ESTIMATION OF HISTORICAL POPULATION-SIZE OF THE EASTERN SPINNER DOLPHIN (STENELLA-LONGIROSTRIS-ORIENTALIS)

An assessment of the population status of the eastern spinner dolphin (Stenella longirostris orientalis) in the eastern tropical Pacific is required by the U.S. Marine Mammal Protection Act (MMPA), because dolp hin are killed in the tuna purse-seine fishery. A pooled estimate of a bundance from recent (1986-90) research vessel surveys, in combination with estimates of fisheries kills from tuna vessel observer data, was used to estimate the historical (pre-exploitation) population size wi th a population dynamics model. Estimates of relative population size (current population size divided by historical population size) were c alculated by using a range of values for the maximum net recruitment r ate and the maximum net productivity level (MNPL). The resulting estim ates of relative population size ranged from 0.32 to 0.58, with a best estimate of 0.44 based on available life history data. Estimates of r elative population size were all below the value of MNPL used to calcu late each estimate. Calculation of confidence limits for relative popu lation size by Monte Carlo simulation showed that the precision of the estimates was sufficient to make a status determination. The results indicated that, as of 1988, the stock of eastern spinner dolphin was d epleted as defined by the U.S. MMPA.