Reducing bycatch mortality in crustacean traps: Effect of trap design on platypus and yabby retention rates

The platypus (Ornithorhynchus anatinus) is known to be vulnerable to drowning in enclosed traps used to capture freshwater crayfish such as yabbies (Cherax spp.). To help quantify the degree of risk posed by such traps, we carried out 113 trials in Victorian streams and larger New South Wales rivers to assess the platypus's ability to escape from standard opera house traps (OH), a second commercially available enclosed trap design (closed-top pyramid traps, CTP), and opera house traps that had been modified by adding an opening in the roof (MOH). All 10 of the animals tested in OH failed to find an exit in the period allowed. In contrast, 82% of subjects tested in CTP (n = 45) and 83% of those tested in MOH (n = 58) escaped from traps within 2 min. Victorian animals took significantly less time to escape from traps (CTP, mean = 31 s; MOH, 33 s) than those tested in New South Wales (CTP, mean = 55 s; MOH, 53 s). In addition, juveniles were less likely than adults to escape from CTP and MOH in a timely manner. The results of trials comparing the number and size of yabbies captured in Victorian farm ponds indicate that MOH should at least equal and potentially exceed the performance of OH when used to harvest yabbies for human consumption.