Farmed Salmon More Likely To Have Otolith Deformities Than Wild Salmon

By on May 3, 2016
A Chinook salmon at Coyote Falls on the Similkameen River in Washington. (Credit: Brian Miller)

A persistent issue in the management of fisheries is that farmed fish seem not to survive as well as their wild counterparts. And though there are guesses, no one really knows why that is.

In a recent investigation, University of Melbourne scientists have put forth another notion as to at least what could be a contributing factor in the survival rates of one popular farmed fish, salmon. The researchers have surveyed many of the world’s largest salmon stocks and found that farmed salmon commonly have deformities in their earbones, called otoliths.

Farmed salmon are about 10 times as likely to have deformities in their otoliths as wild salmon, scientists found. The earbones are important for a number of things, not the least of which are hearing and balance.

Researchers say that the deformity typically occurs at an early age, usually when salmon are still in hatcheries, when the structure of calcium carbonate in the fish earbone is replaced with a different crystal form.

The deformed otoliths are usually large, lighter and more brittle than normal earbones. And scientists say these conditions can cause salmon to lose up to 50 percent of their hearing sensitivity.

The left and right earbones of a juvenile farmed salmon. The left one is normal, and the right one is deformed. The deformed earbone is larger and more opaque. (Credit: Tormey Reimer / University of Melbourne)

The left and right earbones of a juvenile farmed salmon. The left one is normal, and the right one is deformed. The deformed earbone is larger and more opaque. (Credit: Tormey Reimer / University of Melbourne)

To make the finds, researchers at U. of Melbourne, along with others at the Norwegian Institute for Nature Research, sampled salmon from the world’s major salmon-producing nations, including Australia, Canada, Chile, Norway and Scotland.

The team compared the structure of the otoliths from farmed and wild salmon. They also compared the hearing of the fish using a model that predicts what a fish can hear. Regardless of the country of origin, farmed salmon in their study were found to have the deformity much more commonly than wild fish.

Scientists believe their findings indicate that something is amiss with the fish farming process. But they are uncertain as to what it could be. Up next for their team is to dig a little deeper to try and uncover the factors at play.

Over two million tons of farmed salmon are produced every year, with more than a billion fish harvested. With those numbers to work with, researchers estimate that about half of the fish have the deformity.

Given that broad estimate, it does appear that the deformity could at least play a small role in the long-term survivability of salmon. That’s because losing hearing can make it harder for the fish to detect predators as well as restrict their abilities to navigate back to home streams to breed.

Full results of the work are published in the journal Scientific Reports.

Top image: A Chinook salmon at Coyote Falls on the Similkameen River in Washington. (Credit: Brian Miller)

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