SPATIAL STRUCTURE OF RELAXATION EVENTS AND CRAB SETTLEMENT IN THE NORTHERN CALIFORNIA UPWELLING SYSTEM

During spring and summer 1993, we monitored settlement of crabs (prima rily Cancer spp.) and sea urchins Strongylocentrotus spp. in conjuncti on with physical variables associated with coastal circulation to inve stigate how physical conditions influence the spatial distribution of recruitment along the coast. Observations were made along a 100 km str etch of the northern California coast (USA), from the Gulf of the Fara llones north to Point Arena. Temperature, salinity and wind stress dat a indicated fluctuations in upwelling and provided evidence for the al ongshore, northward flow of warm, low salinity water during upwelling relaxation events which typically lasted several days. On a weekly tim e scale, crab settlement was positively correlated with temperature, a nd negatively correlated with salinity, indicating that settlement, oc curred during relaxation events. Correlations were higher north of Poi nt 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 settleme nt was higher south of Point Reyes. On a daily time scale, crab settle ment north of Point Reyes was associated with the sharp increase in te mperature observed as the relaxation current reached that point on the coast. This association suggested that crabs were transported northwa rd alongshore in the thermal front which propagated northward during e ach relaxation event. This alongshore transport mechanism may be respo nsible for the predictable pattern of settlement variability within th is system: with continuous, event-modulated settlement south of Point Reyes and episodic, event-dependent settlement to the north. Similar p hysical/biological interactions may occur at other points along this c oast and along the midlatitude boundaries of other oceans.