Trends in abundance of Australian sea lions, Neophoca cinerea, at Seal Bay, Kangaroo Island, South Australia

Pups of the Australian sea lion have been counted at Seal Bay for 20 pupping seasons, 1973-74 to 2002-03. Temporal changes in counts of live pups over the course of each pupping season were fitted to Gaussian (normal) curves to determine objectively the date when pup numbers reached their peak. The mean interval between pupping seasons was 532 31 days (i.e., 17.5 months). Maximum counts of live pups for 13 pupping seasons averaged 144 (s.d. 14) from 1985 (from when data quality was adequate) to 2002-03. The data show an annual decrease of 0.77% (exponential slope of regression was -0.0077, r(2) = 0.216), or -1.14% per breeding cycle (95% confidence limits -2.47% and +0.20%), but this exponential regression was not significant. Maximum pup numbers for each pupping season were correlated with duration of the interbreeding intervals, such that more pups were counted following shorter interbreeding intervals than following longer intervals. This relationship was not significant, but with one outlier removed it became highly significant, suggesting that pup numbers were influenced by the duration of interbreeding interval. A generalized linear model incorporating three predictor variables (year, interbreeding interval, and their interaction) produced a significant model that explained 51% of the variance in pup numbers, and both year and interbreeding interval had a significant negative effect on pup counts. A generalized additive model (GAM) using cubic spline smoothing functions produced a highly significant model with both terms (year and breeding interval) having negative coefficients. We conclude that year and duration of the interbreeding interval affect pup counts negatively, but that a significant component of the variance is accounted for by the interaction between year and breeding interval. Our best estimate for the rate of decline in the Seal Bay population is from the exponential regression analysis (i.e., 0.77% per year, 12.6% decline between 1985 and 2002-03). These analyses suggest that the reproductive output of Australian sea lions at Seal Bay has declined over the period 1985 to 2002-03. This decrease is contrary to recent increases of New Zealand fur seals, Arctocephalus forsteri, in Australia. The decrease of sea lion numbers at Seal Bay is a cause for concern and deserves further investigation.