Possum monitoring using raised leg-hold traps
Malcolm D. Thomas and Jennifer A. Brown
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Abstract
The Department of Conservation currently measures possum (Trichosurus vulpecula) population abundance with leg-hold traps set on the ground. Protected flightless birds, such as the native weka (Gallirallus australis) and kiwi (Apteryx spp.), are at risk of being maimed or killed if caught in these traps. Consequently the Department has specified that traps be raised above the ground on platforms or brackets to prevent bird captures. However, there is concern that raised traps do not catch possums as often as the traps set on the ground so that comparisons of population indices gained from ground and raised sets may not be valid. Field trials were conducted in three habitat types using ground and raised sets to determine whether differences occurred in catch rates. The trials used the survey intensity recommended for possum monitoring specified in the NPCA trap-catch protocol. Also, the feasibility of using a calibration index to 'correct' the raised-set population indices so they can be compared to ground-set indices was examined. The results showed that there was no significant differences in catch rates in forest. Capture trends indicated that significant differences could occur if sample sizes were larger. Therefore comparisons of population indices from ground and raised sets were not considered valid, and separate protocols for their use are recommended. A standardised protocol for raised traps is recommended. The use of a calibration index is not feasible because the error associated with its calculation makes it meaningless. The study highlighted the need to include error estimates when making comparisons of population indices. Consideration needs to be given to investigating the use of alternative monitoring devices that do not pose a risk to native birds and give more precise estimates of population indices by providing larger sample sizes.
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