Augmenting mark–recapture with beach counts to estimate the abundance of little penguins on Penguin Island, Western Australia

Context Penguin Island supports the largest colony of little penguins in Western Australia. It is subjected to a suite of anthropogenic threats because of its proximity to an increasing urban population. For effective management of the colony, it is necessary to not only have knowledge of the size of the colony, but also the population trend of the colony. Aims To demonstrate a new cost-effective method of estimating the island-wide population of penguins on Penguin Island. Methods We estimated the island-wide population by combining mark–recapture sampling over 2 years on part of the island and beach counts of penguins arriving at night around the entire island. We estimated the abundance using closed population models, allowing for sex and time effects in capture probabilities. We had four capture occasions in 2008 only, and so considered heterogeneity of capture probabilities (Mh), using the Chao heterogeneity moment estimator. The proportion of all penguins counted that arrived at the four mark–recapture sites was then used to inflate the population estimate for the whole island. Key results In all, 62% of all penguins counted used the four mark–recapture sites. In 2007, there were an estimated 2369 ± 198 penguins, and 1543 ± 82 in 2008. When capture heterogeneity was allowed for in 2008, this estimate increased to 2069 ± 172. Conclusions Fewer eggs were laid and all measures of breeding performance were lower in 2008 than in 2007. Hence, the lower population estimate is most likely to represent fewer birds attempting to breed. However, further work on population estimates is required to determine whether capture heterogeneity occurs in both good and poor breeding years. Capture rates were affected by the presence of a full moon and high tides. Implications The estimate of the population can be used as part of the basis of a long-term monitoring program needed for effective management of the penguin colony. However, such studies must be coincident with the monitoring of a suite of reproductive and foraging parameters if short-term impacts of threats are to be recognised and well managed.