Costs and benefits of aerial 1080 operations to Western weka (Gallirallus australis australis)

The impact of aerially applied 1080 poison on a Western weka (Gallirallus australis australis) population was assessed at Tennyson Inlet, Marlborough Sounds, between September 2010 and June 2016. We estimated mortality and the incidence of sublethal poisoning as a direct consequence of two aerial 1080 operations and examined the differences in nest success, chick survivorship and adult survivorship. Most weka in the treated block appear to have been sublethally poisoned but only one of 58 (1.8%) radio-tagged weka died as a direct consequence of 1080 application. Both adult and chick survivorship were higher in the treated block, but nest success was unaffected by the observed reduction in rat and stoat abundances following the 1080 operation. The net effect of aerial 1080 application on this weka population was positive when the operation followed a beech (Nothofagaceae) mast and stoat (Mustela erminea) irruption, as adult and chick survivorship after the operation exceeded adult mortality during the operation. However, 1080 operations carried out when stoats are uncommon may have a minor negative impact on weka populations due to the risk of primary poisoning without compensatory increases in survivorship.