Fisheries Are Hurting Salmon Populations

When we think of evolution we think of biological changes spanning centuries. We think of unbelievable concepts like dinosaurs being ancestral birds and how bat wings and human hands have far more in common than not. Evolution, however, is not defined by a beginning and an end, but it is a dynamic concept— evolutionary mechanisms are constantly in motion and animals are constantly evolving. 

    Recently, research has begun to focus on the impact of human activities on the evolution of wild animals. Human activities, like fishing, sometimes interfere with the “natural selection” that would otherwise dictate a species' evolution. As industries grow, and humans reach farther and farther into nature, it becomes inevitable that the evolution of animals will be affected, no matter how wild.

One animal that caught the attention of scientists was the Atlantic salmon. 

Atlantic salmon in the Teno River, Finland

To understand the influence of humans on salmon evolution one must first understand salmon and their lifecycle. Atlantic salmon (Salmo salar) is an anadromous species, meaning their lives are spent in both freshwater and saltwater environments. Salmon, specifically, are born in freshwater, reach maturity at sea, and then return to freshwater grounds to give birth and die.  

The “sea-age” of a salmon is the number of how many years an individual fish spends reaching maturity in a saltwater environment like the sea— this sea-age is important in determining both the survival and reproductive ability of the individual salmon. Salmon that mature late are typically larger in size with a lower survival rate but high reproductive success. Salmon that mature early are usually small with a high survival rate and low reproductive success. Typically, nature decides which trait is more beneficial, and these genes are the ones that continue to propagate in the salmon population— essentially, fish with traits bad for survival die early and do not reproduce— we call this phenomenon “natural selection”. However, researchers from Finland have just identified two types of fisher-induced evolution that deviate from what is naturally occurring in salmon populations. 

Capelin are fish from the smelt family; they are common prey for salmon

The first way in which human activity affects salmon doesn’t even have to do with the salmon directly— it has to do with a group of fish that make up part of the salmon’s food supply. Capelin are fish in the smelt family, and in the Barents Sea they are being fished and harvested alongside herring and krill, other food sources for salmon. In the study, it was found that increasing biomass of these food sources leads to an increase in the proportion of Atlantic salmon that mature later— leading to salmon with larger size and higher reproductive ability, but low survivability. As fewer salmon survive predators, fewer salmon reproduce, and this could lead to a tragic loss in the salmon population. 

Salmon net-fishing, also called salmon-netting

Net-fishing— a human activity— primarily targets small, early-maturing salmon and the model in the study showed a positive correlation between the number of net fishing licenses and late-maturing salmon, as late-maturing salmon were not fished. Other factors like temperature and rod fishing licenses were taken into account, but none showed as significant an association as the one between net fishing and late-maturation in salmon populations. 

Overexploitation in the fishing industry is a serious problem and could pose shortages and endangerment in the near future. Man-made natural selection for salmon threatens a massive reduction in the salmon population, and overfishing of capelin, their prey, is also linked to this drop-off. It is important to recognize the food cycles prevalent in nature, and the delicate balance that must be maintained to support healthy, vibrant populations that will sustain humans for generations to come.