Salmon conform to beauty standards, but only if they live a natural life
12 August 2025
New study finds that salmon reared in captivity are more asymmetrical when compared to those that get to live their life in the wild, suggesting that captive fish are experiencing more environmental stress.
The global sale of aquatic species reared in captivity (farm to gate) is worth over $300 billion annually, according to the FAO. The Atlantic salmon is the most valuable of these species, but is seeing widespread declines in the wild.
To put the decline into perspective, in 2024 the aquaculture industry produced somewhere in the region of 600 million Atlantic salmon for consumption, but there are less than 2 million returning to rivers every year in the wild. This has led to the Atlantic salmon being classified as 'Endangered' in Great Britain by the IUCN. To address this imbalance, some countries supplement wild populations with cultivated fish produced in hatcheries.
However, hatchery environments affect fish, making them less likely to survive in the wild. When they are released into the wild, their genetics can also pollute local gene pools. Therefore, while hatcheries seem to offer a solution, decades of research has shown that they largely have a negative impact on the wild populations they are trying to save.
Aurora Hatanpää, who carried out the experiments at Natural Resources Institute Finland, said, 'Despite the problems with hatcheries, they can sometimes be the only thing standing between a population and extinction. This very situation is seen with the Lake Saimaa landlocked salmon in Finland, which is now classified as extinct in the wild due to logging and construction of hydropower stations in the 1950s and 1970s.'
Facial symmetry in humans is associated with beauty, but in fish, it indicates low environmental stress. In this new study, photographs taken on the left and right sides of Saimaa salmon, reared using different techniques, were compared to measure asymmetry. The results showed that fish reared in captivity for a year displayed asymmetry. Not only this, but attempts to enrich the rearing environment, by adding shelters and fluctuating water flows, did not prevent asymmetry. The only technique that prevented asymmetry was releasing the fish into a natural river after they had just hatched.
Fish released into the wild after hatching also had larger pectoral fins and lower jaws, relative to body size, when compared to those reared in captive environments. These traits are likely to be beneficial in the wild, where there are more complex river flows and diets.
This research further demonstrates the transformative and rapid impact captive rearing has on fish and suggests that, if hatcheries must be used, they should release fish into the wild as soon as possible after hatching. The paper, Stress-linked morphological change associated with rearing techniques of hatchery-reliant endemic landlocked Atlantic salmon (Salmo salar m. sebago), is published in the Journal of Fish Biology.
