Proceedings of the First Symposium on Marsupials in New Zealand

Trap Catches

Trap Catches

The trap-catch model for estimating possum density was formulated from data collected in 1945 and 1951 from the Pararaki catchment in the Haurangi Forest Park (Batchelor et al. 1967). When the model is used in other areas the estimate must be regarded as an index of density because the probability of capturing animals - an important constant in the formula - may not be the same. The model's principal application is therefore in measuring the effect of control operations, using the change in the density index as a measure of percent kill.

The Pararaki population was measured again in 1975 and in 1976. Each sample consisted of four lines of 20 traps, spaced at 100 m intervals which were run for three consecutive fine nights. Density in 1975 was 11 possums per ha ± 50% and in 1976 12 per ha ± 44%. Smaller errors could be expected if the sample size was increased to six lines of 25 traps - requiring approximately 12 man-days of effort. A typical situation where the trap-catch model could be used would be an easily accessible forested area due to be aerially poisoned. Trap lines would be run before and after poisoning and the mean and 95% confidence limits of the kill assessed from the percent reduction in the density indices of individual lines. Total effort required would be about 20 man-days.