Harbor porpoise distribution and abundance off Oregon and Washington from aerial surveys in 1991
Calambokidis, J., J.R. Evenson, J.C. Cubbage, P.J. Gearin, and S.D. Osmek. 1992. Harbor porpoise distribution and abundance off Oregon and Washington from aerial surveys in 1991. Report to the National Marine Mammal Laboratory, National Marine Fisheries Service, Seattle. 44pp.
Aerial surveys were conducted off Oregon and Washington in the summer of 1991 to help determine the current abundance of harbor porpoise (Phocoena phocoena) in these waters. Effort in 1991 was increased from that in previous years' surveys to reduce the coefficient of variation around the estimated abundance and to cover regions that had not been surveyed in 1990. Several new regions including the entire Strait of Juan de Fuca, the waters around the San Juan Islands, Puget Sound, and the coastal embayments of the southern Washington coast were surveyed for the first time in 1991.
The study area was surveyed along transect lines following a saw-tooth survey design. Surveys of all areas except Puget Sound and the coastal embayments were conducted from a twin-engine, high-wing aircraft (Partenavia P-68) outfitted with bubble windows for side observation and a belly window to search below the aircraft. Surveys were flown at 600 ft at 90-100 kts. Surveys of Puget Sound and the coastal embayments were conducted primarily to verify that few harbor porpoise occurred in these areas and were conducted with a single-engine aircraft.
A total of 1,236 sightings of 4,816 marine mammals were made during the surveys including 579 sightings of 913 harbor porpoise (not including the calibration experiment). A total of 501 sightings of 782 harbor porpoise were made on-effort during transects. Highest sightings rates were in Oregon and southern Washington. No harbor porpoise were seen in Puget Sound (south of Admirality Inlet), Hood Canal, or coastal embayments of the Washington coast (including Columbia River).
Sightings rates declined with increased Beaufort sea state and cloud cover. Statistical tests of the sighting rate by survey line confirmed the effect of both Beaufort sea state and cloud cover and that these were not the by-products of regional differences. Given this finding, density and abundance estimates were conducted using only surveys conducted during Beaufort sea state of 2 or less and cloud cover of 25% or less. A total of 2,387 of the 3,658 nm survey effort (on transect) fell within the acceptable weather criteria.
A primary goal of the analysis of the land calibration experiment was to determine the proportion of harbor porpoise groups being missed by the aircraft. This was complicated by the high density of harbor porpoise in the sample area; 522 harbor porpoise sightings were made from land and 259 sightings from air during the 5 hour experiment. A more general analysis of the calibration data was conducted for the primary viewing area directly north of the land site. We compared the total number of harbor porpoise groups seen by aerial and land teams along a 500 ft strip to either side of the aircraft path. This revealed that aerial observers were seeing a maximum of 40%-45% of the sightings along the transect line; a figure consistent with the correction factor we used in the current study that is based on breath rates summarized by Barlow et al. (1988).
We estimate total abundance to be just under 30,000 harbor porpoise for Oregon and Washington. This includes a correction factor for animals missed because they were underwater. We estimated just over 13,000 harbor porpoise for coastal Oregon and just over 13,000 along the outer coast of Washington. The highest densities were found along the central and southern Washington coast accounting for high abundance for this region. As in previous years, a low number of harbor porpoise (under 1,000) was found on the northern Washington coast where harbor porpoise are caught incidentally. The abundance estimates for the Strait of Juan de Fuca and the San Juan Islands totaled just over 3,000. Comparison of the abundance estimates reported here with those made previously revealed good agreement with past estimates in some areas and disagreement in others. Where they disagreed, the current estimates were higher than previously calculated.
Coefficients of variation for the different regions were below the target of 0.30. To achieve these coefficients of variation we pooled some areas for analysis, such as the waters around the San Juan Islands and the Strait of Juan de Fuca, and 1990 and 1991 data for the low density northern Washington coast. The density estimates for the samples pooled were nearly identical.
Harbor seals were the most numerous marine mammal seen and were encountered in all regions except the limited effort in offshore waters. A total of 76 sightings of 154 Dall's porpoise were made during the surveys, primarily in the Strait of Juan de Fuca. Other marine mamamls sighted included California and northern sea lions, sea otters, northern right-whale dolphins, killer whales, gray whales, minke whales, and a humpback whale.
The surveys in 1991 met the objectives of providing suitable abundance estimates for harbor porpoise in Washington and Oregon. The principal weakness of the abundance estimates is the absence of a tested correction factor with known variance for harbor porpoise missed on or near the transect line. Until this is developed a true minimum abundance must be based on the uncorrected abundance estimate. Given the importance of this factor in accurately assessing the true abundance of harbor porpoise, it warrants more effort than this question has been afforded to date.