Use of genetic mark-recapture to assess the efficacy of thermal aerial control in reducing deer density in mountainous forest

Invasive deer species pose significant ecological threats to New Zealand’s indigenous forests. This study evaluated the effectiveness of a helicopter-based hunting technique, Thermal Aerial Control (TAC), in reducing red deer (Cervus elaphus) density in mountainous forest in Fiordland, New Zealand. Using DNA-based mark-recapture methods, the pre-control deer population was estimated at 79 deer (68–92, 95% CI). The TAC operation killed 60 deer over eight sorties totalling 10.65 hours, reducing density by 76% from 5.6 deer per km2 to 1.3 deer per km2, a level expected to allow regeneration of most deer-palatable plant species if maintained. Thermal Aerial Control detected ≥ 73% of deer within the catchment, of which 91% were shot. Analysis of habitat variables at kill sites showed deer were more likely to be detected and shot on steeper slopes (> 23°). Compared to ground-based hunting assisted by dogs in similar Fiordland forests nearby, TAC was more cost-effective at low deer densities, averaging NZ$595 per kill versus NZ$1428 for ground-based methods. These findings demonstrate that TAC, in the habitat evaluated, is a highly efficient tool for rapidly reducing forest-dwelling deer populations to levels that support conservation outcomes, with potential applicability across similar forest types and terrain throughout New Zealand.