Timing matters: Early enrichment reduces vateritic otolith deformities in hatchery-reared coho salmon

Vateritic otoliths, abnormal calcium carbonate crystals that can replace aragonite in the inner ears of fish, occur far more frequently in hatchery-reared than wild salmonids and are associated with impaired hearing, reduced predator evasion, and potentially lower post-release survival. Because hatchery and aquaculture programs are increasingly important for food supply and stock enhancement, identifying practical rearing interventions that reduce vaterite formation is a priority. We tested whether physical enrichment and stocking density influence the prevalence and percent coverage of vateritic sagittal otoliths in hatchery-reared coho salmon ( Oncorhynchus kisutch ) using two years of production-scale experiments. In Year 1, enrichment during both incubation and post-ponding rearing greatly reduced vaterite prevalence by release (day 400), with 1 to 2% of enriched fish predicted to develop vaterite compared to 45 to 67% of control fish ( p < 0.001). Among fish that developed vaterite, coverage increased substantially over time and reached 66 to 70% in controls and 59 to 63% in enriched fish by release, although treatment differences in coverage were not significant. A crossover design in Year 2 confirmed that incubation enrichment had the strongest and most persistent effect. At first ponding, prevalence was highest in control–control fish (65%), intermediate in control–enriched (enrichment only after ponding) fish (33%), and low in enriched–enriched and enriched–control (enrichment only during incubation) fish (8 to 11%). By release, prevalence remained highest in control–control fish (74%), intermediate in control–enriched fish (33%), and lowest in enriched groups (12 to 16%) ( p < 0.001). Stocking density had a weaker, non-linear effect, with medium density producing the highest prevalence (38 to 75%) and significantly greater risk than low or high densities ( p < 0.01). Otolith asymmetry covaried with tank flow direction, which was balanced across tanks to control for unidirectional swimming effects. Overall, early-life physical enrichment was the most effective and reliable approach for reducing vaterite prevalence and, to a lesser extent, percent coverage, supporting scalable hatchery interventions to improve sensory health and welfare in salmonids. • Adding enrichment early in life reduces abnormal (vateritic) otolith formation in hatchery-reared coho salmon. • The timing of enrichment matters. Enrichment during incubation provides stronger and longer-lasting protection than enrichment added later, post-ponding, in tanks. • Stocking density has a non-linear effect: high density showed the lowest levels of vaterite, medium density the highest, and low density was intermediate. • Water flow direction affects otolith asymmetry. Constant unidirectional flow (clockwise or counterclockwise) leads to side-specific otolith deformities. • Incubation-stage enrichment is a simple, low-cost, and scalable way to improve fish welfare and potentially enhance post-release performance in hatchery systems.