Farmed salmon are “three times more likely to be partially deaf” than wild salmon, according to the University of Melbourne‘s Pursuit publication. That means roughly every second farmed salmon that humans consume has lost a great deal of its ability to hear. And last year, scientists figured out why.

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Rapid growth causes a deformity in a salmon’s ear, leading to partial deafness. The scientists scrutinized salmon farmed in Australia, Norway, Canada, Chile, and Scotland and discovered the impairment was widespread – and that the fastest-growing salmon were three times more likely to be impacted than the slowest-growing ones. Study lead author Tormey Reimer told Pursuit, “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.”

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The otoliths, small crystals in a salmon’s inner ear, are where the deformity happens. Normal otoliths are comprised of aragonite, but deformed ones are partly comprised of vaterite – and fish with deformed otoliths can lose as much as 50 percent of their hearing, per Pursuit.

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Diet, genetics, and longer daylight exposure – some fish farms expose the creatures to bright lights 24 hours every day – seem to cause vaterite. Growth rate was the one factor linking them, according to Pursuit. Since fish farms are noisy, it’s possible hearing loss could actually reduce stress, but even so, study co-author Tim Dempster said their research raises “serious questions about the welfare of farmed fish.”

And it could shine light on the failures of some conservation methods. With wild salmon in decline in some regions, farmed ones have been released into spawning rivers. But fish in the wild might use their hearing for detecting prey and predators.

Reimer said, “Future research may find ways to prevent the deformity without compromising growth rate. Our results provide hope of a solution.”

The Journal of Experimental Biology published the research last year. The University of Melbourne led the research with scientists from institutions in Norway contributing.

+ Pursuit/University of Melbourne

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