Wild salmon are not just rarer than their farmed cousins - they're better looking too. In a new study by my colleagues and I, we found that they are noticeably more symmetrical than those reared in captivity. In other words, they're prettier (by human standards, at least). And that's not just cosmetic.
Author
- William Perry
Postdoctoral Research Associate at the School of Biosciences, Cardiff University
The global sale of aquatic species reared in captivity (known as aquaculture) is worth more than US$300 billion (£225 billion) annually. That's a huge figure that doesn't even include the wholesale or retail markups and revenue generated from other industries such as processing or packaging.
Of all the cultured aquatic organisms, the Atlantic salmon is the most valuable but is seeing widespread declines in the wild.
To put salmon decline into perspective, in 2024 the aquaculture industry produced somewhere in the region of 3 million metric tonnes of Atlantic salmon for consumption, which is roughly 600 million fish (assuming a harvest weight of 5kg per fish ). Yet, there are fewer than two million wild Atlantic salmon returning to rivers every year.
This has led to the Atlantic salmon being classified as endangered in Britain by the International Union for Conservation of Nature. Some countries have turned to hatcheries as a way to boost wild numbers, releasing captive-bred fish into rivers to try to strengthen struggling populations.
But rearing fish in artificial hatchery environments changes elements of their biology, making them less likely to survive in the wild. Not only this, but when they are released into the wild, their genetics can pollute local gene pools.
So, while hatcheries seem to offer a solution to declining wild salmon populations, decades of research has shown that they largely have a negative effect on the populations they are trying to save. Despite these problems, hatcheries can sometimes be the only thing standing between a population and extinction.
We carried out our study on the Saimaa salmon in Finland, a critically endangered, landlocked population that's been around since the last ice age. Once common in the wild, the Saimaa salmon would now go extinct in the wild if it were not for hatchery production sustaining the population. This is because the construction of hydropower stations and deforestation destroyed their natural spawning habitats in the 20th century.
In humans, facial symmetry is often associated with beauty. It's no coincidence that stars like Zendaya and Harry Styles , whose features exhibit remarkable symmetry, are held up as icons of attractiveness.
In fish, symmetry is associated with low environmental stress. So, it can be a valuable tool for understanding how effective hatcheries are in producing more natural fish.
To understand how hatchery conditions shape fish development, we photographed Saimaa salmon from both sides and compared their features. Some fish had spent a year in tanks, others just a few days before being released into a river. We then measured their symmetry as an indicator of stress.
What we found was striking. Even fish reared in captivity for just one year showed clear signs of asymmetry. And efforts to enrich their lives, such as 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, minimising their exposure to the hatchery environment.
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 may be beneficial in the wild where there are more complex river flows and diets. They were reduced in those fish who had spent most of their life in the hatchery.
Letting nature lead
Our research suggests that the hatchery environment - no matter how carefully managed - is no substitute for nature. While hatcheries can act as a stopgap for critically endangered populations like the Saimaa salmon, they are not a long-term fix for species in decline.
If hatcheries must be used, our study show it's vital to get fish into natural conditions as early as possible. But ultimately, if we want wild salmon to not just survive but thrive, we have to fix the problems that caused their decline in the first place.
Humanity often tries to engineer its way out of environmental crises. But nature isn't easily replaced. Sometimes, the best solution is to give it the space to recover on its own.
William Perry does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
