Susan Dufault

A Photoidentification study of the geographic stock structure of sperm whales (Physeter macrocephalus) in the South Pacific

Masters Thesis, Dalhousie University, 1994

ABSTRACT
Photo-identification was used to assess the stock structure of sperm whales (Physeter macrocephalus) in the South Pacific through resighting data and a comparison of regional differences in the occurrence of fluke marks. Individuals identified during a survey of the South Pacific in 1992-93 were compared to identifications in existing Galápagos and Ecuador/northern Perú catalogues. Rates of change in fluke mark patterns were also assessed.

Changes in mark patterns observed in this study were, in general, quite low. However, major changes were found to occur though their frequency is not known and hence their impact on photo-identification studies cannot be assessed. Gain rates were, in general, greater than loss rates of marks and hence animals may accumulate marks with age.

Encountered groups of sperm whales showed significant differences in the incidence of some environmentally-caused marks suggesting that individuals within these groups have similar experiences of predation possibly due to their tendency to maintain close groupings in the face of predators. Tests using fluke notch type (thought to be hereditary) indicated that animals encountered together were more closely related than those within a concentration (500km across) or a region (5,000km across).

No sperm whales seen previously off the Galápagos or Ecuador/northern Perú were found anywhere in the western South Pacific or in the eastern Pacific south of 10°S. Some animals seen previously off the Galápagos (1985 and 1989) were found off the Ecuador/N. Perú area north of 10°S though these areas were found to contain separate stocks. It seems likely that the range of movements of female and immature sperm whales in the South Pacific is limited to distances on the order of 1,000km. No large, mature male seen in this study matched to any known individual. It is possible that the mature males observed in this study (all from the western South Pacific) are from a separate breeding stock from those in the eastern tropical Pacific.

PUBLICATIONS

Dufault S., Whitehead, H. and Dillon, M. In processing. An examination of the current knowledge on the stock structure of sperm whales (Physeter macrocephalus) worldwide.

Dufault S. and Whitehead, H. In press. Regional and group-level differences in fluke markings and notches of sperm whales. Journal of mammalogy

Dufault S. and Whitehead, H. 1995. An encounter with recently wounded sperm whales (Physeter macrocephalus). Marine Mammal Science 11(4):560-563.

Dufault, S. and Whitehead, H. 1995. An assessment of changes with time in the marking patterns used for photoidentification of individual sperm whales, Physeter macrocephalus. Marine Mammal Science 11(3):335-343.

Dufault, S. and Whitehead, H. 1995. The geographic stock structure of female and immature sperm whales in the South Pacific. Annual Report of the International Whaling Commission 45:401-405.

Dufault, S. and Whitehead, H. 1994. Floating marine pollution in 'the Gully' on the continental slope, Nova Scotia, Canada. Marine Pollution Bulletin 28:489-493.

Dufault, S. and Whitehead, H. 1993. Assessing the stock identity of sperm whales in the eastern equatorial Pacific. Annual Report of the International Whaling Commission 43:469-475.

Website created by Sascha Hooker This page updated December 1, 1997 by Susan Dufault