An ear bone deformity has left an estimated half of the world's farmed fish with hearing loss, according to research just published in the journal Scientific Reports.
Hearing is critical for a fish's balance as well as its hearing. If something in the farming process is causing the deformity, the study's authors say, there may be animal welfare issues to address.
Also, the deformity could help explain the underperformance of some fish conservation programs, which breed fish in captivity so they can be released into the wild.
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Thanks to a malformed chemical structure, the deformed ear bones end up being bigger, lighter and more brittle than they should be. This impacts proper hearing in the animal.
The prevalence of the deformity – 10 times more likely in farmed fish than wild, regardless of species - was uncovered by researchers from the University of Melbourne, who chose to study Atlantic salmon farms from the world's top salmon producers: Canada, Chile, Norway, Scotland and Australia.
"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," explained the study's lead author Tormey Reimer in a statement, adding that the deformity can cost a fish up to 50 percent of its hearing.
The scientists compared ear structures in both farmed and wild salmon from the top-producing countries and also used a mathematical model to predict what the fish would be able to hear based upon their ear structures.
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The researchers found that no matter which country's fish they studied the ear bone deformity was much higher in the farmed fish versus wild.
"We estimate that roughly half of these fish have the earbone deformity and thus have compromised hearing," said Reimer. "We don't yet know exactly how this hearing loss affects their performance in farms."
"However," she added, "producing farmed animals with deformities contravenes two of the 'Five Freedoms' that form the basis of legislation to ensure the welfare of farmed animals in many countries."
"We now need to work out what is the root cause, to help the global salmon industry produce fish with acceptable welfare standards," Reimer said.
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The scientists also said the ear bone deformity could be the root of the problem for fish conservation programs that see poor survival rates in captive-bred fish that are placed in the wild.
If fish released into the wild have damaged hearing, their ability to both be aware of predators and navigate to their home stream for breeding could be compromised.
"All native fish re-stocking programs should now assess if their fish have deformed earbones and what effect this has on their survival rates," said study co-author Steve Swearer.