Vateritic otoliths in hatchery-reared Strait of Georgia coho salmon: Variation among stocks, hatcheries, and life stages

Abstract

Coho salmon (Oncorhynchus kisutch) populations have been declining, prompting large-scale hatchery-rearing programs to mitigate these losses. Despite these efforts, hatchery-reared coho salmon exhibit lower marine survival rates compared to their wild counterparts, potentially due to differences in rearing environments. One well-documented difference is the occurrence of abnormal (vateritic) sagittal otoliths, which occur at much higher rates in hatchery-reared salmon (50–80 %) compared to wild salmon (<10 %). Given the critical role of sagittal otoliths in inner-ear function, understanding the factors influencing vaterite occurrence is crucial. Our study aimed to quantify the proportion and percent coverage of vateritic sagittal otoliths in hatchery-reared coho salmon smolts and adults from eleven stocks across eight hatchery facilities and river systems around the Strait of Georgia, British Columbia, Canada, collected over two years. We also compared otoliths from smolts and adults originating from the same brood year to assess potential impacts on survival. Our results show that vaterite proportion and percent coverage vary significantly by hatchery facility, stock, and brood year. Smolts from the Coldwater River stock, reared under unique early rearing conditions, exhibited the lowest proportion of vateritic sagittal otoliths (29 % of fish had at least one vateritic otolith) compared to all other hatchery smolt and adult stocks (43–91 %). The proportion and percent coverage of vateritic otoliths did not differ significantly between outmigrating smolts and returning adults from the same brood year, suggesting that vateritic otoliths did not impact marine survival This research emphasizes the variability in vateritic otoliths across hatcheries and stocks, highlighting the need to evaluate hatchery rearing practices to better understand and mitigate the potential impacts of vateritic otoliths on the health and marine survival of hatchery-origin Pacific salmon.