Whether it was fresh sushi or smoky salmon steak, chances are your latest portion of salmon came from a half-deaf animal.
A study led by researchers from the University of Melbourne has discovered that half of the world’s farmed salmon have a deformity in their earbone, rendering them hard of hearing. This could be impacting the survival of captive-bred fish released into the wild as part of conservation efforts.
It also raises questions about animal welfare for the billions of salmon cultivated around the word in aquatic farms. The results were published last week in Scientific Reports.
"The deformity occurs when the typical structure of calcium carbonate in the fish earbone is replaced with a different crystal form,” says PhD candidate Tormey Reimer, lead author of the study. “The deformed earbones are larger, lighter and more brittle, and the way they perform within the ear changes."
Reimer first noticed the deformity when the researchers were working on tagging salmon earbones with a chemical marker - like a permanent fingerprint that lets track an escaped salmon back to the right farm.
"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,” says Reimer.
Curiously, this ear problem is a global issue - the scientists sampled salmon from Norway, Canada, Scotland, Chile and Australia.
Even though we tend to think of wild salmon and farmed salmon as separate groups, there’s actually a significant amount of captive-bred fish that are released to boost wild populations.
"Enormous numbers of those salmon you see swimming up the great rivers of Canada were released when they were small fish, and they were first grown in hatcheries," says study co-author, Associate Professor Tim Dempster.
"If we're doing things in the hatcheries that compromise their ability to live a wild life, we might actually be not getting those conservation programs right."
Depending on whether the salmon were affected in one ear or both, the researchers found between 28-50% of hearing sensitivity loss.
"One of the reasons that hearing is important for wild fish is to that they can hear and avoid predators," says Dempster. This could help explain why captive-bred salmon have 10-20 times lower survival rates in the wild.
But hearing troubles could be problematic for captive salmon, too. Farmed animal legislation around the world is guided by the “Five Freedoms” for animals - freedom from hunger and thirst; freedom from discomfort; freedom from pain, injury or disease; freedom to express normal behaviour; and freedom from fear and distress.
"If they have an earbone that's deformed, that's effectively the outcome of a disease, and having poor hearing probably affects your behaviour as well," says Dempster. According to him, this means that two of those freedoms could be fundamentally compromised in the affected salmon, and this needs to be researched further.
The scientists are now investigating the cause, and they do have a few ideas for where to start.
"Farmed fish are different genetically from wild fish, particularly with salmon there have been a few generations of selective breeding. So it's possible that it could have a genetic basis, and that's one thing we're looking at," says Dempster.
It could also be a dietary issue, or could just be due to the fact that everything in fish farms is optimised to make them grow incredibly quickly in order to increase revenue. Dempster thinks the culprit for the deformity could be due to fast growth.
"A lot of other deformities that have appeared in salmon aquaculture have been related to that fast growth rate, so it could be as simple as that," he says. “We need to get to the root cause and fix that problem.”