GROWTH AND MORTALITY OF SUBTIDAL RED-SEA URCHINS (STRONGYLOCENTROTUS-FRANCISCANUS) AT SAN-NICOLAS-ISLAND, CALIFORNIA, USA - PROBLEMS WITH MODELS

Red sea urchins, Strongylocentrotus franciscanus, were tagged with tet racycline in 1990 at subtidal sites off San Nicolas Island, California , USA. After one year in the field, the sea urchins were collected and growth increments were measured based on tetracycline marks, which in dicated initial slow growth, a maximum rate, and finally a prolonged p eriod of very slow growth. Small red sea urchins (4 cm diam) were esti mated to be 3 to 4 yr old, which is much older than has previously bee n reported. It is estimated that about 12 yr would be required to atta in 10 cm diam. Survival has previously been modeled assuming a constan t rate. If the population of red sea urchins is assumed to be stable a nd stationary, annual survival rate was between 71 and 77% yr-1. Censu s data for the two years of study have permitted annual survival to be estimated without assuming stable and stationary population structure . Under these conditions, annual survival rate was between 79 and 86% yr-1. Analysis of transitions in the size distributions from 1990 to 1 991 suggested that annual survival may have been size-specific: 91 % y r-1 for individuals 1.1 to 4.0 cm diam, 82 % yr-1 for individuals 4.1 to 7.0 cm diam, and 63% yr-1 for those of 7.1 to 10.0 cm diam. An alte rnative explanation to size-specific survival in our study is size-spe cific immigration.