Early summer fishing can have an evolutionary impact, resulting in smaller salmon

Atlantic salmon are caught by fisheries when the fish are migrating to spawn. A new study led by the University of Helsinki explored how salmon caught at different times during their spawning migration differ from each other genetically. The study on wild salmon in the northern Baltic Sea revealed that especially in the early part of the fishing season, fishing strongly targets salmon carrying a 'large salmon genetic variant'. The variant guides Atlantic salmon to grow large and to mature at an older age, which is an important trait for the fishing and viability of salmon stocks.

Genetic analyses on thousands of wild salmon caught between 1928 and 2020 by fisheries from the northern Baltic Sea region showed that regardless of the year, fishers caught salmon with the 'large salmon variant' more often in the early than late fishing season.

"This finding suggests that the timing of fishing may cause evolutionary changes in the age and size that Atlantic salmon reach before maturation. Intensive fishing especially in the early fishing season may lead to the 'large salmon variant' becoming rarer and to salmon spawning at a younger age and smaller size," explains the lead author of the study Antti Miettinen, PhD, from the Faculty of Biological and Environmental Sciences at the University of Helsinki.

This kind of evolutionary impact resulting in fewer large salmon would be bad news for the diversity and viability of salmon populations and for fishers who highly value large catches.

Valuable information for conservation

The results of the study help to understand how the timing of evolutionary selection pressures induced by human actions, in this case fishing, can affect wild fish populations and properties important to them.

The largest wild Baltic salmon stock spawns in the Tornio and Kalix Rivers in northern Finland and Sweden and has high ecological and societal value. The study found that early-season fishing at sea and in the Tornio River caught salmon that originate from upstream sites in the river system more often than did fishing in the later parts of the season.

"Fishing in the early part of the fishing season targets salmon that spawn in the headwaters of these rivers, which should be accounted for in fisheries management so that it ensures the viability of these salmon populations," Miettinen says.

Over the years, the timing of the legal fishing season in the Baltic Sea and along its salmon rivers has sparked heated debate and questions on national and international levels. The published study addressed a particular concern: whether fishing in the early season can reduce the mean age and size of Baltic salmon.

"By analysing the genetics of samples collected across the northern Baltic over many decades, this study shows how human activities could cause evolutionary changes in wild salmon populations," says senior researcher and senior author of the study Victoria Pritchard, PhD, from the University of the Highlands and Islands.

"This study is a fantastic example of using genetic approaches to answer important questions about the conservation and management of biodiversity. The genetic tools designed during this project can be used to monitor the future impacts of fishing regimen changes," Pritchard says.

The research was done in collaboration with the University of Helsinki, Natural Resources Institute Finland (Luke), Swedish Agricultural University (SLU) and the University of the Highlands and Islands (UHI).

The samples analysed in the study were from the archives of Natural Resources Institute Finland (Luke) and the Swedish University of Agricultural Sciences (SLU). The samples were collected between 1928 and 2020 by fishers along the Tornio and Kalix Rivers and coastal Bothnian Bay in the northern part of the Baltic Sea.

The study was funded by the Ministry of Agriculture and Forestry of Finland, Finnish fisheries management fees, Tornio/Torne River fishing license revenues, Natural Resource Institute Finland, the Alfred Kordelin Foundation, the Kuopio Naturalists' Society (the Betty Väänänen fund), the Raija and Ossi Tuuliainen Foundation, Societas pro Fauna et Flora Fennica, the Finnish Foundation for Nature Conservation (the Baltic Sea Fund), the Swedish Agency for Marine and Water Management, Swedish Research Council Formas, Kempestiftelserna, the Finnish Society of Sciences and Letters, the European Union and the University of Helsinki.