Date of this Version
The status of harbor porpoise, Phocoena phocoena, is reviewed for stocks in Oregon and Washington waters, and the adjacent transboundary waters of southern British Columbia, Canada, emphasizing the most recent data on their geographic range, population structure, distribution, population size, trends in abundance, and reproductive biology. This information is used to determine if the annual rate of incidental mortality and serious injury in gill-net fisheries from 1990-94 could be at a biologically significant level.
In the eastern North Pacific Ocean, harbor porpoise are found near the coast, generally in water depths of less than 200 m. Differences in harbor porpoise mitochondrial DNA and organochlorine pollutant ratios and concentrations indicate that porpoise movements and intrinsic rates of mixing are sufficiently restricted to form geographically distinct groups, but not specific stock boundaries. Two harbor porpoise transboundary management areas, the Inland Washington Stock and the Oregon/Washington Coast Stock, are recommended because incidental takes of harbor porpoise occur in commercial sockeye salmon gill-net fisheries of inland Washington waters.
The corrected 1990-91 best and minimum abundance estimates are N = 3,352 (CV(N) = 0.270) and Nmin = 2,680 for inland Washington/southern British Columbia, and N = 26,175 (CV(N) = 0.206) and Nmin = 22,049 for coastal Oregon/Washington. Except for Puget Sound, where a substantial decline in harbor porpoise abundance has occurred, there are no data to determine population trends in Oregon and Washington waters. The aerial survey data collected during the summer of 1990-91 for water depths less than 91 m (50 fathoms), indicated significant differences (z = 6.9, P<0.001) in harbor porpoise mean densities (1.7 animals/km2 and 0.5 animals/km2, respectively) between the waters of southern coastal Washington/Oregon and northern coastal and inland Washington (i.e., the U.S. and Canadian portions of the Strait of Juan de Fuca and San Juan Islands). Although incidental takes of this species in gill-net fisheries only occur in the regions of lowest harbor porpoise densities, other factors such as variations in habitat quality may be responsible for these observed density differences.
To determine if the annual number of incidental takes in gill-net fisheries was potentially significant for a particular stock, an average mortality rate was calculated for 1990-94 and compared to the stock’s calculated potential biological removal (PBR) level (i.e., the product of the minimum population estimate, one-half the maximum theoretical productivity rate and a population recovery factor). For the Inland Washington Stock, an estimated total of 16 harbor porpoise are killed or seriously injured annually: 15 animals/year (90% C.I. = 2 - 58) are incidentally taken in the commercial and tribal sockeye salmon drift gill-net fishery and 1 is estimated to be taken annually in the tribal northwest Washington chinook salmon set gill-net fishery. This combined rate is less than the PBR, but it exceeds 10% of the calculated PBR (27 x 0.1 = 2.7 animals). This tribal chinook salmon fishery also operates in the waters of the Oregon/Washington Coast Stock. An average of 9 harbor porpoise (1990-93 data) are incidentally taken or seriously injured annually. The annual incidental mortality is less than 10% of the PBR (220 x 0.1 = 22) for the coastal stock.