Abundance estimates of harbor porpoise in Washington and Oregon waters

Citation

Calambokidis, J., J.C. Cubbage, J.R. Evenson, S.D. Osmek, J.L. Laake, P.J. Gearin, B.J. Turnock, S.J. Jeffries, and R.F. Brown. 1993. Abundance estimates of harbor porpoise in Washington and Oregon waters. Report to the National Marine Mammal Laboratory, National Marine Fisheries Service, Seattle, Washington. 55pp.

Executive summary

Numerous surveys have been conducted in recent years to estimate harbor porpoise abundance for Washington and Oregon. The primary goal of this study was to evaluate the past data on harbor porpoise abundance and develop a revised abundance estimate the data from these data.

To this end, we evaluated the suitability and comparability of past surveys. Four data sets from different surveys were selected for development of a revised abundance estimate:

1) Vessel surveys dedicated to harbor porpoise estimation conducted in 1989 off the northern Washington coast (Calambokidis et al. In press),
2) Aerial surveys dedicated to harbor porpoise estimation conducted in 1989 off the coasts of Oregon and Washington (Turnock et al. In press),
3) Aerial surveys dedicated to harbor porpoise estimation conducted in 1990 off the coast of Washington (Calambokidis et al. 1991), and
4) Aerial surveys dedicated to harbor porpoise estimation conducted in 1991 off the coasts of Oregon and Washington and in Washington inland waters (Calambokidis et al. 1992).

The methods used in the dedicated harbor porpoise aerial surveys conducted in 1990 and 1991 were identical and these data were pooled for analysis. Abundance estimates from the dedicated aerial surveys and vessel surveys conducted in 1989 were analyzed separately and then averaged (weighed by the inverse of variance) with the 1990-1991 dataset.

Revised abundance estimates for the different data sets were usually similar to the original estimates. The only exception was the revised estimate of the 1989 dedicated aerial survey data which differed substantially from the original estimates. This was primarily the result of the different detection function we used in the analysis which excluded consideration of the area closest to the transect line. The area closest to the transect line had low sighting rates because of the absence of a center observer.

Revised abundance estimates using all the survey data yielded estimates of 13,014 harbor porpoise off the coast of Oregon (including Heceta Bank), 10,074 off the southern Washington coast, 634 off the northern Washington coast, and 3,298 for the Strait of Juan de Fuca and San Juan Islands area. For the first time revised coefficients of variation (CV) for these estimates included the variance contributed by the correction factor for animals missed on the transect line. These CVs varied between 0.18 and 0.26 for the above estimates.

All the surveys of Oregon and Washington coastal waters used in the revised abundance estimates covered waters out to a water depth of 50 fathoms (91 m). The revised abundance estimates are therefore only valid for the portion of the population inhabiting these waters. Surveys conducted by Ebasco Environmental for the Minerals Management Service surveyed waters much farther offshore for marine mammals and marine birds (Green et al. 1992). Though these surveys were not considered suitable for use in the abundance estimates (partly because of a much lower survey altitude) they did provide information on the proportion of harbor porpoise that occurred within and outside the study area covered by the dedicated surveys. A total of 24% of harbor porpoise sightings made during their systematic survey legs, extending from the coast to up to 100 nm offshore, were outside the area covered by the dedicated surveys used for the revised abundance estimate. This would result in a correction factor of 1.31 to adjust our abundance figures to cover the animals missed outside the study area.

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