GROWTH AND MORTALITY ESTIMATES FOR RED-SEA URCHIN STRONGYLOCENTROTUS-FRANCISCANUS FROM SAN NICOLAS ISLAND, CALIFORNIA

The red sea urchin, Strongylocentrotus franciscanus, is one of the larger if not the largest shallow water sea urchin species in the world, with a maximum test diameter > 16 cm. It is a numerically and functionally important element of benthic assemblages along the west coast of North America and is the basis of a commercial fishery. Despite these points, there are very few data on its basic population biology. We studied 2 intertidal populations of S. franciscanus from exposed sandstone benches on San Nicolas Island, California, USA, and estimated growth and survival. The shapes of its growth curves were similar to many tropical and temperate species in that ca 42 % of asymptotic size was attained during the first year of growth. Sea urchins at Cosign Cove grew more slowly and attained a smaller maximum size than did those at Northwest Point, so at 10 yr diameters would be 6.7 and 7.9 cm, respectively. Annual mortality rate was about 10% for both sites. Although growth rates were different at the 2 sites at San Nicolas Island, relative sizes of the demi-pyramids ('jaws') of Aristotle's lantern were the same. Size of demi-pyramids relative to test diameter has been shown to be responsive to food abundance in other sea urchin species, so the different growth rates at the 2 sites either represent responses to environmental factors other than food or relative growth of jaws in S. franciscanus is more canalized than in other sed urchin species.