Project 0607/13

Project Title: Developing population monitoring protocols for Australian sea lions

Chief Investigators: Dr Simon Goldsworthy and Dr Peter Shaughnessy

Australian sea lions (ASL) were listed as a threatened species under the EPBC Act in February 2005. Information on the size and status of most subpopulations is poor and hampers developing appropriate management strategies for the species. Many aspects of the species breeding biology and ecology are unique among otariids (fur seals and sea lions) and make accurate assessment of pup production challenging.

Goldsworthy et al. (2007a), addressed these challenges by trialling new methodological approaches to estimating pup production at large (>40 pups) and small (<40 pups) ASL subpopulations. At Olive Island they collected individual re-sight histories of pups throughout the breeding season and used Cormack-Jolly-Seber (CJS) models in conjunction with standard mark-recapture (Petersen estimate) methods to improve estimates of pup production for large ASL subpopulations (>40 pups). They also developed a cumulative mark and count (CMC) method for improving estimates of pup production in small ASL subpopulations, and trialled this at the Seal Slide (Kangaroo Island).

This study extends that of Goldsworthy et al. (2007a), by continuing the trails of the Petersen/CJS estimate for large colonies at Olive Island and Lewis Island, and for the CMC method at two small colonies (the Seal Slide and Jones Island), all of which had breeding seasons during 2007. The estimates of pup production for Olive Island were 161 (151-172, ±95% CL), indicating a large decrease (21%) in pup production from the 2006 breeding season. For Lewis Island the 2007 estimate of pup production was 131 (116-146, ±95% CL), representing the first accurate survey of this subpopulation since its confirmation as a breeding colony in 2005. Estimates of pup production at Lewis Island were confounded by dispersal of pups from neighbouring Dangerous Reef. The estimate of pup production at the Seal Slide was 16 (range 15-18), and 15 for Jones Island.

This study has confirmed that mark-recapture methods using the Petersen estimate are likely to accurately estimate true pup abundance if surveys are undertaken at the appropriate times throughout the breeding season with the appropriate effort. The main source of error comes from unaccounted mortality, and CJS estimates of apparent survival provide a simple internal check against which cumulative recovered mortalities can be compared, and if necessary adjusted. The study had also confirmed that CMC and Petersen estimate methods provide suitable means to estimate pup production with confidence limits at small Australian sea lion colonies. Standardisation of methodologies will enhance the accuracy and precision of estimates, and facilitate assessment of trends in abundance at key monitoring sites.