Abstract
The distribution, population structure, social organisation, and behaviour ofsperm whales (Physeter macrocephalus) was investigated off the Kaikoura coast between spring 1990 and summer 1992. Acoustic surveys of uniform coverage design, stereophotogrammetry, and identification photographs were used to carry out this investigation.
The abundance of whales peaked in winter, when significantly more whales were heard during acoustic surveys. Data collected on seasonal distribution and habitat partitioning between large and small males indicate an inshore peak in abundance of large males during winter. Whaling data gathered in this area and knowledge of local fishing trends suggest that this may be in response to movements of groper.
The development of a simple, accurate, boat-based stereophotogrammetry system allowed us to make size estimates for 41 individually identified sperm whales. Length estimates for these whales ranged from 8.69 metres to 14.95 metres with median 95% confidence limits of± 0.46 metres.
Likely sex, age, and maturity levels were assigned for all measured and identified whales in the population. These assignments were based on our length estimates, presence or absence of a dorsal callus and an age/length key. Dorsal calluses were observed on 24.4% of whales believed to be females, immatures and pubertal males. Our data indicate that the population of sperm whales off the Kaikoura coast was largely comprised of young males between the ages of about 10 and 20 years. Most of the whales were in the age range of pubertal males, with five males being in the sexually mature age range, and three in the socially mature age range. There were also a few small whales whose sizes suggest they may be females or immature males.
Associations among whales were weak. The majority of encounters were with single whales. Large groups were rarely seen, and whales seen in groups tended to be of similar size. Breaches, lobtails, and codas, which are believed to serve in social communication, were very infrequent.
The whales appeared to be spending most of their time feeding. Dives lasting an average of 40 minutes were typically followed by a c. 10 minute resting period at the surface. Whales approached while at the surface sometimes exhibited avoidance reactions to approaching vessels by turning away or shallow diving.