Abundance, distribution and feeding ecology of small cetaceans in the eastern Sulu Sea and Tanon Strait, Philippines.

摘要

Cetacean study in the Philippines has only began in the last ten years; to date, the presence of 20 species have been documented. Because of the recent development of Philippine cetology, the biology and ecology of the cetacean species in this area are poorly known. This dissertation focuses on the small cetaceans found in the Sulu Sea and Tanon Strait, two contiguous marine ecosystems which differ with each other mainly in size, maximum depth, bottom topography, and protection from the monsoon winds. Chapter One contains a general introduction and background information. Chapter Two compares and contrasts the structure of cetacean fauna in the eastern Sulu Sea and Tanon Strait in terms of species composition, associations and abundance; and correlates the sighting rates of the commonly occurring species with environmental factors such as sea state, water depth and temperature. In addition, the structure of cetacean fauna in the two study sites was compared with those of other tropical marine ecosystems such as the eastern tropical Pacific, western Indian Ocean and the Gulf of Mexico. Fourteen species were found in the Sulu Sea, but only six of these were found in Tanon Strait. The abundant species were spinner dolphin, Stenella longirostris; Pantropical spotted dolphin, Stenella attenuata; Fraser's dolphin, Lagenodelphis hosei; and short-finned pilot whales, Globicephala macrorhynchus. Significant correlations were found between one or more environmental factors and dolphin sighting rates, except for the spinner dolphins.; Chapter Three investigates the feeding habits of Fraser's and spinner dolphins. Although both feed on mesopelagic fish, (primarily myctophids), squids and crustaceans, there was a significant difference in the vertical distributions of the important prey species that these two dolphins take. Basing on the species and size of its prey, spinner dolphins appear to forage mainly at night on myctophids migrating to the upper 200m and probably occasionally forage down to 400m. Fraser's dolphins appear to feed from near the surface to as deep as 600m, preferentially on larger prey; mesopelagic squids and crustaceans appear to be equally as important as the mesopelagic fishes.; Chapter Four is a comparative study of the myoglobin (Mb) content of the skeletal muscles of Fraser's and spinner dolphins and the pygmy killer whale, Peponocephala electra. Myoglobin concentrations in these animals are among the highest reported for small cetaceans. Fraser's dolphin has the highest value (6.8--7.2g 100g-1 ), followed by spinner dolphin (5--6g 100g-1) and pygmy killer whale (5.7g 100g-1). Myoglobin concentrations were correlated with the activity of the muscles (as determined by other studies) and with age or size of the animal. Results show that swimming muscles have significantly higher Mb concentrations than non-swimming muscles and larger or older animals have significantly higher Mb concentrations than calves and juveniles. The amount of myoglobin stored in the skeletal muscles of adult Fraser's and spinner dolphins suggests that they are capable of diving to the hypothesized foraging depths.; The last chapter contains a summary and recommendations. There is general agreement of the results of the studies on distribution, abundance, feeding, and diving capabilities, at least for Fraser's and spinner dolphins.