The M74 syndrome is a reproduction disorder of salmon (Salmo salar) feeding in the Baltic Sea. M74 manifests as offspring mortality during the yolk-sac fry phase. Before dying, the yolk-sac fry display typical symptoms.  Thiamine (vitamin B1) deficiency in eggs is the immediate cause of M74 mortality. The deficiency can be prevented by thiamine treatments.
The thiamine deficiency syndrome M74 is related to the fat and thiamine content of prey fish. The diet of Baltic salmon leads to thiamine deficiency in eggs and consequently to the mortality of yolk-sac fry: The main prey species of the Baltic salmon are sprat (Sprattus sprattus) and herring (Clupea harangus membras). Average fat content is greater in sprat than in herring. The fat content is highest and the thiamine concentration is lowest in the youngest sprat. The need for thiamine depends on the amount of fat in the diet. 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.
Relationships between fish stock changes in the Baltic Sea and the M74 syndrome: The M74 syndrome is connected to a weak Atlantic cod (Gadus morhua) stock and strong year classes of sprat in the Baltic Sea. Since the collapse, heavy fishing mortality as well as predation on cod eggs by sprat and food competition between sprat and young-of-the-year cod has inhibited cod recovery. Coincidentally with the decline in the cod stock since 1982, and following the consequent reduction in predation pressure, the sprat stock increased rapidly, and salmon therefore had more food. Salmon grew faster, resulting also in a high CF.
- Marja Keinänen; et al. (March 19, 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 (4): 516–528. doi:10.1093/icesjms/fss041.
- Marja Keinänen; et al. (2008). "The M74 syndrome of Baltic salmon: the monitoring results from Finnish rivers up until 2007" (PDF). Riista- ja kalatalous – Selvityksiä. 4/2008: 21.
- P. Koski (June 30, 2002). "Parental Background Predisposes Baltic Salmon Fry to M74 Syndrome". Acta Veterinaria Scandinavica 43 (2): 127–130. doi:10.1186/1751-0147-43-127. PMC 1764190. PMID 12173502.
- Jaakko Mikkonen; et al. (2011). "Relationships between fish stock changes in the Baltic Sea and the reproductive syndrome, M74 of the Atlantic salmon (Salmo salar L.)". ICES Journal of Marine Science 68 (10): 2134–2144. doi:10.1093/icesjms/fsr156.