Effects of aerial 1080 operations on deer populations in New Zealand

Aerially distributed baits containing sodium fluoroacetate (1080) are used in New Zealand for small-mammal pest control over an average of about 600 000 ha each year. This can also kill non-target species, including deer. This incidental mortality of deer generates antipathy to 1080 amongst many hunters, adding to the broader opposition to aerial 1080. Hunter opposition to 1080 baiting has also prompted the development of deer-repellent 1080 bait formulations. Historical estimates of deer mortality varied widely but were sometimes high. However, a recent study showed no adverse impact of 1080 on red deer sighting rates in one area and suggested that modern baiting protocols and frequency may have reduced the risk to deer. Here we provide a broader review of deer mortality observed in 26 aerial 1080 operations (or part operations) conducted since 1999. The estimated mortality of deer ranged from 0% to 100%. Overall, 42%, 38% and 20% of operations were classed as having low (0 to 33%), moderate (34 to 66%) or high (67 to 100%) impacts on deer populations, respectively. Adult males were found dead less often than adult females and all other age/sex classes, suggesting that by-kill risk might sometimes be inversely related to body size. Lower sowing rates (0.25–1.5 kg ha⁻¹) more often resulted in low deer by-kill than higher sowing rates (2.0–3.1 kg ha⁻¹), but there was no indication that using larger (12 g) rather than smaller (6 g) baits increased deer mortality. There was some indication that mortality may be lower where 1080 operation had been repeated within 5 years, possibly because of learned bait aversion in survivors of previous operations. The determinants of incidental mortality of deer are complex and somewhat unpredictable, and some deer are likely to be killed in most if not all operations, irrespective of the baiting strategy. We recommend formally planned experiments (rather than gathering often informal observations as summarised here) to assess whether deer mortality is significantly reduced when low sowing rates and/or short-interval repeat baiting is used, and (if so) whether this jeopardises efficacy in possum and rat control.