Spatial structure of relaxation events and crab settlement in the northern California upwelling system

During spring and summer 1993, we monitored settlement of crabs (primarily Cancer spp.) and sea urchins Strongylocentrotus spp. in conjunction with physical variables associated with coastal circulation to investigate how physical conditions influence the spatial distribution of recruitment along the coast. Observations were made along a 100 km stretch of the northern California coast (USA), from the Gulf of the Farallones north to Point Arena. Temperature, salinity and wind stress data indicated fluctuations in upwelling and provided evidence for the alongshore, northward flow of warm, low salinity water during upwelling relaxation events which typically lasted several days. On a weekly time scale, crab settlement was positively correlated with temperature, and negatively correlated with salinity, indicating that settlement occurred during relaxation events. Correlations were higher north of Point Reyes, where high settlement occurred only during relaxation, than south of Point Reyes, where settlement occurred both during relaxation events and to a lesser degree during upwelling. Overall crab settlement was higher south of Point Reyes. On a daily time scale, crab settlement north of Point Reyes was associated with the sharp increase in temperature observed as the relaxation current reached that point on the coast. This association suggested that crabs were transported northward alongshore In the thermal front which propagated northward during each relaxation event. This alongshore transport mechanism may be responsible for the predictable pattern of settlement variability within this system: with continuous, event-modulated settlement south of Point Reyes and episodic, event-dependent settlement to the north. Similar physical/biological interactions may occur at other points along this coast and along the midlatitude boundaries of other oceans.