Mortality rate of offspring of Baltic salmon suffering from M74 can be predicted using a new thiamine model

The M74 syndrome is a reproductive disorder of Baltic salmon caused by a high-fat fish diet. As a result, the eggs have too little vitamin B1, or thiamine, and the fry die during the yolk-sac phase. Now, the M74 mortality rate for yolk-sac fry can be calculated from the thiamine concentration of eggs using a model developed at the Natural Resources Institute Finland (Luke). Thiamine concentrations can be measured as soon as salmon spawn in late autumn, more than six months before M74 mortality can be detected in yolk-sac fry.

By now, the number of females whose offspring suffer from the M74 syndrome has been predicted from thiamine. The model now compiled enables a cost-effective assessment of female-specific M74 mortality percentages of yolk-sac fry without incubating eggs and hatched yolk-sac fry for more than six months.

– The model is based on a comparison of the results obtained in laboratory incubation on M74 symptoms and mortality of yolk-sac fry over 16 years with free thiamine concentrations in unfertilized eggs. The new model helps in the management and utilization of salmon stocks, says adjunct professor Pekka Vuorinen.

The thiamine model is presented in a scientific article published in July 2021, which also summarizes the variation in the M74 syndrome and the causes of the variation, as well as yolk-sac fry mortality over a 30-year period (1985–2020).

Of the most important prey fish of salmon, sprat and herring, sprat are more fatty, and young sprat have the most fat. M74 syndrome is more prevalent when large year classes of sprat have recruited, and especially when cod stocks exploiting sprat have been weak.

– The size of salmon and their prey fish affects the intensity of the M74 syndrome. Smaller salmon can only eat small fish, that is, mainly small / young sprat. In contrast, large salmon can also eat larger sprat and herring. But when there are a lot of young fatty sprat in the sea, even large salmon mainly eat them, explain PhD Marja Keinänen and docent Pekka Vuorinen.As most salmon in all rivers in the Bay of Bothnia migrate to the southern Baltic Sea on their feeding migrations, the annual M74 mortality of salmon in different rivers varies in the same way. The intra-annual variation is due to differences in salmon migration and thus to differences in prey fish.

In addition to yolk-sac fry, female and male brood salmon can also suffer from thiamine deficiency. In them, the syndrome manifests itself as passivity and unbalanced swimming, and they can die even before spawning.

M74 mortality has varied by nearly 60 percentage points in consecutive salmon spawning years and its incidence can be predicted in only one salmon spawning year at a time.

– The incidence and mortality of M74 must be predicted and monitored annually. M74 syndrome is a threat to both wild salmon and in fish cultivation to salmon production from wild-derived eggs. At worst, most salmon yolk-sac fry died of M74 syndrome in the 1990s, and resuscitation of salmon stocks required major financial efforts. In 2016–2019, the average M74 mortality was 7–27 %, states principal specialist Petri Heinimaa.