Trapping small mammals for research and management: how many die and why?

Trapping programs are an essential part of mammal conservation and management, but trapping carries a risk of death for the study animals. Little information is available on mortality levels and its potential to influence studies. We examined the mortality levels recorded during 68 wildlife surveys within New South Wales, recording mortality rates associated with Elliott aluminium box and wire cage trapping for terrestrial mammals and harp traps for bats. We noted 232 deaths from 13623 small ground mammal captures in box traps, one death from 459 mammal captures in wire cage traps, and 44 deaths from 12,274 bat captures in harp traps. Mortality ranged from 0-13.3%, 0-0.6% and 0-9.1% of captures for box, cage and harp traps, respectively. No deaths were reported for 47% of surveys. Antechinus spp. comprised almost 95% of all trap deaths. Thirteen surveys provided detailed information for small ground mammal trapping, recording 111 deaths from 3651 small mammal captures. Box trap mortality in these surveys ranged from 0-7.5%, deaths being attributed to cold temperatures (generally winter) and multiple captures of individuals. Harp trap mortality stemmed from overheating, overcrowding and predation. Post-capture handling also contributed to mortality. No specific level of trap death can be ?expected? during a study as circumstances vary, but all efforts should be made to minimise the risk of deaths. This study found that mortality above 4% is unusual, resulting mainly from one-off ?extreme? mortality events. Minimising unnecessary recaptures and avoiding weather extremes should reduce mortality of small mammals during trapping studies, but will not prevent all deaths.

Influence of shade covers on pitfall trap temperatures and capture success of reptiles and small mammals in arid Australia

Shade covers for pitfall traps can be used to reduce the amount of solar radiation penetrating to the bottom of pitfall buckets, thereby reducing the number of captured animals dying from heat-stress. We tested the effectiveness of a variety of shade covers for reducing temperatures in pitfalls and trap mortality of small vertebrates, and examined the effect of one cover design on trap success in arid landscapes. Shade covers made of insulation foil were found to reduce core pitfall temperatures by 20–22˚C compared with uncovered buckets, which reached temperatures greater than 66ºC. Other cover types tested (plastic lid or cardboard) were found to be less effective: core bucket temperatures still reached 48–53ºC. While foil covers do reduce temperatures and therefore the probability of heat-stress-related mortality, above-ground foil covers also influence trap success. Traps with above-ground foil covers caught 39–43% fewer small vertebrates and 7–42% fewer species than uncovered traps. Above-ground foil covers had the greatest influence on the sampled abundance of scincid lizards (reduced by 50–52%), reduced the sampled abundance of most other lizard families and mammals, but increased capture success for snakes. If shade covers are required to minimise heat stress and mortality in pitfall buckets we recommend foil covers placed inside the bottom pitfall buckets as they significantly reduce pitfall temperatures and are likely to have minimal influence on trap success. However, regular checking of traps is still one of the most reliable ways to reduce heat-stress- related and other deaths in pitfall traps.