Long-term population dynamics of sea urchins in a central California kelp forest: rare recruitment and rapid decline

Long-term data on density, size structure and microhabitat of 2 species of sea urchins in the Hopkins Marine Life Refuge, Pacific Grove, California (USA), are presented for the period 1972-1981. Densities of Strongylocentrotus franclscanus remained very low (ca 1 urchin m-2) throughout the study period, whereas the population of S. purpuratus had one substantive recruitment event in 1975-76 when densities increased 25-fold from about 2 to about 50 urchins 1 0 m -~ New recruits initially were found throughout the algal-invertebrate turf, but soon they became concentrated in shallow cracks. As the urchins outgrew the cracks they invaded kelp holdfasts. They grew to 20 to 40 mm test diameter within 1 yr, and their biomass reached nearly 500 g 1 0 m -~. In winter 1976-1977, densities declined rapidly by more than 50 %, probably a s result of predation by the sea star Pycnopodia helianthoidesand/or mortality from disease. Most of the remaining sea urchins were restricted to deep crevices, and their densities continued to decrease slowly to about 6 urchins 10m-' in 1981. Sea urchins in the Hopkins Marine Life Refuge of central California appear to recruit at very low levels during most years. Moreover, the single episodic recruitment event observed during our decade-long study led to only a temporary increase in population density, and most of the recruits were killed 1 yr after they settled.