Connectivity between Pacific halibut Hippoglossus stenolepis residing in the Salish Sea and the offshore population, demonstrated by pop-up archival tagging

Fisheries for Pacific halibut (Hippoglossus stenolepis) in the US Pacific Northwest support a variety of harvest sectors including some that are restricted to coastal offshore waters and others confined to the inland waters of the Salish Sea. Such restrictions have generated interest in the degree to which the Salish Sea represents an extension of the offshore population versus its potential to support an isolated stock. Here, twelve female Pacific halibut were tagged during boreal summer with pop-up archival transmitting (PAT) tags that were programmed to report during either the subsequent winter spawning period or late-spring feeding period. High-resolution depth data failed to produce evidence of spawning within the Salish Sea, while depth-temperature profiles were consistent with the departure of migrants between October and January. Final positions and at-liberty longitude estimates suggest that ~60% of the tagged fish departed the Salish Sea, with confirmed migrations to Canada and Alaska. The recapture in the southern Salish Sea of one fish whose tag had reported earlier from offshore Canadian waters demonstrated the occurrence of cyclic migrations. Evaluation of likely maturity status suggests that local demographics reflect either the cumulative effects of spawning migrations in which some emigrants remain offshore after departing the system, or some combination of enhanced growth or delayed maturity. These findings confirm that the Salish Sea does not represent a functionally-independent stock, but suggest that inshore waters may play a unique role by receiving recruits, rearing individuals to maturity, and exporting spawners to the offshore stock components, while retaining a proportion of site-faithful individuals with cyclic seasonal connectivity with offshore spawning groups.