Diurnal Behavior and Group Size Patterns of Common Dolphins (Delphinus sp.) during 2008-201 0 Aerial Surveys off San Diego, California

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Aerial surveys offer an ideal observation platform to document the behavior and group size of dolphin species over a wide range in offshore waters in a short period. Eight aerial surveys were conducted near San Clemente Island off southern California, Oct/ Nov 2008, jun/Jui/Nov 2009 and May/Jui /Sep 2010 to monitor behavior of marine mammal species using line-transect and focal-behavioral circling methods. An estimated 94,867 short-beaked (Delphinus de/phis) and longbeaked (Delphinus capensis) common dolphins were observed during 346 separate events. Number of sightings, mean group size and initial group behavior state were recorded and compared by diurnal periods. Daytime observation hours were divided into three periods, following the methods of Bearzi et al. (1999): "morning" (08:01-11 :59)(n = 71 sightings, individuals= 22,777), "early afternoon" (12:00-15:59)(n = 191, individuals= 52,926), and "late afternoon" (16:00-19:59)(n = 85, individuals= 19,164). Sighting rates were highest in the early afternoon (4.1 indiv/km) followed by late afternoon (3.9 indiv/km) and morning (3.3 indiv/km). Mean group size was highest in the "morning" (321 ± 455.8), followed by "early afternoon" (277 ± 354.5) and "late afternoon" (225.5 ± 276.0). During the "morning", initial group behavior was most frequently surface-active mill (44%) followed by travel (29%) and surface active travel (13%). During the "early afternoon", travel (35%) occurred most frequently, followed by surface-active mill (33%) and surface- active travel (24%). During the "late afternoon", surface-active mill (35%) occurred most frequently, followed by travel (28%) and surface-active travel (27%). Social and apparent foraging behaviors typically occurred during surface-active behavior states. Results suggest that groups of common dolphins of both species aggregate in larger numbers and are generally exhibiting more surface-active mill behavior during the morning than early and late afternoon, potentially corresponding with socializing and foraging strategies. Further data gathering and multivariate analyses are underway to elucidate more specific diurnal trends and behavioral correlations.