News Fish

M74 mortality among the yolk-sac fry of Baltic salmon hatching later in the spring of this year will be higher than last year, predicts the Natural Research Institute Finland (Luke). It is estimated that one third of female salmon that returned to the River Simojoki to spawn in autumn 2016 were M74 salmon. All or a proportion of the offspring of these M74 salmon will die during the yolk-sac fry phase.

In addition to the River Simojoki, samples for M74 monitoring by Luke were collected from salmon ascending the River Iijoki and the Rivers Umeå and Dalälven on the Swedish coast of the Gulf of Bothnia. At worst, more than half of the ascending females were M74 salmon. These estimates are based on the thiamine (vitamin B1) concentrations in the salmon eggs.

In the figure, the light blue dots and line describe the median concentration of free thiamine in eggs of salmon that ascended the River Simojoki in 1994–2016. Red columns depict the proportions of female salmon whose offspring suffered from M74 in spring 1992–2016. The black column represents the predicted proportion of female-specific offspring groups that will suffer from M74 in spring 2017. The prognosis is based on the thiamine concentration in the eggs of salmon that returned to the River Simojoki to spawn in autumn 2016. (Graph Pekka J. Vuorinen / Luke)

The thiamine concentration in salmon eggs indicates the strength of M74

After an absence of several years, the M74 syndrome reappeared in salmon from the rivers of the Gulf of Bothnia last spring. The appearance of M74 mortality could be predicted from the thiamine concentration of the salmon eggs. The thiamine concentration in the eggs of salmon ascending to spawn in autumn 2015 had decreased to roughly one half of that in the previous year and further decreased in autumn 2016. The egg thiamine concentrations of salmon entering the River Simojoki have been measured since 1994. In the eggs of salmon entering the Rivers Umeå and Dalälven, thiamine has been measured during the two previous autumns. The thiamine concentrations in the eggs of these salmon had also decreased from those recorded in autumn 2015, when approximately 20% of salmon of the River Umeå and 40% of the River Dalälven were M74 females.

Ample fatty food depletes the thiamine reserves of salmon

Among the female salmon there were also many ‘wigglers’ that, due to thiamine deficiency, could not swim normally. Some of these salmon, which lay on their side as the spawning time approached, died before or shortly after egg stripping. It is known that wiggling salmon produce offspring that soon after hatching die of thiamine deficiency.

The 2014 year-class of sprat was very strong and the stock of cod, the principal predator of sprat, was weak. Thus, there has been an abundance of young, fatty sprat available as food for salmon in the main basin of the Baltic Sea and the Gulf of Finland. Ample fatty food depletes the thiamine reserves of salmon during the spawning migration and prespawning fasting period.

“However, the M74 mortality in spring 2017 will not yet reach the levels of the worst years in the 1990s, when nearly 80% of yolk-sac fry died of thiamine deficiency in several years,” explains researcher Marja Keinänen from Luke.

Normal newly-hatched yolk-sac fry of salmon from experimental incubation. At hatching, the length of fry is approximately 23 mm, and they develop and grow for several weeks, assimilating parent-derived yolk. If the egg thiamine concentration is too low, the fry die before they begin to swim and take external food. (Photo: Pekka J. Vuorinen / Luke)

Sufficient numbers of salmon must get to spawn

Worsening of the M74 syndrome will inevitably impair the reproductive success of salmon, which will only become evident after several years in the numbers of ascending salmon. In order for the mortality caused by M74 to remain as low as possible, it must be ensured that large numbers of salmon have the possibility to spawn. Fortunately, record numbers of salmon have been detected on the spawning migration in the Rivers Tornionjoki and Simojoki during several recent years as a consequence of strong year classes.

The Baltic Salmon and Trout Assessment Working Group of the International Council for the Exploration of the Sea (ICES WGBAST) annually requires a prognosis and data on the incidence of M74 to assess the status of the salmon stocks and based on this provide recommendations for the salmon fishing quotas. With M74 becoming worse, restrictions on the salmon fishery might need to be tightened and the quotas set at lower levels. The Baltic salmon stocks approached extinction in the 1990s, but recovered because of strict fishing restrictions and large-scale and expensive recovery measures.

Luke is also responsible for safeguarding the natural salmon stocks by keeping salmon brood fish stocks, producing adequate numbers of eggs for compensatory introductions and smolts for strengthening of the natural salmon stocks and for other introductions. In the event of an increase in M74, hatcheries must prepare to bathe eggs or yolk-sac fry in thiamine solutions. Smolt production had to be increased to save the salmon stocks during the 1990s and 2000s.