Abundance and depth distribution of harbor porpoise (Phocoena phocoena) innorthern California determined from a 1995 ship survey

We estimate abundance and describe the depth distribution of harbor porpois e (Phocoena phocoena) in northern California from a November 1995 ship surv ey. This was the first ship survey designed to systematically survey differ ent depth strata out to 91 m (50 fathoms) in this region. Harbor porpoise a bundance in northern California is estimated at 5686 (CV=0.29, log-normal 9 5% CI=3498-9242), based on 377 km of transect effort and 153 porpoise sight ings. Using a confidence interval of differences approach, we determined th at our abundance estimate was significantly different (P=0.08; alpha =0.10) from an aerial survey estimate obtained 1 to 2 months earlier in the same region, where abundance was estimated at 13,145 (CV=0.39, log-normal 95% CI =6316-27,357). Possible explanations for differences in estimates include s easonal movement of porpoise to other areas or depths, insufficient transec t effort during the ship survey, or underestimates of the fraction of porpo ise groups missed on the trackline due to large swells. Porpoise distributi on in northern California was not random with respect to water depth; signi ficantly more porpoise than expected occurred at depths of 20 to 60 m and f ewer porpoise than expected occurred at depths >60 m.