Advection of red king crab larvae in the southeast bering Sea: interactions between changes in spatial broodstock population structure and physical forcing mechanisms

Abstract

Red king crab (Paralithodes camtschaticus) in Bristol Bay once supported the most lucrative fishery in the world, but catches over the last ~20 years have been substantially reduced compared to the 1970s.  Since the stock collapse, harvests have been maintained within strict guidelines, with little effect on stock rebuilding.  Recruitment levels have remained relatively constant and modest, suggesting that factors outside the fishery may exert strong influence on abundance.  Field studies indicate that the species is reliant upon complex habitat for settlement and establishment of early juvenile populations.  Such habitat appears to be rare in Bristol Bay, located primarily nearshore along the Alaska Peninsula and in Kvichak and Togiak Bays.  During the 1980s, a shift occurred in the mature female (broodstock) distribution: once found near Unimak Pass, most broodstock is presently located farther to the northeast, in the center of Bristol Bay.  Altered broodstock distributions were accompanied by changes in spatial recruitment.  Recruitment to southern areas of the Bay, common in the 1970s, has been rare in recent years; the stock now appears to be more reliant upon nurseries Kvichak and Togiak Bays.  We hypothesize that shifts in spatial recruitment were partly the result of altered larval advection.  Female red king crab carry egg clutches throughout incubation.  Thus, adult distribution translates directly into the location of the larval pool at hatch.  A larval advection model was developed to examine the hypothesis that changes in female distribution decoupled the southern nurseries from the larval pool.  The model incorporated long-term averaged coastal flows, wind-driven advection, and temperature-dependent development in predicting advection pathways associated with a variety of hatch scenarios.  Model results were consistent with the hypothesis that broodstock, as presently distributed, is unable to supply southern nurseries with larvae.  However, the model failed to explain the manner in which larvae reach the Kvichak and Togiak regions, suggesting that either an important component of the stock has evaded the assessment surveys, or that advection pathways are more complex than assumed.  Interactions between larval behavior and tidal transport may play a large role in the larval delivery of this species; these processes have not been adequately studied.