Say what? Farmed fish found to be hard of hearing

Atlantic salmon
CC BY 2.0 Timothy Knepp/U.S. Fish and Wildlife Service

It’s not just a concern for fish welfare advocates, but for conservationists as well ... not to mention the poor hearing-impaired fish.

With 85 percent of the world’s marine stocks being either fully exploited or overfished, the farmed seafood industry continues to ramp up at a prodigious pace … like to the tune of $60 billion in revenue a year. While yes, the inhabitants of this teeming planet need their protein, taking factory farming to the water has all kinds of horrid effects – even though strides have been made in sustainable aquaculture.

Now new research from the University of Melbourne adds another wrinkle to the fold: Half of the world’s farmed fish have hearing loss. Huh? It’s a curious thing to consider – unlike say rabbits or dogs with their very obvious and expressive ears, we don’t really equate fish with hearing. Where even are their ears? But sure enough, of course fish have ears … and they use them for hearing and balance, just like we do.

The researchers found that half of the world's most popularly farmed fish, Atlantic salmon, have a deformity of the earbone (otolith), which is much like the inner ear of mammals. When comparing farmed and wild fish, the deformity was found to be much less common in the wild specimens. Lead author Tormey Reimer says that farmed fish are 10 times more likely to have the deformity than wild fish.

"The deformity occurs when the typical structure of calcium carbonate in the fish earbone is replaced with a different crystal form. The deformed earbones are larger, lighter and more brittle, and the way they perform within the ear changes," Reimer says.

EarboneThe left and right earbones of a juvenile farmed salmon. The left one is normal, and the right one is deformed. Photo: Tormey Reimer/CC BY 2.0

"The deformity occurs at an early age, most often when fish are in a hatchery, but its effects on hearing become increasingly more severe as the fish age," she adds. "Our research suggests that fish afflicted with this deformity can lose up to 50 percent of their hearing sensitivity."

To see how widespread the problem might be, the Melbourne team joined up with the Norwegian Institute for Nature Research and examined salmon from the globe’s largest salmon producing nations: Norway, Canada, Scotland, Chile and Australia.

Regardless of where the salmon came from, the problem was always much more common in the farmed fish. With more than a billion salmon harvested each year, the problem affects a tremendous amount of individuals.

"Something about the farming process is causing the deformity. We now need to work out what is the root cause to help the global salmon industry produce fish with acceptable welfare standards," says Reimer. "We don't yet know exactly how this hearing loss affects their performance in farms. However, producing farmed animals with deformities contravenes two of the ‘Five Freedoms’ that forms the basis of legislation to ensure the welfare of farmed animals in many countries."

(The Five Freedoms are a set of animal welfare standards formalized in 1979 by the UK Farm Animal Welfare Council and have since been adopted by agencies across the globe, including the American Society for the Prevention of Cruelty to Animals.)

Even for those who may not be concerned about the welfare of farmed fish, the deformity has broader implications as well. Each year, billions of captive-bred juvenile salmon are released into rivers in North America, Asia and Europe to give a bump wild populations, but their survival is 10 to 20 times lower than that of wild salmon, notes the study. Loss of hearing could certainly explain why fish conservation programs are faring poorly; salmon rely on their hearing to keep tabs on predators as well as to help navigate back to their stream to breed.

Study co-author Steve Swearer from the University of Melbourne says that the hampered survival rates of restocked fish has long been a mystery.

"We think that compromised hearing could be part of the problem. All native fish re-stocking programs should now assess if their fish have deformed earbones and what effect this has on their survival rates," Swearer says.

Tags: Fish


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