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Early-season fishing may shrink the size of salmon

A recent genetic study led by the University of Helsinki has found that heavy fishing at the beginning of the fishing season may result in younger Atlantic salmon that are smaller in size. 

This discovery is vital for conserving large fish, which are essential for the diversity and sustainability of salmon populations.

Large salmon genetic variant 

Atlantic salmon are typically caught during their spawning migration. The study, focusing on wild salmon in the northern Baltic Sea, revealed that early-season fishing predominantly targets salmon carrying a “large salmon genetic variant.” 

This variant is responsible for guiding salmon to grow larger and mature at an older age – an important trait for both the viability of salmon stocks and the fishing industry.

Genetic analyses of thousands of wild salmon caught between 1928 and 2020 by fisheries from the northern Baltic Sea showed that fishers consistently caught salmon with the large salmon variant more frequently in the early part of the fishing season, regardless of the year.

Timing of fishing impacts salmon size

“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,” explained lead author Antti Miettinen, a biologist at the University of Helsinki.

Such evolutionary impacts, resulting in fewer large salmon, would negatively affect the diversity and viability of salmon populations and the fishing industry, which values large catches.

Human-induced evolutionary selection pressures 

The results of the study are crucial for understanding how human-induced evolutionary selection pressures, such as fishing, can affect wild fish populations and their important traits. 

The largest wild Baltic salmon stock spawns in the Tornio and Kalix Rivers in northern Finland and Sweden and holds high ecological and societal value. The researchers found that early-season fishing at sea and in the Tornio River targeted salmon from upstream sites in the river system more often than fishing later in the season.

Evolutionary impacts of fishing on salmon

“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 to ensure the viability of these salmon populations,” Miettinen explained.

“By analyzing 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,” said senior author Victoria Pritchard, a biologist at 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.”

Atlantic salmon 

Atlantic salmon is a species primarily found in the northern Atlantic Ocean and rivers that flow into this ocean. These fish are anadromous, meaning they hatch in freshwater, migrate to the ocean as they grow, and then return to freshwater to spawn. 

Unique life cycle

The life cycle of Atlantic salmon makes them unique among many fish species because of this long migration, which can be quite arduous and fraught with hazards, from natural predators to human-made obstacles like dams.


Their appearance is distinctive, with a silver-flanked body during their ocean phase and darker, more pronounced coloring when they return to spawn. 


Atlantic salmon are also known for their ability to “leap” upriver, a necessary skill to navigate upstream to their spawning grounds.

Ecological role 

Historically, Atlantic salmon have been a vital part of both the ecosystem and regional economies, supporting commercial and recreational fishing industries. 

Population decline 

However, their populations have faced significant declines due to overfishing, habitat loss, and pollution, leading to strict regulations and conservation efforts in many areas they inhabit. 

These efforts include hatchery programs and habitat restoration projects to support their numbers and maintain their role in the biodiversity of their native regions.

The study is published in the journal Evolutionary Applications.


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