Snaring to control feral pigs Sus scrofa in a remote Hawaiian rain forest

  • Published source details Anderson S.J. & Stone C.P. (1993) Snaring to control feral pigs Sus scrofa in a remote Hawaiian rain forest. Biological Conservation, 63, 195-201.


Kipahulu Valley within Haleakala National Park (Maui Island) contains some of the most intact remaining rainforest in Hawaii. Amongst other things, it supports over 400 native plant species (45% endemic to the islands). Feral pigs Sus scrofa probably began to invade this area in the late 1970s. Due to habitat damage being caused, it was decided to attempt pig eradication by snaring within two purpose-built large fenced areas within the Valley.

To develop a control strategy, pig home ranges and movement patterns were assessed by radio-telemetry studies during November 1979-March 1980 (13 adults captured in box traps, fitted with radio collars, and released) and April 1985.
The Valley was divided into two fenced management units (120 cm hog wire mesh with a strand of barbed wire at ground level, anchored to fence posts). Fencing above the Valley headwall (2,380 m) was completed in June 1986, preventing pig ingress into upper areas.
Two fences were constructed along contours at 960 m (completed September 1986) and 1,400 m (completed March 1987), with adjoining side fencing, utilizing natural barriers (e.g. steep sidewalls and deep ravines) to complete the pig-proof units. These created the two management units, the lower containing 6.2 km² of habitat (bounded by the 960 and 1,400 m elevation fences); the upper 7.8 km².
During the 45 months, 1,978 snares were set (equivalent to 1.6 million snare nights). Scouting and pig activity transects were used to assess eradication success.

Initial pig densities were estimated at 6/km2 in the upper unit and 14.3/km2 in the lower (based on population reconstruction from individuals killed and aged). An average effort of 43 worker hours/pig was used to remove 53 pigs from the upper unit (21 (40%) boars, 23 (43%) sows, and nine (17%) too young to determine sex), and 7 hours/pig to remove 175 pigs (38 (22%) boars, 76 (43%) sows and 61 (35%) undetermined sex) from lower unit.
Snare density was 96/km² in the upper unit and 200/km2 in the lower unit, at the end of the study. Near eradication in the upper unit was achieved over 45 months; during the first three months, nearly 60% (31) of the pigs present or born during the study were removed. In the lower unit whilst only 10% (17) were removed initially, eradication after 45 months was still nearly achieved.
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