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Early fishing season actually alters the genetic evolution of Atlantic salmon size

A new study by the University of Helsinki raises concerns about the long-term impacts of early summer salmon fishing. The research suggests a potential link between fishing pressure during the early season and a decline in the average size of Atlantic salmon. This change could be attributed to evolutionary processes influenced by human activity.

Genetic factor in salmon size and growth

Atlantic salmon possess a genetic variant that influences their growth and maturity patterns. Individuals carrying this variant adopt a life history strategy characterized by larger body size and later maturation. This strategy allows salmon to invest more energy in growth before diverting resources towards reproduction.

Larger females can produce more eggs, and larger males have a competitive advantage during spawning. This increased potential for reproductive success over their lifetime provides an evolutionary benefit to salmon carrying the “large salmon variant.” Larger fish also offer a more substantial catch, making them highly desirable to anglers.


“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 Dr. Antti Miettinen.

The study revealed a concerning trend. Salmon possessing the “large salmon variant” were more likely to be caught during the early part of the fishing season. This isn’t random. Early-arriving salmon tend to be larger and older, making them more attractive targets for anglers. Over multiple seasons, the consistent removal of larger fish with the valuable growth gene creates a form of artificial selection.

Fish populations naturally exhibit a range of genetic variations. When fishing pressure heavily targets a specific trait, like the larger size associated with the “large salmon variant,” it disrupts the natural balance of these variations.

Over time, fish with the variant become less common in the population. Conversely, salmon that mature earlier and are smaller in size – traits not targeted by early season fishing – survive and reproduce at a higher rate.

Long-term consequences for salmon populations and size

If this trend of targeting larger fish during the early season continues unchecked, it could have serious consequences for salmon populations in the long run. Here’s why:

Reduced genetic diversity

Within a population, a wide range of genetic variations improves resilience. Different gene combinations provide a wider array of traits, offering better chances of some individuals adapting to diseases or changing environments.

Targeting larger fish disproportionately removes those carrying the “large salmon variant.” This gene, which promotes size and later maturation, becomes less common over generations. The loss of this beneficial trait limits the population’s potential to rebound when conditions improve.

Less genetic diversity can lead to inbreeding and make the population more susceptible to diseases. It can also hinder their ability to adapt to changes like warming water temperatures or shifts in prey availability.

Smaller average salmon size

As larger salmon are consistently removed, the average size of the population gradually declines. Over many generations, this decrease could become substantial.

Salmon play a crucial role in their ecosystems, both as predators and prey. Smaller salmon may be less effective hunters and more vulnerable to their own predators. These changes can disrupt the delicate balance of the food web.

Large adult salmon carry a significant amount of nutrients back to freshwater when they spawn. These nutrients boost the growth of insects and other organisms, which in turn support birds and other wildlife. Smaller salmon contribute fewer nutrients, potentially affecting the entire ecosystem.

Reproductive success

In fish, there’s a strong correlation between body size and egg production. Larger females produce a higher number of eggs, which translates to more potential offspring.

When smaller salmon dominate the population, fewer eggs are produced overall. Without sufficient new members joining the population, it can gradually decline over time.

Reduced reproductive success makes it harder for populations to recover from natural fluctuations or additional pressures like habitat loss or reduced food availability.

Case of the Baltic salmon

The researchers focused on wild salmon in the northern Baltic Sea, where the impact was noticeable. Salmon caught early in the season, especially in upstream areas of the Tornio and Kalix Rivers, were more likely to have the “large salmon variant.”

“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,” said Miettinen.

Implications for conservation and fishing

This study highlights the complex ways our actions can shape wild animal populations. Fishing, if not carefully managed, can inadvertently drive evolutionary changes over time. Policymakers and fishing communities face the challenge of balancing immediate needs with the long-term health of salmon stocks.

“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 study co-author Dr. Victoria Pritchard.

The researchers emphasized the need for continued monitoring and research. The genetic tools created in this study provide a way to track the effects of changing fishing practices. It’s vital to find solutions that protect salmon populations while supporting sustainable fishing.

The study is published in the journal Evolutionary Applications.


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