Local movements, migrations and habitat use of humpback whales off the west coast of South Africa, including observations of southern right whales

Abstract

The migration of Southern Hemisphere humpback whales Megaptera novaeangliae between their feeding and breeding areas is considered a highly predictable and seasonal event. The west coast of Africa is host to Breeding Stock (BS) B, which has been divided into sub-stocks B1 and B2 based on different catch histories observed between Gabon in the north, and other stations to the south – a notion supported by recent regional genetic analysis, some samples of which were collected during this project. It has thus been assumed that the west coast of South Africa (WSA) functions as a migratory corridor for BS B2 whales. While catch data from Saldanha Bay in 1911-12 supported the existence of two migration peaks, a pilot study in the spring of 1993 (the first dedicated study in over 80 years) suggested a more atypical pattern, with some whales apparently abandoning or suspending their migration, possibly to feed. This has been complicated further by the detection of direct transits of 10 individual humpbacks through microsatellite matches between Gabon and WSA, and has raised questions about the exact function and relationship of WSA to BS B humpback whales. This thesis presents the results from a study based at Saldanha Bay that included shore-based observations of whale groups during two field seasons (July – December 2001, May 2002 - February 2003), and photographic and genetic data collected during boat intercepts from 1983 to 2008. The observed relative abundance of humpback whales again did not support a classical migration pattern, with the highest sighting rates from mid-spring through summer. Movement patterns of humpback groups tracked by theodolite showed mid-spring to be a turning point in their behaviour, after which they swam significantly slower, showed an increase in ‘non-directional’ movement, and were found farther from shore. Data on group composition and sex showed a significantly female-biased sex ratio during mid-spring, unlike most low-latitude areas where males predominate. The individual identification of humpback whales by means of photographs of ventral tail flukes, left and right dorsal fins, and through microsatellites, yielded numerous resightings at intervals of a year or more, indicating a high level of fidelity to the region and temporary residency by some individuals. Population estimates were calculated using open and closed capture-recapture models and suggest that about 500 animals are present in the area during the spring/summer season. Direct observation of humpback whales feeding on crustacean prey, short-term association patterns, and the resighting of individuals participating in feeding aggregations in multiple years confirmed this area to be a feeding ground for humpback whales during spring and summer months. Concurrent shore-based observations on southern right whales Eubalaena australis showed that this species was present virtually throughout the study period, also utilising feeding opportunities during summer. The potential benefit of this mid-latitude feeding area for humpback females is illustrated by a record of a cow that produced calves in three consecutive years, each of which survived to at least six months of age - the first observation of post-partum ovulation for this species in the Southern Hemisphere. The return of three known calves to the same area is strongly suggestive of maternally derived site fidelity.