Possible Stock Structure of Coastal Bottlenose Dolphins off Baja California and California Revealed by Photo-Identification Research

Abstract

Boat-based photo-identification research has been carried out on bottlenose dolphins in the eastern North Pacific coastal waters of northern Baja California, Mexico and southern and central California, USA from 1981 to 2001. Within these waters, bottlenose dolphins routinely travel back and forth between coastal locations while generally staying within a narrow coastal corridor of about 1 km or less from the shore. Inter-area match rates between 616 dolphins photo-identified in Ensenada, San Diego, Orange County and Santa Barbara between 1981-2000 averaged 76%. To explore possible southern range limits for these California coastal dolphins, photo-identification surveys were carried out in the coastal waters off San Quintín, Baja California, Mexico between April-August 1990 (n=8) and July 1999 to June 2000 (n=12). The 207 individual dolphins identified in San Quintín were compared to the 616 dolphins identified in the California coastal study areas (CCSAs) of Ensenada, San Diego, Orange County and Santa Barbara. The inter-area match rate between San Quintín and the CCSAs was 3% (n=7). This low rate contrasts sharply with the much higher average match rate of 76% observed between the CCSAs. These differences in match rates suggest that both a California coastal stock and coastal Northern Baja California stock may exist, with only a limited degree of mixing between them.