UC San Diego

Documenting year-round diversity and distribution of marine mammals off Southern California is important for assessment of effects of potentially harmful anthropogenic activities. Although the waters off Southern California have been surveyed extensively for marine mammals over the past 18 years, such surveys have been periodic and were conducted primarily from summer to fall, thereby missing potential seasonal shifts. We examined seasonal abundance and population density of cetaceans off Southern California from 16 shipboard line-transect surveys conducted quarterly during 2004-08. The study area consisted of 238,494 km2 of coastal, shelf, and pelagic oceanic habitat from nearshore waters to 700 km offshore. Based on 693 encounters of 20 cetacean species, abundance estimates by seasonal period (summer-fall or winter-spring) and depth (shallow: <2000.5 m; deep: ≥2000.5 m) were determined for the 11 most commonly encountered species. The following are values of uncorrected density (individuals/1000 km2, coefficients of variation in parentheses) for the seasonal period and depth with greatest density for a selection of the species in this study: blue whale (Balaenoptera musculus), summer-fall, shallow, 3.2 (0.26); fin whale (B. physalus), summer-fall, shallow, 3.7 (0.30); humpback whale (Megaptera novaeangliae), summer- fall, shallow, 3.1 (0.36); short-beaked common dolphin (Delphinus delphis), summer-fall, shallow, 1319.7 (0.24); long-beaked common dolphin (D. capensis), summer-fall, shallow, 687.9 (0.52); and Dall's porpoise (Phocoenoides dalli), winter-spring, deep, 48.65 (0.28). Seasonally, density varied significantly by depth for humpback whales, fin whales, and Pacific white-sided dolphins.