Human fishing activity has evolutionary consequences on Atlantic salmon maturity that work in opposite directions, a new study finds. The results reveal the impacts of size-selective fishing techniques and large-scale harvesting of a key salmon prey species.
Human activities can impose powerful selective pressures leading to rapid evolutionary changes in wild species. Heavily exploited species like fish provide some of the best examples of human-induced evolutionary change.
For example, large-scale fishing has caused early maturation and changes in adult body size in fish populations. Despite this, identifying the evolutionary consequences of human activities remains a challenge. This is especially true for indirect impacts – where a third species mediates the impact of one species on another.
Atlantic salmon have a complex life cycle; Born in rivers and streams, salmon spend the first years of their life in the ocean. When they mature, they leave the ocean and return to their ancestral freshwater ecosystems to spawn. The age at which mature Atlantic salmon return to freshwater – their age at sea at maturity, or age at sea – is an important and genetically relevant vital trait. H
Here, Yann Czorlich and his colleagues identify two types of fishing-induced evolution in Atlantic salmon – both direct and indirect – that have opposing effects on salmon age at sea. Using a multi-species, multi-factor model and four decades of fishing data from a particularly biodiverse native Atlantic salmon population of northern Europe.
Czorlich et al. show that net fishing in river systems has a direct impact on sea age by selecting against early maturation. The authors also found an indirect effect related to the harvesting of a salmon prey species – tiny marine fish called capelin.
According to the authors, the way in which capelin are harvested as a source of food for aquaculture indirectly opposes the late maturation of salmon. “Commercial harvesting of an important salmon prey species, capelin, appears to have indirectly induced the evolution of the age of Atlantic salmon at maturity to younger, smaller individuals,” the authors write.
“Our results therefore identify a novel indirect pathway by which Atlantic salmon aquaculture may affect wild populations of the same species and underscore the importance of identifying alternative and sustainable sources of protein for the aquaculture industry.”