The size of feral pig populations that survive conventional (food bait) trapping campaigns on two sites, and the tendency of trapping to preferentially remove sows, were examined. The use of traps containing oestrus-induced sows to enhance the trapping response obtained with conventionally baited traps was also investigated. Estimates of percentage reduction achieved by conventional trapping on the two sites were derived from two indices, proportional bait take and spotlight counts, using index-manipulationindex measures of pig abundance. Proportional bait take indicated reductions in pig abundance of 100% in 16 nights and 93% in 14 nights for the two sites from conventional trapping, while spotlight counts estimated reductions of 81% and 83%, respectively. Sex ratios of pigs on both sites were at parity prior to trapping, but strongly biased in favour of males after trapping. There was a coincident female bias in the sex ratio of trapped pigs. Subsequent to conventional trapping, no pigs were trapped using oestrous sows as bait, indicating that the use of oestrous sows does not enhance the trapping response achieved using conventional techniques. Trapping data are used to derive a compartmental model of the trapping programme. The model is used to identify potential strategies for improving the efficacy of feral pig trapping programmes.

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Recent developments in the application of line-transect models to aerial surveys have used double-observer sampling to account for undercounting on the transect line, a crucial step in obtaining correct population estimates. This method is commonly called the mark-recapture line-transect sampling method and estimates the detection probability at zero distance to correct line-transect estimates of abundance. An alternative approach, which uses the same methodology during data collection, is to use a range of covariates, including distance from the transect, in a mark-recapture model. This approach overcomes the implicit assumption of uniform distribution of distances in line-transect estimators. In this paper, we use three alternative approaches (a multiple-covariates distance method, a distance method incorporating adjustment for incomplete detection on the transect line using mark-recapture sampling, and a mark-recapture method with distance as a covariate) to estimate the abundance of several medium-sized mammals in semiarid ecosystems. Densities determined with the three estimators varied considerably within species and sites. In some cases distance estimates were larger than mark-recapture estimates and vice versa. Despite large numbers of observations, distance uniformity was not observed for any species at any site, nor for any species where sites were combined. Possible reasons, which include sampling variability, movement in response to the aircraft and failure of the mark-recapture independence assumption, are discussed in detail.

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Factors that influence bait uptake by feral pigs will determine the efficacy of poisoning and trapping programmes for the control of pigs and have the potential to introduce bias to indices of pig abundance requiring bait consumption. In this study, the influence of pasture availability on uptake of bait trails consisting of soaked wheat by pigs in the semi-arid rangelands of north-western New South Wales was investigated. Percentage uptake of bait trails, pig density and pasture biomass were estimated for six sites along the Paroo River on six occasions: two each when pasture biomass was relatively high, moderate and low. The influence of pasture biomass on the relationship between percentage uptake of bait trails and pig density was examined by linear regression analysis. The analysis demonstrated that increasing pasture biomass significantly reduced the density of pigs corresponding to a given percentage uptake of bait trails, suggesting that, as pasture biomass increased, fewer pigs consumed bait trails andlor the number of bait trails each pig consumed declined. Assuming the former, the effect of increasing pasture biomass on the relationship between percentage uptake of bait trails and pig density indicated that, for every increase in pasture biomass of 100 kg ha-1, the percentage of pigs consuming bait declined by about 10%. The implications of these results for pig control and bias associated with indices of pig abundance requiring bait consumption are discussed.