Abnormal otoliths: Causes and consequences of vateritic otoliths in hatchery-reared coho salmon
Date
2026
Authors
Gaffney, Leigh P.
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Abstract
Pacific salmon (Oncorhynchus spp.) are ecologically, culturally, and economically vital throughout the North Pacific, yet many populations have experienced long-term declines in productivity and marine survival. To offset these losses, large-scale hatchery programs such as Fisheries and Oceans Canada’s Salmonid Enhancement Program (SEP) rear and release millions of juveniles each year. However, hatchery-origin coho salmon (O. kisutch) consistently exhibit lower marine survival and behavioural competence than their wild counterparts, suggesting that artificial rearing environments can impair sensory and physiological development. One well-documented difference is the high prevalence of vateritic otoliths—abnormal calcium carbonate formations in the inner ear that disrupt hearing, balance, and orientation. This dissertation investigates the environmental factors that drive vateritic otolith formation in hatchery-reared coho salmon and examines the behavioural and ecological consequences of these deformities. To evaluate the scope of the problem, vaterite prevalence and percent coverage were quantified in smolts and adults from eleven stocks across eight SEP hatcheries in the Strait of Georgia, British Columbia (Chapter 2). Vaterite occurred in 29–91% of hatchery fish, with consistent hatchery-specific patterns indicating that local rearing conditions are primary drivers. Because unidirectional flow, barren rearing environments, and high stocking densities are standard but welfare-limiting hatchery practices, their effects were tested directly. Alternating tank water flow (reversed weekly) reduced both vateritic and asymmetrical otoliths relative to constant clockwise or counterclockwise flow (Chapter 3). Early-life structural enrichment using artificial gravel redds produced the strongest reduction in vaterite prevalence—from over 60% to less than 5%—while post-ponding enrichment had smaller benefits and density effects were weaker and non-linear (Chapter 4). Finally, behavioural trials showed that fish with vateritic otoliths displayed weaker and shorter-lived predator-avoidance responses to harbour seal vocalizations and boat noise (Chapter 5). These results link a structural deformity to reduced antipredator performance, suggesting a potential mechanism contributing to the poor marine survival of hatchery-origin coho salmon. Collectively, this dissertation provides the first comprehensive assessment of vateritic otolith formation, its causes, and its consequences for hatchery-reared coho salmon. The results demonstrate that small, practical adjustments to common rearing practices—particularly early enrichment and alternating water flow—can substantially reduce sensory deformities and have the potential to improve welfare, behaviour, and post-release performance.
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Keywords
Vateritic otoliths, Coho salmon (Oncorhynchus kisutch), Salmonid enhancement, Fish welfare, Environmental enrichment, Post release survival, Predator avoidance