Endemic to Southern New Zealand, Otago (Leucocarbo chalconotus) and Foveaux shag (L. stewartia) are classified as threatened, but there is little recent information on their population status and trends to inform conservation management. The aim of this project was to conduct a breeding population census of Otago shags in 2021 (Part A) followed by Foveaux shags in the following breeding season, 2022 (Part B). The attached report covers the Otago shag component.
Comprehensive surveys were conducted in targeted visits of current breeding sites. Aerial photographs for Otago shag counts were taken using a drone where appropriate (six colonies) or vantage-point DSLR photographs where a drone could not be flown (one colony). Building on animal response trials in previous work, these drone overflights during the breeding season first determined the drone flight height appropriate at each site to cause minimal disturbance. Survey flights were all taken within a week of each other, at the start of the breeding season in September 2021.
Photographs were stitched and counted, recording the number of apparently nesting Otago shags. To correct counts of apparently nesting shags (apparently on nest, or AON), we collected ground-truthing data assessing nest contents at one colony (Pukekura), finding that 0.74 of apparently nesting Otago shags were actually breeding at the start of September. The size of the breeding population is then calculated as the raw count of apparently nesting pairs multiplied by the nest-contents correction. Since surveys took place at the very start of the breeding season, we expect to have missed some birds yet to lay, so figures should be understood as minimum breeding population estimates.
The breeding colonies ranged in from the small southern colony at Kinakina Isl (estimated 32–33 breeding pairs) to the very large colony at Sumpter Wharf comprising some 504 breeding pairs (best estimate; range 496–511). The Otago shag population estimate—at least 1,275–1,332 breeding pairs at the start of the 2021 breeding season—is roughly similar to the last whole-population count in 2007. Despite different methods used, we believe this comparison is reasonable because independent counts of five colonies in 2021, using the vantage-point methods of earlier work, gave comparable numbers to our estimates from aerial photographs. However, for re-assessment of population trends to be robust the population size estimate should first be repeated, considering the 15-year interval since the last regular colony counts and the unknown population dynamics in that interval.