(…) Harbor porpoise, Phocoena phocoena, have been caught incidentally in set gill nets off the central California coast since at least 1958 (Norris and Prescott, 1961). The annual mortality of harbor porpoise caught in gill nets in this region peaked in the mid-1980’s and then gradually declined (Barlow and Forney, 1994) as fishing effort decreased following the implementation of restrictions and area closures in order to protect marine mammals, sea birds, and sport fisheries (Barlow et aI., 1994). In 1986, a series of aerial line-transect surveys was initiated jointly by the California Department of Fish and Game and the National Marine Fisheries Service to monitor trends in abundance of the central California harbor porpoise population. Harbor porpoise in this region are managed separately from animals found off northern California and Oregon, because movement of animals along the U.S. West Coast appears limited, and fishery-induced mortality is restricted to central California (Barlow and Hanan, in press). An analysis of covariance model applied to the first five annual surveys (1986-90) failed to detect a significant trend in abundance (Forney et aI., 1991); however, simulations revealed that statistical power to detect trends, given the level of variability observed in the time series, was low with only five
survey years. A minimum often survey years was estimated as necessary to provide sufficient power.
Additional surveys utilizing the same methodology were conducted in 1991 and 1993, completing an eight-year time series. In updating the analysis of trends in central California harbor porpoise abundance for the period 1986-93, I anticipated either that 1) no significant trend would be identified because of low power, or 2) an increase in abundance might be detected because the population was expected to be recovering after the reduction in fishery-induced mortality. However, a declining trend in central California harbor porpoise abundance was identified for the period 1986-93. Because of the surprising nature of this result and because of the management implications, this report presents the updated 1986-93 analysis and includes additional data from the 1989-1993 aerial surveys in northern California for the first time. (…)