Otoliths, commonly known as “ear stones,” are essential sensory structures within the inner ear of teleost fish like salmonids. In the case of hatchery-reared salmonids, they frequently develop vateritic otoliths (Figure 1), which are an abnormal, less stable polymorph of calcium carbonate 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.
Figure 1. Demonstration of vateritic otoliths.
A group of researchers located in Canada has investigated how tank water flow direction affects otolith composition, asymmetry, and survival to adult return (SAR) in hatchery-reared coho salmon (Oncorhynchus kisutch) during two years. Fish were reared in circular tanks (Figure 2) and these were subjected to three distinct flow treatments:
- constant unidirectional clockwise flow
- constant unidirectional counterclockwise flow
- alternating flow (water direction was reversed weekly).
Figure 2. Fish tanks of the experiment.
Vaterite coverage and otolith asymmetry were assessed via sagittal otolith analysis, and SAR was tracked using passive integrated transponder (PIT) tags.
The results showed that fish reared under alternating flow had significantly lower vaterite formation and fewer asymmetrical otolith pairs compared to unidirectional flow treatments. In terms of survival, the SAR did not differ significantly among treatments. These findings suggest that a simple, no-cost change (switching tank water flow every 7 days) can decrease harmful otolith deformities in hatchery-reared coho salmon, improving welfare and release success without extra infrastructure.
Some British Columbia hatcheries have already begun incorporating alternating flow direction into their rearing practices. “Additional research is needed for wider applications; its simplicity and benefits suggest it should be adopted now,” researchers said.
Reference:
Gaffney, L. P., Polard, E. B., Walton, L. N., Kraemer, C., Molder, Z. A., Covernton, G. A., Atkinson, J., Elmer, L., James, S., Negrin, T., Quindazzi, M., Duguid, W., & Juanes, F. (2026). Hatchery-reared coho salmon develop less otolith deformities in tanks with alternating water flow directions. Aquaculture, 611. https://doi.org/10.1016/j.aquaculture.2025.743037
