New South Wales Department of Agriculture, Veterinary Research Centre, Glenfield, N.S.W.; present address: School of Applied Science, Canberra College of ...
Aust. Wildl. Res., 1983, 10, 269-75
A Short-Term Evaluation of Feral Pig Eradication at Willandra in Western New South Wales Jim Hone New South Wales Department of Agriculture, Veterinary Research Centre, Glenfield, N.S.W.; present address: School of Applied Science, Canberra College of Advanced Education, P.O. Box 1, Belconnen, A C T . 26 16.
Abstract An evaluation was conducted of an attempted eradication of feral pigs by poisoning and shooting, in an area of 50 km2 at Willandra in western New South Wales. Poisoning with 1080 killed 73% of the feral pigs. After the poisoning, 95 of 98 feral pigs seen in the area were shot. The results and their implications for the control and eradication of feral pigs are discussed.
Introduction Feral pigs can be controlled by a variety of methods including poisoning and shooting (Hone et al. 1980). Poisoning with sodium monofluoroacetate (compound 1080) is a common method that has been used in New South Wales since 1973. Giles (1976) considered that 1080 poisoning had been more successful than other methods for controlling crop damage and lamb predation due to feral pigs. A high level of control or eradication may be necessary if exotic diseases such as foot-and-mouth disease, swine fever or African swine fever enter Australia (Geering 1981; Hone and Bryant 1981). Little information has been published, however, on the effectiveness of 1080 poisoning for control or eradication of feral pigs. Hone and Pedersen (1980) reported a 58 8% reduction in feral pig numbers after poisoning with 1080. The effects of poisoning were quickly lost, as the population nearly doubled over the next year. This paper reports an evaluation of the effects of 1080 poisoning and shooting on the abundance of feral pigs and non-target species. The study aimed to determine: (1) if feral pigs in a limited area could be eradicated or a high level of control achieved in a short period; (2) the percentage reduction of feral pigs due to 1080 poisoning; (3) the effect of 1080 poisoning on non-target species; (4) the utility and cost of shooting. Methods Site The study site was two adjacent areas, each of 50 km? along Willandra Billabong near Hillston in south-western New South Wales (33" lo's., 145"E.). The area is very flat, and at an altitude of 76 m above sea level. Willandra Billabong traverses the area in an east-west direction. The dominant vegetation along the creek was black box Eucalyptus largiflorens, river cooba Acacia stenophylla, and lignum Muehlenbeckia cunninghamii. This dense vegetation occurred on both sides of the creek for up to 1 5 km. The surrounding areas were grassland. The study was conducted in November and December 1978. Previous studies (Hone, unpublished data) showed that most feral pigs were distributed close to the creek with its associated thick vegetation. Few were more than 2 km from the creek.
J. Hone
Poisoning Method Feral pigs were offered pellets placed along a furrow trail, cut by a bait layer. The bait (Canonba Pig Cubes: Furneys, Dubbo, N.S.W.) was compressed pellets consisting of wheat, sorghum, lucerne, barley and bone meals, bran, pollard, salt and artificial flavouring. The minimum crude protein level was lo%, and maximum crude fibre content 17O/o, according to label specifications. The trail was placed along the edge of the timber line which divided the creek area from the surrounding grassland plain. Within the poisoned area the trail was 32 km long. It extended 2 km into each of the adjacent non-poisoned areas to the east and west, where pigs were only offered non-poisoned pellets. As there was no watercourse or sizeable waterhole or cover for at least 10 km to the north or south, it was assumed that these areas were devoid of feral pigs. A total of 2100 kg of free-feed was offered over 9 days before the poisoning: 1750 kg in the poisoned area, and 350 kg in the non-poisoned area. All free-feed was eaten. Poisoned pellets were then offered for three consecutive days. The density is^) of poisoned pellets, as measured on 80 randomly selected 1-m lengths of trail, was 2 . 6 1 0 . 2 pellets per metre. Pellets remaining on the trail after poisoning were covered. The 1080 concentration in the pellets was 0.05% w : w (0.045% pure 1080). Data on the length of trail fed upon, and duration of feeding by adults (> 12 months) and juveniles (< 12 months) were recorded during counts from observation hides. Differences in length and duration of feeding between ages of pigs were examined by analysis of variance of log transformed data. Pig carcasses found after the poisoning were sexed, weighed, aged (Matschke 1967) and examined for the presence of pellets in the stomach. Samples were collected for determination of the weight of oven-dry pellets in the stomachs. The weight of pellets was assumed to be an index of the amount eaten, as some bait would pass on to the intestine (Clemens and Stevens 1980) before death, or be lost as vomitus, a common symptom of 1080 poisoning (Hone and Kleba, unpublished data). Conversely, small amounts of pellet material may have been in the stomach from the previous day's feeding. Due to the thick vegetation along the billabong, only a small percentage of dead pigs were expected to be found. Estimates of Population Size Estimates of feral pig abundance were obtained by three methods, each of which gave an index of abundance, not an estimate of total population size. Spotlight counts were conducted on two successive nights in each area immediately before and after the poisoning. Counts were conducted from a vehicle travelling at 10 km h-I, to 80 m either side. The transect was 5 km long in the poisoned area and 4 km long in the non-poisoned area. Counts were conducted in the first hour after sunset. In part of the poisoned area feral pigs were also counted from two hides on the night before poisoning. Hides were placed adjacent to the bait trail. Observers counted all feral pigs observed on or near the trail during the 105 min before sunset. Aerial surveys were conducted from a Cessna 172, travelling at 120 km h-I, at an altitude of 46 m, with a strip width of 80 m for each of the two observers. Equal-sized transects were selected at random, without replacement, with a sampling intensity of 18% in each area. Indices of population size were obtained from the equation Y = N y (Jolly 1969) where Y is the index, N is the number of possible transects, and jj is the average number of pigs seen per transect. The surveys were flown during the second hour after sunrise. The post-poisoning counts were conducted after the first (hide counts) or third (spotlighting and aerial survey) nights of poisoning. In either case pigs destined to die from 1080 would have done so (Hone and Kleba, unpublished data). Analysis ofthe Efects of Poisoning The statistical significance of changes in feral pig abundance in each area after poisoning was determined by independent t-tests (Snedecor and Cochran 1967). The population variances of the means or estimates at each time were assumed to be different. This is expected in population estimation, as the variance is often related to the mean. The spotlight and hide methods counted feral pigs on or near the bait trail, so estimated the percentage reduction of feral pigs that ate the bait. The aerial survey counted pigs on randomly selected areas throughout the study site, and so estimated the percentage reduction of all feral pigs. These two percentages allow three statistics to be estimated: x, the proportion of feral pigs that ate the poisoned bait and died; y, the proportion of feral pigs that ate the poisoned bait and survived; z, the proportion of feral pigs that did not eat the poisoned bait.
Feral Pig Eradication
By definition, the percentage reduction in numbers of feral pigs that ate the bait is: The percentage reduction in numbers of all pigs is:
Since x+y+z = 1 0, we have three equations with three unknowns. They can be solved simultaneously to estimate x,y and z. The above model makes several assumptions. Firstly, that following poisoning the surviving pigs are not bait-shy and have no aversion to being near the bait. If this is not so, the percentage reduction of pigs that ate the bait is overestimated. Secondly, the model assumes that the population is closed (no immigration or emigration); thirdly, that pigs die only from the poison bait and not from secondary poisoning.
Non-target Effects of Poisoning The effect of the poisoning on non-target species was estimated by spotlight counts, carcass counts and bait take. Spotlight counts of red kangaroo Macropus rufus, western grey kangaroos M. fuliginosus, rabbits Oryctolagus cuniculus and hares Lepus capensis were conducted during counts of feral pigs. Counts of the two species of kangaroos and those of rabbits and hares were combined, due to difficulties in identifying species. For 3 days after poisoning, extensive searches were conducted for approximately 16 h, and dead non-target animals collected and identified. Bait take by non-target species was investigated in habitat similar to that in the poisoned area, but from which feral pigs had been removed by shooting. A total of 484 pellets were laid 1 m apart in a trail, and pellets taken or moved were recorded. Shooting Two weeks after poisoning, pigs in the same area were shot with a shotgun and SG shot from a Gazelle helicopter. This method is faster and more efficient than ground shooting. The area was completely flown over three times, in a systematic search path. The helicopter flew at 50-100 m altitude and 25-40 km h-l. When a feral pig was sighted, the helicopter descended to 10-40 m and shooting began. Shooting continued through each of two days, and subsequently, spotlight counts were repeated. Table 1. Estimates of feral pig population size, obtained by three methods, before and after poisoning Values are means
+ standard errors; per night for spotlight counts and per observer for hide counts
Area
Time
Spotlight counts
Hide counts
Poisoned
Before After Before After
13 0+4.0 1.010 1 . 0+0 1 510.5
26.0+5 0 1 Oil.0
-
81.5+44.2 21 8110.5 81.5+25.5 70.6+26.0
92
96
73
Non-poisoned
Reduction (%), poisoned area only
Aerial survey
Results
Poisoning The percentage reduction in numbers of feral pigs, associated with the poisoning, was estimated at 92% by the spotlight counts, 96% by the hide counts and 73% by aerial survey (Table 1). The estimates of numbers of feral pigs in the non-poisoned area were slightly higher from the spotlight counts and slightly lower from the aerial survey at the second than at the first count. Due to the low precision of the estimates, there were no significant differences between pig abundance before and after poisoning.
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J. Hone
The average percentage reduction from the spotlight and hide counts (94%) and the estimate from aerial survey (73%) were used in the poisoning model equations. From the model outlined earlier:
The solution of these equations gives values of 73% for x (percentage of all pigs that ate baits and died), 4% for y (percentage that ate baits and survived), and 23% for z (percentage that did not eat the baits). Twenty-two dead feral pigs were found after poisoning (Table 2), but the total number that died is not known; 15 (68.2%) pigs were under 13 months of age. The average weight of dead pigs was 2 7 . 1 kg. Seventeen pigs (77.3%) had pellets in their stomachs, the mean (ISE) oven-dry weight of which was 210.1+88.5 g (n = 13, as four stomach contents decomposed before analysis). The average wet weight was 368.8 g, so that 43.0% of the wet weight was water and digestive fluids. For these 13 pigs, there was a highly significant (P
