Testing the effectiveness of integrated pest control at protecting whio (Hymenolaimus malacorhynchos) from stoat (Mustela erminea) predation in beech forest (Nothofagaceae)

The introduction of mammalian predators, particularly stoats (Mustela erminea), to New Zealand led to the decline in whio (Hymenolaimus malacorhynchos), an endemic riverine duck. Stoat control for whio in the South Island has focused on valley floor trapping along waterway margins but increasing survival and productivity for whio using this method is complicated by irruptive predator dynamics caused by occasional masting of beech species (Nothofagaceae). We investigated the effect of integrating stoat trapping with pulsed aerial 1080 toxin operations to counter predator irruptions on whio survival and productivity in c. 40 000 ha of beech dominated Kahurangi National Park. We collected data on adult female whio survival, probability of breeding, nesting success, and duckling survival for seven years through two full beech mast events. We found a positive relationship between distance into the interior of the treatment block and whio population growth, nesting success, duckling, and adult survival. Heavy female whio were more likely to breed than light ones, and adult female survival was higher during the breeding season than during non-breeding. Nesting success was greatest in the breeding season following a 1080 operation. Duckling survival was higher at lower river flows. Positive population growth was only predicted near the centre of the study area (c. 13 km from the edge) with both traps and 1080. We conclude that in the centre of treatment blocks at least as large as our study block, integrated pest control should be enough to ensure long-term population persistence of whio in a beech-dominated system.