Response to change in the environment : population dynamics of Weddell seals in east Antarctica
Creators: name: Lake, SE
Publisher: name: University Of Tasmania
Year: 2023

description: In this thesis, two inter-related hypotheses were explored: The first hypothesis was that Weddell seals forage over small spatial scales (10s to 100s ofkilometers) and therefore experience, rather than move to avoid, resource limitation. The second hypothesis was a consequence of the seals experiencing periods of resource limitation i.e. that changes over time in resource availability affect the seals' annual rates of survival and reproduction so that these parameters can be used as indicators of foraging conditions. Diet study supported the premise that Weddell seals forage over relatively small spatial scales. Prey types were characteristic of the continental shelf. Many prey types were from inshore habitats. There was regional, temporal and fine-scale variation in the diet indicating that seals were foraging nearby where seats were collected. However, the wide variety in potential prey types may minimize coupling between Weddell seal population dynamics and the abundance of any particular prey species. Ground studies and satellite tracking from mid-winter showed that Weddell seals rest in areas of fast ice. There was indirect evidence for seals foraging further offshore in that many haul-out sites were adjacent to a lead of open water within which the seals could have traveled through the dense sea ice zone. The three tracked seals did rest regularly on coastal fast ice, suggesting that they were foraging within a limited area. Limited movement may be a behavioural strategy to navigate by spatial memory in the dark of Antarctic winter. Furthermore, evidence suggests that local seals communally use and maintain breathing holes in static ice, which may make access to and from the water reliable, despite temperatures well below freezing. Multi-strata models showed that the proportion of females in breeding state did change over time. Breeding proportion averaged 0.70 per annum, and ranged from 0.53 to 0.88 per annum. This contrasts with the relatively small magnitude of temporal variability in probability of parous females surviving from year to year, and is qualitatively similar to temporal variation in probability of pups surviving first year of life. The magnitude of inter-annual variability in both the probability of producing a pup, and the probability of that pup surviving, demonstrates how tenuous each reproductive event is for this long-lived mammal species. Better understanding of the teleconnections between El Nino - southern oscillation (ENSO) and local physical and biological processes might elucidate the loose connections sometimes observed between ENSO and Weddell seal reproduction. At this stage, it is simply noted that the longest El Nino (warm) event in the record of 113 years (1990-1995), closely followed by another very severe El Nino event (1997), did coincide with low reproductive rates of Weddell seals at the Vestfold Hills throughout the last decade of monitoring. In general, evidence suggests that the purported trend for increasing frequency and intensity of El Nino events could be detrimental to Weddell seals and the inter-dependent aspects of Antarctic ecosystems.