Ecosystem Services 3.2
Assessment of pup production and maternal investment strategies of the Australian sea lion Neophoca cinerea at Dangerous Reef in the 2006-07 breeding season
| ID Number | Project Number | Manager | Start Date | End Date | Total Project Funds ($) |
| SVE-5453G | RD06/0402 | Goldsworthy, Simon | 01-Jun-2006 | 28-Feb-2007 | 8,500 |
Executive Summary
Australian sea lion pup abundance was determined for the Dangerous Reef population during the 2006-07 breeding season. Using mark-recapture methods (including cumulative mortality), pup production was estimated to be 708 (95% CL 632-779). The exponential rate of increase in pup production from 1999 until 2006-07, based on maximum live-pup counts plus cumulative mortality and mark-recapture methods, ranged between 6.7-9.9% per breeding season, or 4.4-6.4% per year. This provides further evidence of strong positive growth in the population, which has been occurring since 2000.
A study to characterise the maternal strategies of Australian sea lions at Dangerous Reef was also undertaken. The parameters of maternal investment assessed included female mass, the duration of foraging trips and shore attendance bouts, the percentage time that females spent ashore, the birth mass and growth rates of pups to 100 days of age, and the lipid content of milk. These were compared to results from an earlier study undertaken at Seal Bay. No differences were detected in the mass of adult females, or the duration of foraging trips (~1.8 days) between sites. However, the duration of attendance bouts was significantly longer at Seal Bay (1.6 days vs. 0.9 days), and as a consequence, females at Seal Bay were ashore for a greater percentage of time (48% vs. 32%). Although the estimated birth mass of pups at Dangerous Reef was greater than that at Seal Bay, the growth rates of pups at Seal Bay were 27% higher than at Dangerous Reef. Consistent with this, the estimated milk-lipid content for females at Seal Bay was about 27% greater than that estimated for Dangerous Reef females. Based on these results, conditions for foraging and for raising pups would appear more optimal at Seal Bay than at Dangerous Reef, although there is uncertainty in this conclusion due to difference in the timing of the studies.
Available data suggest a marked difference in the respective population trends of the Australian sea lion populations at Dangerous Reef and Seal Bay. The Dangerous Reef population is currently increasing by about 5% per year, while Seal Bay population has been declining by about 1% per year, for at least 20 years. Comparison of the maternal strategies at each site are contrary to expectations based on the differing population trajectories, suggesting that differences in population trajectories are not related to differences in foraging conditions at each site. There is a growing consensus that the most likely cause for the lack of recovery in Australian sea lion populations is principally due to anthropogenic related mortality, especially from fishery bycatch. The recent pronounced increase in pup abundance at Dangerous Reef has occurred coincidentally with the cessation of shark fishing in Spencer Gulf in 2001. This provides circumstantial evidence that positive growth has followed a reduction in anthropogenic mortality for this population. In contrast, demersal gillnet fishing effort remains significant in waters adjacent to the Seal Bay population, and entanglement rates in monofilament gillnets and a declines in pup abundance suggests that mortality related to fishery bycatch may be significant in this population.