Seasonal patterns in the abundance and distribution of California cetaceans, 1991-1992

This study presents a detailed seasonal comparison of the abundance and distribution of cetaceans within 100-150 nmi (185-278 km) of the California coast during 1991 and 1992. The results of a shipboard line-transect survey conducted in July-November 1991 ("summer") were compared to those from aerial line-transect surveys conducted in March-April 1991 and February-April 1992 ("winter"). Using a confidence-interval-based bootstrap procedure, abundance estimates for six of the eleven species included in the comparison exhibited significant (alpha = 0.05) differences between the winter and summer surveys. Pacific white-sided dolphins (Lagenorhynchus obliquidens), Risso's dolphins (Grampus griseus). common dolphins (Delphinus spp.), and northern right whale dolphins (Lissodelphis borealis) were significantly more abundant in winter. The abundance of blue whales (Balaenoptera musculus) and gray whales (Eschrichtius robustus) reflected well-documented migratory patterns. Fin whales (B. physalus) were significantly more abundant during summer. No significant differences in seasonal abundance were identified for Dall's porpoises (Phocoenoides dalli), bottlenose dolphins (Tursiops truncatus), killer whales (Orcinus orca), sperm whales (Physeter macrocephalus), or humpback whales (Megaptera novaeangliae). Significant north/south shifts in distribution were found for Dall's porpoises, common dolphins, and Pacific white-sided dolphins, and significant inshore/offshore differences were identified for northern right whale dolphins and humpback whales.