Populations of coastal and offshore bottlenose dolphins (Tursiops truncatus) are considered to differ ecologically, with implications for their protection and management. We assessed occurrence, distribution and behaviour of coastal and offshore populations of dolphins during a photo-identification study in Santa Monica Bay and nearby areas (1997–2007). Bottlenose dolphins occurred year-round and were encountered on 44.2% of all surveys (n = 425). We photo-identified 647 individuals; of these, 375 (58.0%) were coastal (<1 km from shore), 241 (37.2%) offshore (1–65 km from shore) and 31 were both (4.8%). Dolphins mostly travel (69.0%) and travel-dive (61.5%), with offshore dolphins socialising more (22.6%) than coastal. There were low re-sighting rates for both coastal and offshore dolphins. Low re-sighting rates of coastal individuals provide little evidence of year-round site fidelity, suggesting their range is greater than the study area. Several individuals were re-sighted between and over 1-year periods, often foraging. This suggests that coastal dolphins: (1) are highly mobile within inshore waters, but spend time foraging in the study area, and (2) range >1 km from shore, contrary to what has been previously reported.

This initial research is a component of the first comprehensive study on the occurrence, movement patterns, and behavioral ecology of bottlenose dolphins (Tursiops truncatus gilli) in the coastal waters

Skin lesions and physical deformities on coastal and offshore bottlenose dolphins (Tursiops truncatus) were assessed during a photo-identification study conducted between 1997 and 2007 in Santa Monica Bay and adjacent areas in California. During 425 boat surveys, 647 individuals were identified based on marks on their dorsal fins. Of 637 individuals examined for skin lesions and deformities, 79.0% exhibited at least one type of lesion. Offshore animals showed more lesions than coastal animals (offshore: 87.8%, n=209; coastal: 73.4%, n=270). Only one individual showed a physical deformity. Results show that skin lesions affect a large portion of the coastal and offshore dolphin populations in the study area. When considering that lesions and physical deformities can be a sign of disease and may be related to anthropogenic factors, their high presence on dolphins must be a cause of concern.

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