Introduction

For many years it has been common practise for field officers involved in control of common brushtail possums Trichosurus vulpecula to estimate killing rates from the difference between takes on bait acceptance lines before and after a poison operation. Bamford (1970) developed this practise into a bait interference method which involved a device which would exclude other animals such as rats from the baits. He defined the sample strategy by drawing an analogy between the encounters with bait stations and trapping models where the Poisson model is applied to estimate density (Seber 1973):

Relative density d = - log_e (1-f)

where f is the frequency of baits taken.

A major problem is that if bait stations are placed too close together an animal is able to follow along the lines. However Bamford (1970) showed that if bait stations are placed at distances of 40 yards there was little evidence of such contagion.

The bait interference method has been used extensively by N.Z. Forest Products Ltd. and more recently in the Wellington Conservancy of the N.Z. Forest Service to estimate poisoning success. Baiting was usually carried out page 186 nightly, over extended periods of weeks or even months. Both operators found that the frequency of baits taken rose with each assessment, with a break occurring at the date of poisoning. This led to two ways of estimating the kill rate: (1) Use of the mean number of baits taken over several days before and after poisoning, from which estimated densities were calculated; (2) Use of only those values immediately before and after poisoning if the rise in baits taken over successive nights prior to the kill was steep. Neither approach was very satisfactory; with the first method the frequency of baits taken was often still rising immediately before poisoning and almost always rose from the low point after poisoning - it usually gave a value well below visual estimates of kill; with the second method many data were ignored and it had characteristics of a 'shot in the dark' estimate. Neither method gave a value as high as that obtained from trapping or spotlighting where comparison could be made (Table 1). Also the larger labour content involved in baiting lines over an extended period (arising from the uncertainty of the poison date) posed a serious limitation for large operations, such as at Kaingaroa where up to 48000 ha are poisoned each year.