EFFECTS OF KELP FORESTS MACROCYSTIS-PYRIFERA ON THE LARVAL DISTRIBUTION AND SETTLEMENT OF RED AND PURPLE SEA-URCHINS STRONGYLOCENTROTUS-FRANCISCANUS AND STRONGYLOCENTROTUS-PURPURATUS

It has often been observed that the abundance of sea urchins is lower inside kelp forests and along their nearshore edges than outside their offshore boundaries. We tested the hypothesis that this distributiona l pattern is a reflection of settlement patterns by monitoring settlem ent on artificial surfaces at 3 kelp forests near San Diego, Californi a, USA. We estimated settlement at paired sites under the canopy and o utside the offshore edge of the kelp forests and at paired sites locat ed 20 m outside the canopy near the inshore and offshore boundaries of each kelp forest. There was no evidence of an effect of dense stands of giant kelp on the settlement of purple sea urchins. The results for red sea urchins were ambiguous; in 3 of 7 comparisons, average settle ment was higher or observations of higher settlement were more frequen t offshore from the kelp forest than elsewhere. Although this could re sult from different effects on the 2 species, we think it probably was due to accidents of sampling. We conclude that the distributional pat terns of purple and red sea urchins relative to kelp forests are unlik ely to reflect larval availability or settlement, but are more probabl y a function of post-settlement events. We qualify our conclusions bec ause of the tremendous temporal variability in the physical and biolog ical factors that can potentially affect the local distribution of sea urchin larvae. The 2 years of this study may be short relative to nat ural cycles.