Comparison of breeding population survey methods for the Auckland Island shag (Phalacrocorax colensoi)

The Auckland Island shag (Phalacrocorax colensoi) is endemic to New Zealand and restricted to the subantarctic Auckland Islands. The species is classified as "Vulnerable" under the New Zealand threat classification systems. The total breeding population is estimated at <1000 pairs, with Enderby Island considered to be the stronghold for the species. The trend of the population is unknown, and it has been recommended a census of the entire adult population be conducted once a suitable methodology has been developed. The objective of this research was to examine census methods to estimate population size for the Auckland Island shag. Three census methods were trialled on Enderby Island during the 2011/2012 breeding season: (a) boat-based counts; (b) aerial photography using a helicopter and; (c) direct counts from the ground. The aerial photographic survey recorded 1889 pairs of Auckland Island shags breeding on Enderby Island during the 2011/2012 breeding season. In comparison, boat-based and ground count surveys recorded lower population sizes by 27 and 26%, respectively. This research provides the first comparable methodology and baseline population estimate for the breeding stronghold of this threatened seabird and forms the basis for long-term population monitoring. All methodologies examined here could be used for other cliff-nesting seabird species; however, these results indicate a level of bias may be present for any survey method.