Harbor porpoise studies in the Gulf of the Farallones
Calambokidis, J., C. Ewald, G.H. Steiger, S.M. Cooper, I.D. Szczepaniak, and M.A. Webber. 1990. Harbor porpoise studies in the Gulf of the Farallones. Report to the Gulf of the Farallones National Marine Sanctuary, San Francisco, California. 58pp.
We report on research to examine the distribution, abundance, reproduction, mortality, and pollutant levels in harbor porpoise (phocena phocena) in the Gulf of the Farallones. High mortality of this species in recent years and their elimination from other portions of their range has raised concerns about their status and vulnerability.
Vessel surveys were conducted for harbor porpoise from December 1987 through September 1989 in the Gulf of the Farallones area. Just under 1,000 nm of line-transect surveys were completed. We also conducted calibration experiments and photographic identification of individual harbor porpoises.
Harbor porpoises were predominantly distributed in coastal waters at water depths under 40 m. Highest sighting rates were in the summer and fall. Harbor porpoise were consistently seen just outside San Francisco Bay. In September 1989, the area west of Pt. Reyes had some of the highest harbor porpoise densities seen in the study. The presence of harbor porpoise in this area was variable, however, with few sightings at other times.
Best estimates of harbor porpoise abundance were obtained in the fall of 1988 and 1989 when an estimated 2,100 harbor porpoise were present in the study area. This is considerably higher than estimates reported previously. This difference is more the result of greater effort and the use of different methods than an indication of a change in harbor porpoise population size. We restricted our analysis of population size to only include survey effort in ideal weather conditions (Beaufort sea state of 0 or 1).
Sightings of harbor porpoise made from Oceanic Society nature trips showed a significant increase from 1983 to 1987. These differences remained after the effects of weather were incorporated. These trips only reflect the density of harbor porpoise along the route traveled from San Francisco to the Farallon Islands.
The number of strandings of harbor porpoise reported in the Gulf of the Farallones area has decreased substantially since the high numbers of strandings in 1983 and 1984. This appears to reflect a decline in incidental mortality of harbor porpoise in set nets as a result of protective measures adopted starting in 1984. Concentrations of the stable chlorinated hydrocarbon contaminants PCBs, DDT, and HCB were found in blubber tissues of stranded harbor porpoise from the Gulf of the Farallones. These concentrations, though high in comparison to most animals, were within the range found in harbor porpoise and other coastal small cetaceans in other regions. Contaminant ratios provide an indication of the interchange of harbor porpoise between different areas. Ratios determined in harbor porpoise from the Gulf of the Farallones were highly variable and overlapped with those found in harbor porpoise in Monterey Bay and to a lesser degree Oregon.