Gut content analysis of trapped mustelids and trend analysis for ship rats and stoats over six trapping seasons at Mount Taranaki : A thesis submitted in partial fulfilment of the requirements for the degree of Master at Lincoln University

Abstract

Stoats (Mustela erminea), weasels (Mustela nivalis), ferrets (Mustela furo) and ship rats (Rattus rattus) are responsible for some of the greatest losses to native biodiversity in New Zealand. Common methods to control these species include trapping, poison bait stations and aerially applied sodium fluoroacetate (1080). However, effective management through sustained control requires a complex understanding of the target species regarding population dynamics and their responses to environment and resources. Consequently, it is vital to better understand the diet of mustelids, identify spatial trends, use of habitats, and the effect of elevation and habitat type on capture rates of stoats and ship rats, so that trapping networks can be focussed in areas where they are most needed. In this study, the gut content of 31 stoats, nine weasels and five ferrets were examined from Mt Taranaki and 82 stoats, 25 weasels and 28 ferrets from the surrounding farmland (ring-plain). There was a significant difference in the gut contents of the mustelids caught on Mt Taranaki and the ring-plain. It was found that 94% of stoats sampled from Mt Taranaki had rodents in their gut content, 39% had invertebrate remains and 23% had bird remains. Of the individual weasel guts examined for prey contents, 100% of them contained rodent remains, 22% invertebrate remains and 22% bird remains. The ferret gut contents consisted of 40% rodent remains, 80% invertebrate remains and 20% bird remains. The gut content of stoats on the ring-plain consisted of 64% rodent remains, 22% invertebrate remains, 36% bird remains and 3% lagomorph remains. Weasel gut content consisted of 61% rodent, 22% invertebrate and 33% bird. Ferret gut content consisted of 63% rodent, 13% invertebrate, 4% bird, 21% lagomorph and 13% mustelid. Based on all trapping events on Mt Taranaki, from March 2014 to March 2020, there were significant ‘hotspot’ capture areas for stoats and ship rats, and significant ‘cold spot’ areas avoided by ship rats. The capture frequency of rats was 7.79% per trap night and 0.81% for stoats. At the individual trap level, there was no significant relationship between traps that had a high probability of catching rats and traps that had a high probability of catching stoats. Double capture events (two pests caught in a double set trap) were infrequent (0.09% occurrence of all trap nights). There was a significant positive relationship between stoat capture frequencies and elevation and also between ship rats and elevation. Stoats were caught throughout all elevation bands (400-1599 m a.s.l) within the trapping network, with the highest capture frequencies between 900-1299 m a.s.l. Rats were also caught throughout all elevation bands with highest capture frequencies between 800-1199 m a.s.l. Habitat types on Mt Taranaki are largely determined by elevation. As such, differences between preferred habitat types for stoats and ship rats may be driven by elevation. The habitat type with the highest frequency of stoat captures was tall tussock grassland. The habitat type with the highest frequency of ship rat captures was broadleaved indigenous hardwoods. Overall, rodents were the main food source for stoats and weasels and invertebrates were the main food source for ferrets (albeit with a low sample size) on Mt Taranaki. Rodents were the main food source for all mustelids on the ring-plain. Capture frequencies were highest for stoats between 900-1299 m a.s.l and in tall tussock grassland habitat. For ship rats capture frequencies were highest between 800-1199 m a.s.l in the broadleaved indigenous hardwood habitat. Together, these results provide important insight into areas on Mt Taranaki that should be targeted to achieve more efficient pest mammal control.