Hatchery-reared coho salmon develop less otolith deformities in tanks with alternating water flow directions

Hatchery-reared salmonids frequently develop vateritic otoliths—abnormal calcium carbonate structures that impair hearing, balance, and survival. Although vaterite formation has been linked to artificial rearing conditions, the influence of water flow direction has been largely overlooked. This study investigated how tank water flow direction affects otolith composition, asymmetry, and survival to adult return (SAR) in hatchery-reared coho salmon ( Oncorhynchus kisutch ). Fish were reared in circular tanks under three water flow regimes: clockwise (unidirectional), counterclockwise (unidirectional), or alternating (reversed every seven days). Vaterite coverage and otolith asymmetry were assessed via sagittal otolith analysis, and SAR was tracked using passive integrated transponder (PIT) tags. Fish reared under alternating flow had significantly lower vaterite formation and fewer asymmetrical otolith pairs compared to unidirectional flow treatments. While SAR did not differ significantly among treatments, survival estimates were highest in the alternating flow group. These findings suggest that water flow direction is a simple, widely applicable hatchery-rearing modification that may reduce otolith deformities, enhance fish welfare, and improve post-release outcomes. Given the widespread use of unidirectional flow in hatcheries and the high prevalence of vateritic otoliths in hatchery-reared salmonids, alternating flow may help mitigate vaterite-related sensory impairments and contribute to more effective conservation and stock enhancement programs across species and systems. • Alternating water flow reduces vateritic otolith formation in hatchery coho salmon. • Otolith asymmetries are significantly lower under alternating flow conditions. • Unidirectional flow leads to side-specific otolith deformities in circular tanks. • Alternating flow may enhance fish welfare and improve post-release survival. • Flow direction is a simple, low-cost enrichment applicable across hatchery systems.