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

Issue:

Author(s): 
Caretta, James V., Barbara L. Taylor, and Susan J. Chivers
Cover date: 
2001
Pages: 
29–39
Abstract: 

We estimate abundance and describe the depth distribution of harbor porpoise (Phocoena phocoena) in northern California from a November 1995 ship survey. This was the first ship survey designed to systematically survey different depth strata out to 91 m (50 fathoms) in this region. Harbor porpoise abundance in northern California is estimated at 5686 (CV=0.29, log-normal 95% CI=3498–9242), based on 377 km of transect effort and 153 porpoise sightings. Using a confidence interval of differences approach, we determined that our abundance estimate was significantly different (P=0.08; α=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 seasonal movement of porpoise to other areas or depths, insufficient transect effort during the ship survey, or underestimates of the fraction of porpoise groups missed on the trackline due to large swells. Porpoise distribution in northern California was not random with respect to water depth; significantly more porpoise than expected occurred at depths of 20 to 60 m and fewer porpoise than expected occurred at depths >60 m.