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The odds are that every second farmed salmon we eat has lost much of its ability to hear.

Although fish senses aren’t usually a consideration when they’re on a plate, researchers now know that deafness in farmed salmon is due to a deformity in the ear, caused by accelerated growth in aquaculture.

Faster growing salmon in farms are three times more likely to have the deformity. Picture: Wikimedia

The findings, published in the Journal of Experimental Biology, raise significant welfare issues and may also explain the poor survival of farmed hatchlings in conservation programs.

Scientists from the University of Melbourne looked at salmon farmed in Norway, Chile, Scotland, Canada and Australia and found the deformity was widespread.

The study’s lead author, Ms Tormey Reimer, says when they went looking for the cause of the deformity they found that the fastest-growing fish were three times more likely to be afflicted than the slowest, even at the same age.

“We also found that we could reduce the incidence of the deformity by reducing how fast a fish grew. Such a clear result was unprecedented,” says Ms Tormey, a masters graduate from the School of BioSciences at the University of Melbourne.

The deformity occurs in the otoliths explains Ms Reimer, which are tiny crystals in a fish’s inner ear that detect sound, much like the ear bones in humans. So even a small change can cause massive hearing problems.

She says normal otoliths are made of the mineral ‘aragonite’, but deformed otoliths are partly made of ‘vaterite’ which is lighter, larger, and less stable.

The team showed that fish afflicted with vaterite could lose up to 50 per cent of their hearing.

Otoliths have been used for decades to determine a wild fish’s age and life history, but since the age and life history of farmed fish is always known, there has never been a reason to examine them.

The deformation was first recorded in the 1960s, but in 2016 this team was the first to show it affects more than 95 per cent of fully-grown hatchery-produced fish globally.

Study co-author Associate Professor Tim Dempster says that the deformity is irreversible, and its effects only get worse with age.

“These results raise serious questions about the welfare of farmed fish. In many countries, farming practices must allow for the ‘Five Freedoms’, which are freedom from hunger or thirst; freedom from discomfort; freedom from pain, injury or disease; freedom to express (most) normal behaviour; and freedom from fear and distress,” says Dr Dempster, from the University of Melbourne.

The issue is that farmed salmon lead very different lives to wild salmon.

Scientists studied salmon farmed in Norway, Chile, Scotland, Canada and Australia. Picture: Pexels

Generations of selective breeding have created fish that are genetically distinct from their wild ancestors. The food pellets given to farmed fish are not the same as a wild diet, and since fish only eat and grow during the day, many farms expose their stock to bright lights 24 hours a day.

The team found that vaterite was seemingly caused by a combination of genetics, diet and exposure to extended daylight. But there was one factor that linked them all: growth rate.

Dr Dempster says that producing animals with deformities violates two of the freedoms: the freedom from disease and the freedom to express normal behaviour.

“However, fish farms are noisy environments, so some hearing loss may reduce stress in hatcheries and sea cages. We simply don’t yet know what hearing loss means to production,” he says.

But the deformity could also explain why some conservation methods aren’t very effective.

Between habitat destruction and overfishing, wild salmon are actually in decline in many areas. One strategy used to boost stocks is to release millions of farmed juveniles into spawning rivers.

Farmed juveniles are often larger than their wild peers at the same age, and would theoretically stand a better chance of surviving.

However, the actual survival rate of farmed juveniles is between ten and 20 times lower than that of wild salmon. In the wild, fish may use their hearing for finding prey and avoiding predators, and for a migrating species like salmon, hearing could help them navigate back to their home stream to breed.

Study co-author Professor Steve Swearer explains that the next step will be to determine if vaterite affects the survival rate of hatchery fish released into the wild.

“Stocking rivers with hearing-impaired fish may be throwing money and resources into the sea,” Professor Swearer says.

Farmed salmon are now genetically distinct from their wild ancestors. Picture: Walter Baxter

Ms Reimer says that since vaterite is irreversible once it’s begun, the key to control is prevention.

“Future research may find ways to prevent the deformity without compromising growth rate,” she says.

“Our results provide hope of a solution. The close link with growth rate means that the prevalence of the ear stone deformity is within the control of hatchery managers. Producing slower growing fish for release into the wild may actually increase their chances of survival in the long run.”

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