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

Abstract

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.