Project Title: Developing population monitoring protocols to determine the abundance of Australian sea lions at key subpopulations in South Australia
Chief Investigators: Dr Simon Goldsworthy and Dr Peter Shaunessy
The Australian sea lion was 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 surveys at Blefuscu, Lilliput and Breakwater Islands and Gliddon Reef in the Nuyts Archipelago using a combination of mark-recapture (Petersen estimate) and cumulative mark and count (CMC) methods. Other breeding and haul-out sites in the Nuyts Archipelago were also surveyed by single ground and aerial surveys undertaken at the end of the breeding season. Surveys of the Dangerous Reed and English Island colonies in southern Spencer Gulf were also undertaken using mark-recapture and CMC methods, respectively.
Three mark-recapture sessions were undertaken at Liliput and Blefuscu Islands during the 2007-08 breeding season, giving estimates of 64 (95% CL, 62-69) and 99 (95% CL, 92-106) pups, respectively. Due to sea conditions, landing at Breakwater Island and Gliddon Reef was only possible on one out of three occasions, when 15 and 7 pups were surveyed, respectively. Single ground surveys for pups at Lounds (34 pups), Purdie (95), West (39) and Fenelon (40), produced a total minimum pup production estimate for the Nuyts Archipelago of 393.
The 2008 breeding season at Dangerous Reef lasted nine months (early February to November). Four surveys were undertaken roughly corresponding to the first, third, fourth and fifth month of breeding. The maximum mark-recapture estimate (520 pups, 95% CL 509-535) was less than the maximum pup count of 537. This represents a maximum decline of around 24% since the 2006-07 breeding season and a departure following at least four consecutive breeding seasons of sustained increases in pup production. This decline most likely reflects a drop in fecundity rates between seasons, rather than a reduction in female population size, although surveying of subsequent breeding season will be needed to assess this. Even with the reduction in pup production in the 2008 breeding season, the overall trend in pup abundance is still increasing at around 6.9% per season, or 4.5% per year, since 1996.
Estimated mortality rates of pups to the fifth month of breeding at Dangerous Reef were high (43%). A very clear pattern of alternating high (mean 38%) and low (mean 18%) pup mortality between winter and summer breeding seasons has now been established at Dangerous Reef. Based on studies from other sea lion and fur seal populations, we propose that climate and season induced fluctuations in hookworm (Uncinaria spp.) infection in combination with enteritis-bacteraemia complexes explains the observed marked inter-seasonal fluctuation in pup mortality. Given these fluctuation could induce a large variance in recruitment and age-structure, there is critical need to understand the role of disease and parasites on pup survival, and on the broader population structure and demography of Australian sea lion populations.
Surveys undertaken at English Island were compromised by influxes of dispersing pups into the colony from Dangerous Reed (25km away), with an unknown number of untagged pups present by the second survey when pup production to June 2008 was estimated to be 23.
For surveys undertaken during this study, we are confident that mark-recapture methods provide accurate estimates of pup production at Lilliput and Blefuscu Islands, however those undertaken at Dangerous Reed underestimated total pup production, with the largest estimate being provided by counts of marked, unmarked and cumulative dead pups. The long nine-month breeding season meant that at least one additional survey should have been undertaken there, but limited fund prohibited this. Future surveys should build in some contingency to conduct additional surveys if required due to extended breeding seasons.
For the small colony surveys undertaken at Breakwater Island and Gliddon Reef, and English Island, two major problems were uncounted. Firstly, access at Breakwater Island and Gliddon Reef was difficult, with landings being only possible on one occasion over three attempts. The second major problem was at English Island, where the presence of dispersed Dangerous Reef pups undermined the cumulative count and mark (CMC) method, because an unknown proportion of unmarked pups present were from Dangerous Reef. The only way to eliminate the inclusion of dispersed pups from these surveys is to modify the CMC method to only include new black-coat and brown pups and increase the number of surveys (eg. Monthly from the second month of breeding.
A workshop to develop a national survey strategy for monitoring Australian sea lion populations is proposed to occur during 2009. Results from this and previous studies will be important in the selection of appropriate monitoring sites, survey methodology and frequency.
