Influence of nearshore hydrodynamics on larval abundance and settlement of sea urchins Strongylocentrotus franciscanus and S-purpuratus in the Oregon upwelling zone

Patterns of larval abundance and early juvenile recruitment of the sea urchins Strongylocentrotus franciscanus and S. purpuratus were examined throughout spring and summer of 1994 and 1995 at 2 sites in southern Oregon, USA, lying within the northern region of the California Current System. Sea temperature and coastal wind records were used as indicators of local upwelling and relaxation events and combined with a method of aging young sea urchin recruits to link specific settlement events with hydrographic events. In 1994 sea urchin larvae were found in plankton collections from April to July, while in 1995 larvae were found from March to July and occurred in a higher percentage of the collections than in 1994. At the 2 sites, settlement occurred from April to August, was variable between years and sites, but most consistently occurred in June. Larval occurrences in the plankton and settlement events in 1994 and 1995 were associated with water masses between 11 and 12 degrees C. Peak settlement in both years was associated with warm water events characterized by a mixed water column and northward wind stress. Settlement rates were low during warm events characterized by a stratified water column and low wind stress. During the summer flow regime, onshore transport of larvae may be linked to relaxation events forced by wind from the south and onshore Ekman flow.