Why Are So Many Farmed Salmon Partially Deaf?

Researchers have found out the reason that so many farm-raised salmon are partially deaf, pointing to a deformity in their ears caused by accelerated growth in aquaculture. The findings raise significant welfare issues and may also explain the poor survival of farmed hatchlings in conservation programs.

University of Melbourne scientists looked at salmon farmed in Norway, Chile, Scotland, Canada, and Australia and found the deformity was widespread.

The study’s lead author, Tormey Reimer, a master’s graduate from the School of BioSciences at the University of Melbourne, says when they went looking for the cause of the deformity they found that the fastest-growing fish were three times more likely to be afflicted than the slowest, even at the same age.

says Reimer.

Deformed Otoliths

The deformity occurs in the otoliths, explains Reimer, which are tiny crystals in a fish’s inner ear that detect sound, much like the ear bones do in humans. So even a small change can cause massive hearing problems.

She says normal otoliths are made of the mineral aragonite but deformed otoliths are partly made of vaterite, which is lighter, larger, and less stable. The team showed that fish afflicted with vaterite could lose up to 50 percent of their hearing.

Otoliths have been used for decades to determine a wild fish’s age and life history, but since the age and life history of farmed fish is always known, there has never been a reason to examine them.

The deformation was first recorded in the 1960s, but in 2016 this team was the first to show it affects more than 95 percent of fully-grown hatchery-produced fish globally.

Study coauthor Tim Dempster says that the deformity is irreversible, and its effects only get worse with age.

says Dempster, an associate professor at the University of Melbourne.

The issue is that farmed salmon lead very different lives to wild salmon.

Genetics, Diet And Sunlight

Generations of selective breeding have created fish that are genetically distinct from their wild ancestors. The food pellets given to farmed fish are not the same as a wild diet, and since fish only eat and grow during the day, many farms expose their stock to bright lights 24 hours a day.

The team found that vaterite was seemingly caused by a combination of genetics, diet, and exposure to extended daylight. But there was one factor that linked them all: growth rate.

Dempster says that producing animals with deformities violates two of the freedoms: the freedom from disease and the freedom to express normal behavior.

The deformity could also explain why some conservation methods aren’t very effective.

Between habitat destruction and overfishing, wild salmon are actually in decline in many areas. One strategy used to boost stocks is to release millions of farmed juveniles into spawning rivers.

Farmed juveniles are often larger than their wild peers at the same age, and would theoretically stand a better chance of surviving.

Vaterite Impacts

However, the actual survival rate of farmed juveniles is between ten and 20 times lower than that of wild salmon. In the wild, fish may use their hearing for finding prey and avoiding predators, and for a migrating species like salmon, hearing could help them navigate back to their home stream to breed.

Study coauthor Steve Swearer explains that the next step will be to determine if vaterite affects the survival rate of hatchery fish released into the wild.

Reimer says that since vaterite is irreversible once it’s begun, the key to control is prevention.

she says.

Original Study: Rapid growth causes abnormal vaterite formation in farmed fish otoliths

Image: Zureks CC BY-SA 3.0

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