The age structure and growth of female sperm whales (Physeter macrocephalus) in southern Australian waters

Abstract

The age of 86 individuals derived from groups of female sperm whale Physeter macrocephalus involved in three mass strandings on the north and west coasts of Tasmania in 1998 was determined from the number of dentinal growth layer groups in the teeth of individuals. Dorsal total lengths were also measured. Ages of females ranged from 0.75 to 64 years, with the majority (77%) aged between 20 and 45 years. Total lengths of female sperm whales ranged from 417–1200 cm, with 68% of females 1050–1200 cm long. Constraints associated with the age structure observed in this study and the representativeness of the age structure to that of the greater population are discussed. In an effort to assess the effects of underestimation of age estimates on age-specific demographic parameters, a model simulating changes in age structure as a result of tooth wear was developed and the resulting survival rates compared to those derived from the original age structures of the samples. Survival did not change significantly between the modelled and original age structures. Also, the survival rate calculated from Australian mature female sperm whales was compared to that calculated from sperm whales caught in whaling operations from Japanese waters. Australian mature female sperm whales demonstrated significantly higher survival rates (mean=0.905±0.046 SD range: 0.856–0.986) than mature females from Japanese waters (mean=0.885±0.034 SD; range=0.859–0.970), possibly the reflection of higher fishing mortality on the Japanese whales. Growth equations using Gompertz and von Bertalanffy models were calculated for female Australian sperm whales and compared. Both models described growth in female sperm whales similarly. Changes in the demographic parameters of sperm whales with the cessation of whaling may be reflected in the growth rates of individuals and as such, these equations may provide a useful tool for monitoring continuing changes in the demographic parameters of this species. Without long-term mark–recapture studies on sperm whales in this region, the frequent stranding of sperm whales in south-eastern Australia provides an important opportunity to collect data on the life history and demography of this species.