Human fishing activity has direct and indirect consequences on salmon size

Human fishing activity has evolutionary consequences for Atlantic salmon maturity that work in opposing directions, according to a new study. The findings reveal impacts from size-selective fishing techniques and from the large-scale harvesting of a key salmon prey species. Human activities can impose powerful selective pressures resulting in rapid evolutionary change in wild species. Heavily harvested species like fish provide some of the best examples of human-driven evolutionary change. For example, large-scale fishing has resulted in 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 particularly true for the indirect impacts – where a third species mediates the impact of one species on another. Atlantic salmon have a complex life history; born in rivers and streams, salmon spend the first several years of their lives 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 fresh water – their sea-age at maturity, or sea age – is an important and genetically relevant life-history trait. Here, Yann Czorlich and colleagues identify two types of fisheries-induced evolution in Atlantic Salmon – both direct and indirect – that have opposing effects on salmon sea age. Using a multi-species, multi-factorial model and four decades of fishery data on a particularly biodiverse native Atlantic Salmon population from northern Europe, Czorlich et al. show that net-fishing in river systems directly impacts sea age by selecting against early maturation. The authors also discovered an indirect effect linked to the harvest of a salmon prey species – tiny marine fishes called capelin. According to the authors, the way capelin are harvested as an aquaculture feed source indirectly selects against late maturation in salmon. "Commercial harvesting of an important salmon prey species, capelin, appears to have indirectly induced evolution of Atlantic salmon age at maturity toward younger, smaller individuals," write the authors. "Our results therefore identify a new indirect path by which Atlantic salmon aquaculture can affect wild populations of the same species and emphasize the importance of identifying alternative, sustainable, protein sources for the aquaculture industry."