WIDESPREAD DEAFNESS IN FARMED SALMON: HALF OF THEM HAVE HEARING LOSS DUE TO DEFORMED EAR BONES
The rapid growth of aquaculture raises questions about the welfare status of mass-produced species. Salmon have otoliths (ear bones) made from aragonite, a crystal form of calcium carbonate. These are the main hearing structures in the inner ear. However, individuals can possess a deformity in which the aragonite structure is replaced with another crystal form of calcium carbonate, vaterite.
Australian researchers studied this otolith defect in wild and farmed Atlantic salmon in Norway, as well as farmed fish from Australia, Scotland, Canada and Chile. Farmed fish were 10x more likely to have the otolith deformity than wild fish. Further, average levels of vaterite replacement have a major physiological effect on individuals: a 28–50% loss in hearing sensitivity.
The underlying causes of vaterite formation remain unknown, but the prevalence of hearing impairment in farmed fish has important implications for animal welfare, the survival of escapees and their effects on wild populations, and the efficacy of restocking programs based on captive-bred fish.
Image: Juvenile Atlantic salmon otoliths. The left is normal, while the right has 90% vaterite deformation. Credit: Tormey Reimer