M74 is the designation given to a reproduction disorder affecting salmon (Salmo salar) returning from their feeding migration in the Baltic Sea. The syndrome manifests as symptoms and increased mortality during the yolk-sac fry phase. The mortality of yolk-sac fry is due to thiamine (vitamin B1) deficiency in the eggs.

Research conducted by the Natural Resources Institute Finland (Luke) indicates that thiamine deficiency develops when the fat content of prey fish is very high. Then the thiamine concentration is too low in relation to the dietary energy and unsaturated fatty acid content.

The cause is abundant fatty diet

An exceedingly large proportion of young sprat (Sprattus sprattus) in the food of salmon intensifies the occurrence and severity of the M74 syndrome. At times when there is an abundance of young sprat in the sea, salmon feed on fat prey with high energy content. The higher the fat content of their food, the more thiamine salmon need.

An abundance of unsaturated fatty acids and a scarcity of thiamine increases susceptibility of lipids to oxidation reactions which consume thiamine. The thiamine reserves of salmon are thereby depleted during their long spawning run and pre-spawning fasting. Too little thiamine is transferred from the parent fish tissues to the eggs to last to the end of the yolk-sac fry phase.

M74 can be prevented by maintaining the cod populations stable and controlling the size of the sprat population.

The M74 syndrome was at its worst in the early and mid-1990s. Restoration of the few naturally breeding salmon populations required massive investments. The value of merely stocked smolts and younger fry currently stands at over 10 million euros.

Luke predicts and monitors the M74 syndrome

The prediction and annual monitoring of the M74 syndrome is important, because it can suddenly intensify at any time. For this reason, Luke annually catches salmon on their spawning run into the River Simojoki. The thiamine content of unfertilised eggs is determined in order to predict the incidence of M74 and verify that the mortality of yolk-sac fry is attributable to M74. The control patches of the eggs are hatched at the Keminmaa fish farm. Depending on the situation, salmon ascending the River Torniojoki and Kymijoki can also be monitored. The eggs of salmon running into Swedish rivers emptying into the Gulf of Bothnia are also sampled to be analysed for thiamine in Finland in order to foresee M74.

Data on the annual occurrence of the M74 syndrome is required in order to assess the status of the naturally spawning salmon populations, the planning and regulation of salmon fishing, the establishment of broodfish shoals for aquaculture, and the production of smolts for mandatory introductions.

Luke is responsible for safeguarding the preservation of naturally spawning salmon populations. It maintains broodfish stocks and produces juveniles for stockings to strengthen naturally spawning populations, or introduces salmon to waters from where it has disappeared. If M74 worsens, eggs/yolk-sac fry need to be treated with thiamine in order to avoid fry mortality when broodfish stocks are renewed, or when new fry are produced for further breeding for stocking purposes, for example.

The Assessment Working Group on Baltic Salmon and Trout of the International Council for the Exploration of the Sea (ICES WGBAST) requires annual prognosis of and data on mortality attributable to M74, in order to assess the state of salmon populations and draw up recommendations for salmon fishing quotas. The Working Group has used Finnish monitoring information on M74 to model its impact on the salmon populations of all rivers emptying into the Gulf of Bothnia. Data on M74 can probably be used retrospectively, to describe the abundance ratios of the pelagic fish populations of the Baltic Sea.

The incidence of M74 in salmon from rivers of the Bothnian Bay of the Baltic Sea in 1985/1986–2017/2018 (spawning year / hatching year). Graph: Pekka J. Vuorinen.

Explanations for the figure:

Thiamine = median concentration (nmol/g) of free thiamine (vitamin B1) in unfertilized eggs.

M74(100%)Females  = the proportion (%) of females with the concentration of free thiamine in unfertilized eggs < 0.2 nmol/g or whose offspring have all died.

M74 Females = the proportion (%) of females with the concentration of free thiamine ≤ 1.0 nmol/g in unfertilized eggs or whose yolk-sac fry have shown M74 symptoms and some of them have died.

YSFM = the mean annual yolk-sac fry mortality rate of all females; the vertical line indicates the standard error of the mean (SE).
The numbers of females or egg batches in M74 monitoring are given in parentheses below the date axes.

Female-specific mortality among yolk-sac fry of Simojoki salmon has been recorded from spring 1986 onwards. The concentration of thiamine has been analyzed for all monitored females from autumn 1994 onwards. Data for salmon ascended the Rivers Tornionjoki, Iijoki and Kemijoki are also included.

Picture on top of the page: Pekka J. Vuorinen.