Majority of the farmed Atlantic salmon experience hearing loss due to an ear bone deformity, a new study has revealed.

Researchers from the University of Melbourne have found that almost half of the world's farmed Atlantic salmon have otolith deformity, which is hardly found in wild fishes.

Tormey Reimer, the study's lead author, said that farmed fish have a likelihood of developing a deformity by up to 10 times compared with wild fish. Reimer explained that the deformity develops when the aragonite, a crystal form of calcium carbonate in the fish's ear bone, is replaced by vaterite, which is a more irregular and less dense crystal form of calcium carbonate.

Farmed fish ear bones have vaterite that makes the bone notably lighter, larger, and more brittle, which ultimately affects the ear functions. With the deformity, sounds are absorbed and processed differently, which could pose negative effects on their survival instincts.

For their study, Reimer and her team collaborated with the Norwegian Institute for Nature Research by sampling the fishes from salmon producing regions, including Australia, Canada, Chile, Norway and Scotland, to check whether the ear bone deformity occurs on a global scale.

A comparison of otoliths from wild and farmed salmon was done and result showed that deformity is indeed more pronounced in farmed salmon than the wild ones, regardless of what region they came from. This has led the researchers to conclude that hearing loss in farmed fish could be affecting the salmon's survival rate in the wild.

Associate professor at University of Melbourne's School of BioSciences and co-author Tim Dempster said the farming process contributes to the development of the deformity. He also said that efforts toward identifying that causative factor must be heightened to help the salmon industry produce fishes within the acceptable standards.

University of Melbourne professor Steve Swearer, who also co-authored the study, said that hearing loss poses difficulty in predator detection, feeding, breeding, and navigation of those released in the wild.

Swearer suggested that restocking programs should assess salmon for ear bone deformity and the potential effects it has on survival rates while in the wild.

"If we don't change the way fish are produced for release, we may just be throwing money and resources into the sea," said Swearer.

At present, more than 2 million tons of farmed salmon are produced each year. It is estimated that more than half of these farmed fish have the deformity, which Reimer said violates the Five Freedoms - a campaign for animal welfare under human control.

Salmon are often farmed because of their rapid growth rate compared with those in the wild. About a month ago, environmental advocates sued the U.S. Food and Drug Administration for its approval of farming modified salmon for human consumption.

The study was published online in Scientific Reports on April 28.

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