Interspecific associations between two or more species of the family Delphinidae have been reported by many scientists, but the sympatric ecology of such dolphin associations has not been studied in great detail. A few field investigations have been conducted on this subject in different parts of the world on species such as bottlenose dolphins (Tursiops spp.), short-beaked common dolphins (Delphinus delphis ), and killer whales (Orcinus orca ). Sympatric dolphins seem to use different strategies to co-exist when resources appear to be limited, including dietary divergence (different prey preference, slightly diverse diet, different feeding time) and/or different habitat use (shallow versus deep waters, flat areas versus submarine canyons and escarpments, different travel routes). This paper presents a review of some well-studied dolphin species found in sympatry and discusses the nature of habitat and resource partitioning as well as studies on aggressive behaviour displayed by species living in the same habitat.

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.

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.

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

Habitat partitioning by bottlenose dolphins (Tursiops truncatus), shortbeaked common dolphins (Delphinus delphis), and long-beaked common dolphins (D. capensis), were assessed during 178 surveys conducted between 1997-2000 in Santa Monica Bay, California. Bottlenose dolphins were found year-round within 0.5 km from shore in 80.0 % of the sightings (n ϭ 137) but they were also found in deeper waters further offshore. The two common dolphin species were observed year-round (n ϭ 83) far from shore and near escarpments; they were sympatric but never seen in mixed schools. This study suggests that habitat partitioning in the bay probably relates to resource partitioning among three dolphins species with roughly similar ecological needs.