The migrations of East Coast Humpback whales (Megaptera novaeangliae)

Abstract

Southern hemisphere hwnpback whales undertake annual migrations from summer Antarctic feeding grounds, to breeding grounds in coastal sub-tropical and tropical waters in winter. En route the whales utilise extreme coastal waters as migratory corridors. After severe depletion by commercial whaling between 1904 and 1963, the species is showing a recovery off the east coast of southern Africa. The close proximity of the migration to the coast allowed for a recovery - monitoring programme to be carried out through shore-based visual surveys from Cape Vidal, northern Natal. Surveys of the northward migration were undertaken each winter from 1988 to 1991, and a survey of the southward migration was undertaken in 1990. Daily observations were carried out from an approximately 60 m high vantage point on a headland, during which attempts were made to measure migration characteristics for each group observed, including distance offshore, speed and bearing (measured by theodolite) and group size ( estimated by observers and confirmed by interception of groups by ski-boat). Characteristics of the northward migration were unaffected by environmental parameters (sighting conditions, wind speed or direction, and time of day) or interactive parameters (other migration characteristics). Each year's northward migration was comprised of waves, and apart from the distance offshore, there was no difference in the migration characteristic of each wave. Significant differences were found in group size, distance offshore and speed of the northward migration between years. No differences were found between the mean group sizes or speeds of the northward and southward migrations in 1990, although the southward migration was found to be significantly further offshore than its northward counterpart. The measurement of migration characteristics has played an integral part in population estimation, and in the choice of optimum periods to monitor annual abundance. Results of the 1988 and 1989 surveys suggested that not all groups within view were sighted by observers. Independent-observer surveys were therefore carried out in both 1990 (22 days) and 1991 (51 days), to determine the proportion of the population within the survey area that was being missed by observers using a single mark-release model. Results were stratified into three distance intervals from the shore and three sighting - condition intervals; there were constant sighting probabilities from the south tower under different sighting conditions, while those from the north tower increased slightly as sighting conditions improved. Sighting probabilities of both towers were highest in the intermediate distance interval, and decreased in both the inshore and offshore regions. Daily densities (expressed as the number of groups crossing the midline in front of the towers), adjusted for groups missed with distance from the shore and under different sighting conditions, were multiplied by the mean group size of the survey year, and resulting daily densities of individuals were summed to provide totals of whales passing the towers during each survey. The abundance estimate of 1711, made during the northward migration of 1990, was considered the most reliable estimate of population size made, although no associated cv could be calculated. The estimate of 1777 made during the 1991 survey had an associated cv of O .12 calculated by bootstrapping of daily data. The time series of four years is too short to provide an estimate of the recovery rate of the population, but the numbers sighted show the population to have undergone considerable recovery since protection in October 1963.