Fat content of prey fish linked to decline in numbers of salmon ascending the Rivers Tornionjoki and Simojoki
The high-fat fish diet consumed by salmon during their first year of migration in the Baltic Sea appears to impair their survival and reduce the number of salmon returning to spawn after two years. The findings from 12 years are presented in a newly published scientific article by Keinänen et al. 2025.
"Salmon post-smolts have survived well in the sea in years when the main basin of the Baltic Sea has had an abundance of 0-year-old herring hatched in the same summer and a scarcity of youngish sprat from the previous years. For example, in 2014, a record-high year-class of herring hatched, and two years later, in 2016, record-high numbers of salmon ascended the Rivers Tornionjoki and Simojoki. In contrast, the herring year class hatched in 2021 was exceptionally small, and the number of sprat relative to the number of 0-year-old herring was the highest in the study period. This may have caused a collapse in the number of ascending salmon in 2023," says Dr. Marja Keinänen.
Most salmon post-smolts from the rivers of the Bothnian Bay in the Gulf of Bothnia migrate to feed to the main basin, which encompasses the southern part of the Baltic Sea. There, small herring are better food for young salmon than sprat. Sprat have too much fat in relation to protein for the growth of young salmon. In addition, the intake of thiamine, or vitamin B1, in relation to the calorie content of the diet is lower from sprat than from herring. Too much fat in the diet causes thiamine deficiency and impairs the survival of post-smolts.
The study only examined the connection between the diet in the main basin of the Baltic Sea and the numbers of ascending salmon. Jari Raitaniemi, a senior researcher at the Natural Resources Institute Finland (Luke), ponders “whether post-smolts might have had survival difficulties in the Bothnian Sea of the Gulf of Bothnia in 2021, when small herring, which are suitable prey for young salmon, were fatty there, even though large herring were starving”.
Baltic herring, sprat and cod stocks affect salmon survival
In the main basin of the Baltic Sea, the proportion of sprat has been larger than that of herring since the 1990s. As a result, sprat has accounted for a considerably larger proportion of the post-smolts’ prey than herring.
In their first year of migration in the main basin, post-smolts need herring of the same year for their prey, instead of youngish sprat, which is too fatty for them. “It is important that there are enough small herring in the southern Baltic Sea so that the post-smolts survive to become ascending salmon,” says researcher Jukka Pönni from Luke.
Older salmon in particular also benefit from sprat when the herring stock is small, especially in the southernmost Baltic Sea, where herring quickly grow too large.
– Also cod indirectly affects salmon stocks, as a weak cod stock allows sprat to increase in abundance. When regulating the fish stocks of the Baltic Sea and setting fishing quotas, the effects of sprat, herring and cod on salmon survival should be taken into account, Raitaniemi points out.
The data used in the study on herring and sprat stocks for 2010–2021 and salmon for 2012–2023 were obtained from publications of the International Council for the Exploration of the Sea (ICES). Research institutes in the Baltic Sea countries, such as Luke, produce data for ICES publications. The study also utilized Luke's data on ascending salmon and M74 monitoring.
Article:
Reduced numbers of returning Atlantic salmon (Salmo salar) and thiamine deficiency are both associated with the consumption of high-lipid prey fish. Marja Keinänen, Jari Raitaniemi, Jukka Pönni, Tiina Ritvanen, Timo Myllylä, Pekka J. Vuorinen. Fishes 2025, 10(1), 16; https://doi.org/10.3390/fishes10010016
The M74 syndrome
The survival of post-smolts began to decline around the same time as the M74 syndrome in salmon broke out violently in the early 1990s. M74 is the name given to the thiamine deficiency caused by a diet rich in fatty fish, which affects fatty predatory fish, such as Baltic salmon. The cause and mechanism of thiamine deficiency have been solved by Dr. Marja Keinänen and Associate Professor Pekka Vuorinen, who conducted M74 studies at Luke.
“When too little thiamine is transferred to the eggs from a female salmon that has eaten too much fatty fish, the offspring die from thiamine deficiency during the yolk-sac phase. In the worst case, adults die before spawning. It now seems likely that post-smolts are also sensitive to a fish diet that is too fatty,” says Keinänen.
Young salmon or post-smolts
After leaving their home river in May-June, migratory young salmon, i.e., post-smolts, move from the Gulf of Bothnia to the main basin in early autumn after growing to 25–35 cm.
In the Baltic Sea, species diversity is low, unlike in the Atlantic Ocean, where salmon feed on many fish species and many low-fat invertebrates. In the main basin of the Baltic Sea, young salmon feed on small fish, herring, sprat and some three-spined stickleback, all of which are fatty.
“The majority of salmon return to their home river to spawn after a two-year sea migration. The differences in the annual variation in the numbers of ascending salmon between rivers are probably due at least in part to differences in the diet of the post-smolts of the rivers. Since the post-smolts of the River Tornionjoki are larger, upon reaching the main basin they appear to eat mainly 1–3-year-old sprat and 0-year-old herring and the smallest 1-year-olds. The post-smolts of the River Simojoki, being slightly smaller, eat younger, 1–2-year-old sprat and the smallest 0-year-old herring,” says Associate Professor Pekka Vuorinen.
