HARVESTING RED-SEA URCHINS - RECENT EFFECTS AND FUTURE PREDICTIONS

The red sea urchin, Strongylocentrotus franciscanus, is a conspicuous member of subtidal communities in the north Pacific. Within the last d ecade, this ecologically important species has been exposed to intense harvesting for the first time ever. Analysis of population census dat a suggest that harvestable size urchins have rapidly declined in shall ow regions while catch-per-effort and landings data suggest that diver s have maintained high landings by exploiting more distant and difficu lt fishing areas, including deeper areas. We present a size-structured model for the red sea urchin both to estimate what previous levels of harvesting mortality were and to explore what effect future harvestin g strategies might have on population trajectories. Using population c ensus data, we explore three models: one that would result in an equil ibrial population size in the absence of fishing, one that includes po sitive density dependence (an ''Allee effect''), and one that incorpor ates realistic variability in recruitment. Our principal findings are that annual fishing mortality levels that best fit the observed census data in the past (1984-1993) were 0.38-0.49 and represent a 70-90% de crease in the survivorship of harvestable urchins in the years of fish ing. Under a variety of fishing strategies, 100-yr projections indicat ed that the inclusion of an Allee effect or variability in recruitment could drive the harvestable population to < 50% of present estimated population size at relatively low annual fishing mortality values, Our simulations also indicated that, although a yearly fishery would have higher yields, a rotational fishery would maintain populations at a l evel less likely to cause irreversible decline. The wealth of empirica l evidence that sea urchins are an important component of nearshore co mmunities should encourage management strategies that emphasize long-t erm population viability.