The life history of sperm whales, Physeter macrocephalus, from southern Australian waters

• Main Author: Evans, Karen J.(Karen Jennifer)
• Format: Thesis
• Language: English
• Published: 2003
• Subjects: Sperm whale; Physeter macrocephalus
• Online Access: https://eprints.utas.edu.au/19772/; https://eprints.utas.edu.au/19772/7/whole_EvansKarenJennifer2003_thesis_ex_pub_mat.pdf; https://eprints.utas.edu.au/19772/1/whole_EvansKarenJennifer2003_thesis.pdf

One hundred and fifteen sperm whales (97 female, 15 male, 3 unknown gender) were involved in three mass stranding events during the month of February 1998 along the west and north-west coastlines of Tasmania, Australia. Sixty six of these whales stranded at Ocean Beach, Strahan, 35 at Greens Beach, Marrawah and 11 at Black River Beach, Stanley. The remaining three whales stranded singly along the coastline. Three mass strandings of this species in such close temporal proximity have not been reported in this area before, and this is the first time that samples have been systematically collected from complete or near-complete groups of sperm whales from Tasmanian waters. The broad aim of this project was to examine the life history of complete or near complete female groups of sperm whales in an effort to contribute to the paucity of knowledge on two key areas of research identified for sperm whales in the Australian Government's Action Plan on Australian Cetaceans: the basic biology of this species and pollutant impacts on this species. This study is made up of three components: (i) by developing modifications to current methods of age determination, an assessment of the demographics and growth of the female component of these groups was undertaken; (ii) by collecting stomach contents and blubber samples, an investigation into energy acquisition and storage was undertaken and (iii) concentrations of organochlorines were determined from blubber samples and concentrations and interpreted in light of ecological factors such as diet and demography. The three stranding groups were composed primarily of adult females. Total lengths of all animals ranged from 417-1200 cm and ages ranged from 0.75-64 years. Female sperm whales, unlike many other mammals, demonstrate high, relatively stable survival throughout their entire life span. Overall, growth is prolonged in female sperm whales, not reaching asymptotic length until around 20 years. The longevity, low fecundity, slow growth, delayed sexual maturation and high input of resources into young over a protracted period define sperm whales as extreme K-selected animals. Survival in mature female sperm whales in this study was higher than those observed in mature females from Japanese waters and similar to that observed in female sperm whales deriven from Western Australian whaling operations. This suggests survival in female sperm whales from Australian waters may have undertaken little change post whaling. However, there appear to have been some changes in the age structure of female sperm whale groups and additionally increases in the total lengths of individuals, which indicate some postwhaling demographic changes. The diet of southern Australian sperm whales in late summer was dominated by oceanic cephalopods. Cephalopod beaks from stomach contents represented 48 species from 14 families of Teuthids, two species from two families of Octopods and the single Vampyromorph species. Subtropical and muscular species of cephalopods dominated the diet of southern Australian sperm whales, but a high level of both inter- and intra-group variability in the diet was apparent. Common cephalopod prey species were similar to that of sperm whales elsewhere in the Southern Hemisphere and included members of the Histioteuthidae, Onychoteuthidae, Ommastrephidae, Architeuthidae, Cranchiidae and Pholidoteuthidae families. While smaller species (<300 mm dorsal mantle length) were numerically abundant, larger species (>1,000 gm) were clearly important to sperm whales (comprising 78.6 % of the total estimated wet mass) and are likely to provide an efficient means of acquiring energy for this species. Differences in diet composition and prey size between sperm whales reflect individual variability in foraging success and perhaps also foraging groups related to the social structure of this species. Individual variability in foraging success and therefore, in the acquisition of energy will be reflected in an individuals energy stores. High individual variability characterised both blubber thickness and blubber lipid content in the sperm whales in this study, which suggests both deposition (acquisition) and mobilisation (utilization) of energy stores differed between individuals. The social structure and foraging ecology of this species may serve to minimize the need to rely on stored energy reserves to meet reproductive energy requirements. Continual foraging of lactating mothers facilitated by communal care of young by other members of a pod would aid in facilitating flexibility in the acquisition of energy in an effort to meet energetic costs associated with reproduction, minimising the need to draw on energy reserves to meet those demands. Additionally, the broader role of blubber for structural, buoyancy and insulative functions coupled with high individual variability may cause a lack of obvious relationships between these variables and body size, age, sex and reproductive state in this species. Organochlorines were present in the blubber of all sperm whales sampled in this study. The relationships between organochlorines, sexes, age and reproductive groups were marked by high individual variability and highlight the complexity of organochlorine accumulation in this species. Differences in organochlorine concentrations were observed between two stranding groups and are likely to be the result of differences in the dietary composition and foraging areas of the groups. As a result of this smaller geographic scale of variation, it is therefore difficult to determine temporal changes in organochlorine concentrations positively in highly mobile species across large regions. Organochlorine concentrations were on the whole lower than those observed to be linked with deleterious effects in cetacean species elsewhere. However, it is difficult to draw clear conclusions from this due to species-specific intake, differences in metabolism and differences in physiological reactions to pollutant concentrations. A life history involving low fecundity, high longevity, slow growth rates and delayed , attainment of sexual maturity, a high input of resources into young, and high sociality involving communal care of young and communal defense all under-pin the aspects of the life history sperm whales observed in this study. Yet despite the high dependence on life as a social "unit" this study highlighted the influence of individuality on the life history of female sperm whales. The sociality of female sperm whale groups allows for flexibility in life history traits providing these animals with individual means to sustain their fitness in an aquatic environment and allows them to ride out temporal and geographical fluctuations in the environment. The identification of present day threats and their impacts on sperm whale populations (e.g. chemical and noise pollution, competition with fisheries), moreover establishing the identity and interactions between sperm whale populations in the Australian region, are essential for determining current pressures on populations and should be a high priority for environmental managers in ensuring the conservation of this species.