An active platypus can hold its breath for less than three minutes (Bethge et al. 2003; Grant et al. 2004). Platypus deaths due to drowning therefore can occur in many types of nets or enclosed traps designed to capture fish or crustaceans (such as crabs, crayfish or yabbies). Several individuals may drown in a submerged net or trap if this is set in a place where platypus population density if high, or if the gear is abandoned after being deployed. For example:
At least eight platypus drowned in two rectangular mesh nets that had been illegally strung between dead trees in the middle of a large public reservoir in north-central Victoria (shown at right). The weight of numerous large fish also trapped in the nets (which presumably were the reason the nets had been set in the first place) made it impossible for the platypus to rise to the surface to breathe after becoming entangled.
Seventeen platypus skulls and other bones were found in a single unlicensed fyke (or eel) net that had been illegally set and then abandoned along a small creek in southwestern Victoria (as shown at left) (Serena and Williams 2010). This type of net contains one-way funnels that make it impossible for a platypus to escape. The net would have been left unattended in the creek for many months. Animals entering the net in winter and spring (when it was submerged) would have drowned. Those entering the net in summer and autumn (when the top of the net extended above the water) would have starved to death.
Seven platypus drowned in an illegal spring trap (shown at right), which would have been set to capture yabbies or crayfish in the Werribee River in southern Victoria. The animals are thought to have died over a period of about a week (the patchy white appearance of the platypus at the top of the pile is due to fur falling out as skin starts to decompose), so the trap presumably had been left unattended for the same period. The reason why this trap was so lethal is that the entrance is fitted with a tapered funnel pointing inwards. Research has shown that a platypus held inside a trap only searches for an exit in the outer netting – it doesn’t expect an opening to be located inside the trap and so fails to find it there (Serena et al. 2016).
Opera house yabby traps (as shown above) also have an internal funnel fitted around each entrance. As in the case of spring traps, this means that a platypus will drown after it enters an opera house trap either accidentally or because it’s been attracted by edible yabbies inside. Opera house traps are also known to drown rakali/Australian water-rats (as shown below at left) and freshwater turtles (as shown below at right). For more information about the impact of enclosed yabby traps on rakali mortality, see Rakali conservation status and concerns.
Fortunately, if you want to catch some yabbies to eat at home, there are some effective options that pose no risk to air-breathing animals and are also usually legal (though always check local fishing regulations to confirm this is true). Apart from dangling a chunk of meat in the water so yabbies are attracted to it and can be scooped up using a dip net, various types of open-top lift nets can be purchased (some collapsible, others with fixed mesh walls as shown below). To maximise success, keep in mind that the number of yabbies held in a baited lift net generally peaks within 3 to 6 hours of setting the net – after that time, captures decline as yabbies leave after eating their fill of bait (Serena et al. 2016).
Photos courtesy of P. Gerolemon (paragraph 2), Brad Smith (paragraph 4), Mal Doreian (paragraph 5), J. Spirek (paragraph 6 at left). Other photos: APC
LITERATURE CITED
Bethge P, Munks S, Otley H and Nicol S (2003) Diving behaviour, dive cycles and aerobic dive limit in the platypus Ornithorhynchus anatinus. Comparative Biochemistry and Physiology Part A 136, 799-809.
Grant TR, Lowry MB, Pease B, Walford TR and Graham K (2004) Reducing the by-catch of platypuses (Ornithorhynchus anatinus) in commercial and recreational fishing gear in New South Wales. Proceedings of the Linnean Society of New South Wales 125, 259-272.
Serena M and Williams G (2010) Factors contributing to platypus mortality in Victoria. The Victorian Naturalist 127, 178-183.
Serena M, Grant TR and Williams GA (2016) Reducing bycatch mortality in crustacean traps: effect of trap design on platypus and yabby retention rates. Fisheries Research 175, 43-50.