Cross-shelf distributions and recruitment of crab postlarvae in a region of strong upwelling

Larvae are thought to be highly vulnerable to offshore transport in productive upwelling regions, which increases cross-shelf and alongshore dispersal, limits recruitment, and reduces the strength of community interactions. We investigated whether the last planktonic stages of shallow-water crabs (1) occur far offshore during the peak upwelling season in a region of strong upwelling that is recruitment limited, (2) occur farther offshore during upwelling than relaxation conditions, (3) occur farthest from shore at a major headland where currents are deflected offshore, and (4) recruit less during years of stronger upwelling. Crab postlarvae were collected from between 1 and 70 km from shore at 3 locations across an upwelling cell during upwelling and relaxation conditions in northern California, USA, for 3 yr, and recruitment was measured at 2 sites during 2 of these years. Postlarvae of all species collected were most abundant on the inner continental shelf (84%) and were scarce in the open ocean. Postlarvae of 3 taxa only occurred on the inner shelf, whereas postlarvae of the other 3 taxa collected occurred in low abundances on the outer shelf even during prolonged upwelling. They were abundant close to shore at the Point Reyes headland where flow is deflected offshore. Postlarvae did not occur farther from shore during a year of very strong upwelling, and 4 of 7 taxa recruited more then. Thus, crab postlarvae do not appear to be advected far offshore or recruit less during strong upwelling conditions, and the cause and extent of recruitment limitation warrants further investigation.