., and Vuorinen, P. J. 2011. Relationships between fish stock changes in the Baltic Sea and the M74 syndrome, a reproductive disorder of Atlantic salmon (Salmo salar). – ICES Journal of Marine Science, 68: 2134–2144. The M74 syndrome of Baltic salmon (Salmo salar), which appears as increased yolk-sac fry mortality (YSFM), impairs the reproduction of salmon stocks. Changes in the prey stocks of Baltic salmon in its two feeding areas, the southern Baltic Proper (BPr), where sprat (Sprattus sprattus) was the main prey species during the high incidence of M74, and the Bothnian Sea, where herring (Clupea harengus) is the dominant species, were analysed in relation to salmon growth and size and in relation to the incidence of M74. The high condition factor (CF > 1.05) of prespawning salmon predicted high YSFM. From the various stock factors of sprat and herring in the southern BPr, the biomass of sprat had the strongest positive relationships with the CF of prespawning salmon, and the total prey biomass with YSFM. It is concluded that the ample but unbalanced food resources for salmon in the BPr, primarily sprat, induce M74. By reducing the fishing pressure on cod (Gadus morhua) and by more effectively managing the sprat fishery in years when the cod stock is weak, the incidence of the M74 syndrome could be reduced and even prevented.