SEASONAL PATTERNS IN THE ABUNDANCE AND DISTRIBUTION OF CALIFORNIA CETACEANS, 1991-1992

This study presents a detailed seasonal comparison of the abundance an d distribution of cetaceans within 100-150 nmi (185-278 km) of the Cal ifornia coast during 1991 and 1992. The results of a shipboard line-tr ansect survey conducted in July-November 1991 (''summer'') were compar ed to those from aerial line-transect surveys conducted in March-April 1991 and February-April 1992 (''winter''). Using a confidence-interva l-based bootstrap procedure, abundance estimates for six of the eleven species included in the comparison exhibited significant (alpha = 0.0 5) differences between the winter and summer surveys. Pacific white-si ded dolphins (Lagenorhynchus obliquidens), Risso's dolphins (Grampus g riseus). common dolphins (Delphinus spp.), and northern right whale do lphins (Lissodelphis borealis) were significantly more abundant in win ter. The abundance of blue whales (Balaenoptera musculus) and gray wha les (Eschrichtius robustus) reflected well-documented migratory patter ns. Fin whales (B. physalus) were significantly more abundant during s ummer. No significant differences in seasonal abundance were identifie d for Dall's porpoises (Phocoenoides dalli), bottlenose dolphins (Turs iops truncatus), killer whales (Orcinus orca), sperm whales (Physeter macrocephalus), or humpback whales (Megaptera novaeangliae). Significa nt north/south shifts in distribution were found for Dall's porpoises, common dolphins, and Pacific white-sided dolphins, and significant in shore/offshore differences were identified for northern right whale do lphins and humpback whales.