CETACEAN ECOLOGY IN SANTA MONICA BAY, CA, WITH EMPHASIS ON 1997-1998 EL NINO YEARS

The occurrence, spatial distribution and behavior of different cetacean species were investigated in Santa Monica Bay, California. Inshore surveys (n=47), offshore surveys (n=41), and combined inshore/offshore surveys (n=98), conducted between 1997-2001 documented a diverse and rich cetacean fauna. Three species inhabit the bay year-round: common bottlenose dolphin (Tursiops truncatus), long-beaked common dolphin (Delphinus capensis), and short-beaked common dolphin (Delphinus delphis), and seven species occur only occasionally (Pacific white-sided dolphin, Lagenorhynchus obliquidens; Risso's dolphin, Grampus griseus; Dall's porpoise, Phocoenoides dalli; gray whale, Eschrichtius robustus; minke whale, Balaenoptera acutorostrata; blue whale, Balaenoptera musculus; humpback whale, Megaptera novaeangliae). Common bottlenose dolphins were found mostly in shallow waters within 0.5 km from shore 80% of the sighting time (n=157), but they were found also in deeper waters further offshore. All other species were generally seen far from shore. The majority of cetaceans were found traveling, foraging, and feeding near changes in sea bottom reliefs such as escarpments and submarine canyons, showing a preference for these bathymetric features. In contrast with other studies, cetacean abundance overall did not decrease significantly during the strong 1997-98 El Niño event, perhaps due to unique physical oceanographic features of Santa Monica Bay. These features may have shaped a suitable habitat for several species of prey and, consequently, also for cetaceans with opportunistic feeding habits during periods of increased sea surface temperature. Two exceptions to the general trend displayed by most cetaceans in the bay were recorded for short-beaked common dolphins, long-beaked common dolphins and Risso’s dolphins. The two species of common dolphins showed shifts in abundance prior, during and after the El Niño event whereas Risso’s dolphins disappeared from the bay during El Niño.

CALIFORNIA SEA LIONS USE DOLPHINS TO LOCATE FOOD

Aggregations by three species of dolphins (the common bottlenose dolphin, Tursiops truncatus, the short-beaked common dolphin, Delphinus delphis, and the long-beaked common dolphin, Delphinus capensis) and California sea lions (Zalophus californianus) were investigated in Santa Monica Bay, California. Over 200 surveys conducted during 1997-2001, documented that California sea lions were seen 37.33 % of the sightings (n bottlenose dolphin sightings = 150) aggregated with common bottlenose dolphins, and 71.42% of the sightings (n common dolphin sightings = 98) aggregated with the two species of common dolphins. Mixed schools of common bottlenose dolphins and sea lions were seen both in inshore and offshore waters, whereas common dolphins and sea lions were observed only in offshore waters. These aggregations were often recorded feeding near escarpments and submarine canyons, showing a striking preference for these bathymetric features versus flat areas, plateau and inshore waters (<500 m from shore; c2 = 27.961, DF = 5, P < 0.001). Observations for Santa Monica Bay show that 1) sea lions initiate aggregation and departure from dolphin schools, 2) sea lions imitate dolphin behavior, and 3) no aggressive behavior between sea lions and dolphins was ever recorded. This paper discusses the hypothesis that sea lions may take advantage of the superior food-locating abilities of dolphins, and provides the first detail description of mixed-species aggregations and habitat use by three dolphin species and sea lions.

HABITAT PARTITIONING BY THREE SPECIES OF DOLPHINS IN SANTA MONICA BAY, CALIFORNIA

Spatial distribution of and habitat partitioning by three species of dolphins, bottlenose dolphins (Tursiops truncatus), short-beaked common dolphins (Delphinus delphis), and long-beaked common dolphins (Delphinus capensis), were investigated in Santa Monica Bay, CA. Inshore surveys (n=40), offshore surveys (n=35), and combined inshore/offshore surveys (n=81), conducted in 1997-2000, documented that all three species inhabit the bay year-round. Bottlenose dolphins were found in shallow waters within 0.5 km from shore 80% of the time (n=137), clearly separated from the distribution of both common dolphin species, but some of the same individuals were also found in deeper waters over the continental shelf off Palos Verdes and pelagic waters outside the bay. Short-beaked common dolphins and long-beaked common dolphins were observed year-round (n=87) mostly far from shore in pelagic waters. Both species of common dolphins were found near escarpments, changes in bottom reliefs and submarine canyons. Although long-beaked and short-beaked common dolphins were observed in similar locations of the bay, confirming their sympatric distribution, these two species were never seen in mixed schools. In Santa Monica Bay, bottlenose and common dolphins inhabit different geographic ranges, suggesting that resource partitioning probably occurs.

DOLPHIN SYMPATRIC ECOLOGY: A REVIEW

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 behavior displayed by species living in the same habitat. A deeper understanding of sympatric associations between small odontocetes may help to shed light on the ecology of these species, providing a new perspective on the evolutionary context of social complexity.

DOLPHINS AND GREAT APES: COMPARISONS OF SYMPATRIC ECOLOGY

and C.B. Stanford
Jane Goodall Research Center, Department of Anthropology, U. S. C.
Los Angeles, CA 90089-0032, U.S.A.

Cetaceans and African apes are distantly related mammalian taxa that exhibit striking convergences in their socioecology. In both cetaceans and African apes, two or more closely related species sometimes occur in sympatry. However, detailed reviews of the ways in which sympatric associations of cetaceans and great apes are similar have not been done. As field studies of dolphins and great apes have accumulated, comparisons of how the two groups avoid direct food competition when in sympatry have become possible. In this paper we review sympatric socioecology among cetaceans and African apes, and examine convergences in multiple-species-associations in each taxa. We review evidence for hypotheses that seek to explain avoidance of food competition, and consider whether ape-dolphin similarities in this area may be related to the way in which social groups in both taxa optimally exploit their food resources.