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You’re swimming at the beach on a boiling hot summer day—your shoulders and back exposed to the sun’s harmful rays. Fortunately, the sunscreen you slapped on earlier is keeping you safe from a nasty burn. Farmed fish in unshaded outdoor pens face the same threat, but they have no equivalent protection.
Fish can suffer from sunburns just like we can. And whether in caged environments or garden ponds, they aren’t always able to get away from strong sunlight.
“I have visited some sea cages and seen fish with lesions,” says Lynne Sneddon, a bioveterinary scientist at the University of Liverpool in England. “It’s quite a disaster, really.”
Sunburned fish likely experience pain and physical discomfort, says Sneddon. But excessive exposure to sunlight can also weaken their immune systems. This means the animals become more susceptible to disease, and infections can spread quickly among fish that have been penned together, Sneddon says.
A new review pulls together the existing science on the issue to explain the range of effects excessive light can have on fish. For instance, the authors refer to studies on juvenile rainbow trout that showed how exposure to various doses of light—specifically, ultraviolet (UV) radiation—caused them to lose the mucus on their skin, a key part of their defense against pathogens. And a study of carp found that UV exposure caused a reduction in the number of bacteria-killing cells in an organ called the head kidney. Another showed that when Mozambique tilapia were exposed to UV light, they suffered DNA damage and produced less antibodies.
While more research needs to be done to fully understand immune system changes in fish, these studies have already shown that UV light has a variety of detrimental effects.
“People don’t realize that it’s such a big problem,” says Karl Lawrence, a coauthor of the review and a photobiologist at King’s College London in England.
Sneddon, who was not involved in the review, points out that sun damage also hurts farmers’ wallets. Fish with visible damage to their skin are less marketable and will likely be disposed of rather than sold. But protecting fish isn’t as simple as you might imagine.
“A lot of these farmed fish are very visual feeders. If you cover the tanks, they can’t see the food very well,” she says.
Plus, if outdoor pens are fitted with structures that segregate them and provide shade, the changed environment can provoke aggressiveness in territorial fish, which is also bad for their welfare. Simply covering the pens on sunny days when such protection is sorely needed would be a good idea instead, suggests Sneddon.
But research into the effects of sunlight on fish has revealed something surprising. Unlike people, fish have special compounds called mycosporine-like amino acids (MAAs) that mitigate the harmful effects of UV exposure.
“These compounds basically absorb UV photons and convert them to heat,” says Lawrence.
MAAs accumulate in fish’s bodies because the compounds are found in abundance in the protozoa, algae, and cyanobacteria that fish eat. But even a fish’s defenses will eventually be overpowered by prolonged exposure to UV light. (In previous research, Lawrence showed that MAAs can be used by humans as an eco-friendly alternative to synthetic sunscreens.)
Sneddon argues that it’s time farmers made greater efforts to ensure their fish’s welfare.
“I think it’s one of those issues that the public could really drive with their opinions,” she says, much like how shoppers began to favor free-range eggs once they realized that hens on battery farms live in cramped conditions.
Maybe sun-safe fish will be all the rage next.