Dorsal fin height is not an effective tool to distinguish hatchery and wild steelhead in the field

ObjectiveTo maximize harvest of hatchery steelhead Oncorhynchus mykiss and Pacific salmon Oncorhynchus spp. while reducing impacts to wild conspecifics, fishery managers rely on the ability of anglers to distinguish between the two. Fin erosion, including dorsal fin erosion, is a common result of hatchery rearing of juvenile salmonids. Since the 1980s, managers in the Pacific Northwest have utilized adult steelhead dorsal fin height as a regulatory control rule, requiring anglers to release steelhead with dorsal fins over a height threshold. This approach assumes that all or most hatchery fish have shorter dorsal fins than their wild counterparts. This study was designed to test this assumption.MethodsWe compared steelhead origin (hatchery or wild) determined using scale analysis and adipose fin clipping (hatchery n = 127, wild n = 71) with origin estimated using dorsal fin height.ResultOverall, a static dorsal fin height threshold (2.25 in, 5.72 cm) correctly identified 90% of hatchery-origin steelhead but misidentified nearly half (44%) of wild fish as hatchery. Dorsal fin height of known hatchery fish correlated with fish length, indicating a weakness of using a static dorsal height threshold as a management tool. The accuracy of distinguishing wild fish improved, but only marginally (56-76%), using a classification model where dorsal fin height of known origin fish from this study was a continuous predictor of origin. In comparison, adipose fin status correctly identified 97% of unmarked fish as wild.ConclusionThe low accuracy of the classification model and static dorsal fin height thresholds at distinguishing hatchery and wild steelhead due to a strong correlation between dorsal fin height and fork length and the high fork length variability of hatchery origin fish. Given the low cost and accuracy of adipose fin clipping and the potentially high conservation risk associated with misidentification, we recommend discontinuing the use of dorsal fin height as a harvest control rule for steelhead.