Bioacoustics of Balaenoptera musculus

Movements, Home range and Social organization. Worldwide, Blue Whales have a cosmopolitan distribution, occurring in all major ocean basins except the Arctic. Precise information on migration routes and home ranges is limited and comes from a variety of data sources including historical whaling records, stranding records, and results of modern sighting and acoustic surveys. Data for some subspecies and populations are more extensive than for others, and general biogeographic patterns have undoubtedly been altered by intense, worldwide exploitation of Blue Whales that occurred from 1904 to 1973. Extremely depleted populations of Antarctic Blue Whales retain a nearly circumpolar distribution in Antarctic waters during austral summer and early autumn feeding season in November—March. During this period, their distribution appears to be strongly linked to latitude, with feeding individuals concentrated along latitudinal bands south of 60° S (generally south of the Antarctic Polar Front). In contrast, distribution of Antarctic Blue Whales during austral winter seems to be more clustered, localized, and compressed along low-latitude continental margins and offshore submerged volcanic platforms in areas associated with upwelling. Historical whaling records suggest that during austral winter, Antarctic Blue Whales formerly were concentrated in the eastern South Atlantic Ocean off the western coast of Africa from South Africa to DR Congo, with their distribution likely linked to waters of high primary and secondary production associated with the upwelling system of the north-flowing Benguela Current. A similar distribution of Blue Whales is proposed in the eastern South Pacific Ocean off Chile and Peru. Although clearly most Antarctic Blue Whales migrate into low latitudes during austral winter,it is not certain if they all do. In fact a close examination of whaling records suggests that there may have been a resident population of Antarctic Blue Whales in and around South Georgia Island (54° S) that routinely overwintered in the high-productivity waters surrounding this oceanic island. Migratory patterns of Pygmy Blue Whales are poorly known. It appears, however, based on historical whaling records and sighting and acoustic surveys, that the population of these whales in the south-western Indian Ocean south of Madagascar moves southward during austral winter and then disperses longitudinally across the subantarctic region from Africa to Australia. Evidence indicates that some of these individuals may migrate northward to Diego Garcia (6° S) in the central Indian Ocean during austral winter. Additional evidence, however, suggests occurrence of a year-round resident population of Pygmy Blue Whales in the northern Indian Ocean, another year-round resident population off southern Madagascar, and a possible third year-round resident population in waters around Australia and New Zealand. In the Northern Hemisphere, different populations of Blue Whales have been recognized in eastern and western parts of the Atlantic and Pacific oceans. The western North Atlantic population of Blue Whales ranges from Greenland (60-70° N) in boreal summerto South Carolina (34° N) in winter, while the eastern population has its boreal summer range as far north as the Barents Sea and Svalbard (80° N) and winters in the region around the Cape Verde Islands (15° N). The population of Blue Whales in the eastern North Pacific Ocean appears to spend boreal winter in waters off Baja California (25° N), south to the Costa Rica Dome (10° N), and in the summer,it ranges north to the Aleutian Islands (55° N), with some resident individuals remaining off the central California coast (34° N). The population of Blue Whales in the western North Pacific Ocean spends boreal summer feeding in waters off Kamchatka and the Kuriles Islands (45-60° N) and migrates south in winter to the area south ofJapan around the Bonin Islands (27° N). Social structure of Blue Whales is largely unknown. Most observations at sea are solitary individuals or pairs. Aggregations of 50 or more individuals have been reported on summer feeding grounds, but these concentrations are most likely because of the high productivity of polar seas. With the advent of new technologies, such as digital tagging,satellite tracking, and biopsy sampling of living whales, the opportunity for answering a number of basic questions concerning population structure and identity of Blue Whales may be at hand. Another method potentially useful in understanding population structure involves acoustic studies of vocal sounds of Blue Whales, which may represent the most powerful (188 dB) and lowest frequency (16—-100 Hz) sounds produced by animals in nature. These vocal sounds can be classified as either songs or calls, with songs representing a limited number of stereotypic sound types produced in a regular sequence and forming a recognizable pattern, while calls are sounds consisting of individual short-lived vocalizations produced at irregular intervals or as reciprocal calls and counter-calls between two or more individuals. Songs of Blue Whales can be detected for hundreds and perhaps even thousands of kilometers, and researchers have distinguished nine song types worldwide, each associated with a different geographical region. Four song types are recognized in the Pacific Ocean, at least three in the Indian Ocean, and one each in the Atlantic Ocean and Southern Ocean. These song types, some of which have remained stable for more than 30 years, probably reflect distinct populations and thus serve as another potential means for characterizing population structure. Although functions of Blue Whale songs are currently unknown,it is likely that, at a minimum, they serve as social signals between interacting individuals.