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| Photo Credit: Courtesy of Cardiff University |
The stress of captivity is likely to be causing reared salmon to be less symmetrical in appearance, according to a new study.
Research by the University of Eastern Finland, Natural Resources Institute Finland, and Cardiff University has found that salmon reared in captivity are more asymmetrical in appearance compared to wild salmon, suggesting that captive fish are more stressed, and their appearance might have impacts on salmon in the wild.
Currently, hatcheries are used in some countries to help boost wild populations with captive reared salmon. Global sales of aquatic species reared in captivity for food are also worth over $300 billion annually, with the Atlantic salmon being the most valuable of these species.
Dr William Perry, Cardiff University’s School of Biosciences, said: “In 2024, the aquaculture industry produced somewhere in the region of 600 million Atlantic salmon for consumption, but there are fewer 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. While hatcheries seem to offer a solution to the decline, decades of research have shown that they largely have a negative impact on the wild populations they are trying to save.
“For example, 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."
"Our new research looked at the appearance of salmon to assess how much they have been altered by captivity and whether their release could impact wild salmon populations negatively – rather than help restore them."
Dr William Perry Postdoctoral Research Associate
Whilst facial symmetry in humans is associated with beauty, in fish it indicates low environmental stress. In the new study, the researchers took photographs of the left and right sides of salmon from the Lake Saimaa (Finland) population, reared using different techniques. They compared the symmetry of the appearance of the captive-bred fish with fish that were allowed to grow in the river under natural conditions.
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| Photo Credit: Courtesy of Cardiff University |
The results showed that fish reared in captivity for a year displayed asymmetry. The comparisons also showed that attempts to enrich the captive 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.
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.”
"Our research looking at the appearance of salmon has shown the 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."
Dr William Perry Postdoctoral Research Associate
Authors: Aurora Hatanpää, Hannu Huuskonen, Jorma Piironen, Raine Kortet, and William Bernard Perry
Source/Credit: Cardiff University
Reference Number: ich092625_01
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