Abstract

Harbour porpoise (Phocoena phocoena) in California are subject to incidental mortality in gillnet fisheries. The status of this population is uncertain. We review information on population structure and conclude that porpoises in central California should be managed as a separate stock. We review information from several surveys and estimate that the porpoise abundance in this area is approximately 3,300. The impact of several fisheries is considered and the fishery mortality from 1969-89 is estimated for the halibut gillnet fishery. In the mid-1980s, this fishery took 200-300 harbour porpoise per year. Estimates for the 1970s and early 1980s are extrapolated from current mortality rates and historic catch records. Based on the maximum plausible life history parameters, the maximum population growth rate for harbour porpoise is estimated to be 9.4% per year. Using this estimate and a model of density dependence, the abundance in the year 1969 is back-calculated by adding estimated harvests and subtracting estimated population growth. Using ranges in parameters to express some of the uncertainty in their estimation, the population in 1989 is estimated to be at 30-97% of its 1969 abundance. Given the broad range of this estimate and the fact that it does not incorporate all sources of variation, we conclude that it is impractical to manage this population based on its status relative to carrying capacity.