Growth of the southern elephant seal Mirounga leonina (Linnaeus 1758) at Macquarie Island
thesisposted on 2023-05-27, 01:01 authored by Bell, CM
Body growth was an essential component of early biological research undertaken to develop sustainable harvesting methods of the southern elephant seal (Mirounga leonina Linnaeus 1758). Since slaughter ceased in most areas earlier this century, a substantial decline in numbers of elephant seals has been recognised at several of the main breeding sites, stimulating interest in life-history attributes which may help provide an explanation for these declines. Growth of elephant seals from the Macquarie Island population was investigated to quantify the current growth pattern and to compare with the stable population at South Georgia, where elephant seals were known to be larger, grow faster, and breed earlier. A method was developed to indirectly measure body mass using photogrammetry. Predictive models were developed using photographic images alone, but when compared with models based on measured morphological characteristics, the best predictor of body mass was a combination of body length and girth squared. Growth during the first foraging trip was described and found to reflect the hindrances of early life at sea. Growth was hypothesised to comprise primarily of adjustment to body composition, rather than increase in net body size. Both sexes may exploit similar marine resources during the first foraging trip, as no significant difference in body growth was found. Interspecific comparison suggested that southern elephant seals grow considerably more than northern elephant seals (Mangustirostris) during this period, and that this may be related to prey abundance and distribution. A cross-sectional growth study was undertaken for females up to 15 years of age. Dental cementum layers were counted to estimate the age of some animals, which proved to be a relatively accurate and useful technique. The snout-tail length of adult Macquarie Island cows has increased significantly since the 1950s and 1960s, and is now similar to that of the current South Georgia population. The temporal changes at in snout-tail length at Macquarie Island are consistent with either a population equilibration process or long-term environmental change reducing population density. These findings provide little supporting evidence for a particular cause of the Macquarie Island population decline, but the most likely scenario is that several factors are implicated, these having resulted in changes in marine resource availability. Body growth of southern elephant seals is determined by many factors, but prey abundance and distribution are fundamental influences on all growth stages. Other factors which may play a role include harem disturbances, behaviour of weaners, disease, juvenile body size and stored energy reserves.
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