Measuring rat relative abundance using camera traps and digital strike counters for Goodnature A24 self-resetting traps

Invasive ship rats (Rattus rattus) pose a threat to the biota of Goat Island (9.3 ha), New Zealand. In June 2016 we installed 10 Goodnature A24 CO2 powered self-resetting rat and stoat traps equipped with digital strike counters (Goodnature Ltd., Wellington, NZ) to control rat numbers on the island. The self-resetting traps were monitored with motion-activated cameras to develop a measure of rat abundance from camera traps. All devices were checked on 10 occasions from August 2016 to October 2017. The videos revealed high rat activity on the island, which reduced over time. Counting only the number of videos that showed rats did not result in any loss of information when compared to more laboriously counting individual rats in videos and was therefore the preferred method for obtaining an index of relative rat abundance. We also found that digital strike counters designed to record the number of times an A24 is triggered, accurately reflected the number of individuals killed by A24s. However, measuring rat abundance in number of rat videos per 100 camera nights was shown to be of greater value when rat abundance was low and A24s failed to detect the remaining individuals.