Australian Platypus Conservancy

A platypus is adapted to feed in an aquatic environment – it will starve in the absence of adequate surface water. Less extreme reductions in flow may well result in reduced reproductive success and smaller population size. Although animals can survive for a time in isolated pools scattered along a drying water course, mortality is likely to increase due to stress from crowding and food competition, along with increased exposure to predators, particularly if animals have to move between pools .

The platypus breeds once a year, with regular recruitment of new animals essential to compensate for adult mortality. A positive relationship exists between the number of females that breed in a given year and antecedent flow in the months between weaning and the onset of breeding. This likely reflects the fact that females fail to breed unless they’ve accumulated enough fat reserves to support a successful reproductive outcome (Serena et al. 2014, 2017). Adequate surface water is also needed through summer to support foraging to meet the high energetic demands of lactation (Thomas et al. 2020), maintain close physical proximity between nesting burrow entrances and the water to reduce predation risk for both mothers and their offspring (Grant and Bishop 1998), and allow recently weaned juveniles to grow rapidly before the onset of cold weather in winter (Grant and Temple-Smith 1983).

Occasional high to very high flow events help maintain the quality of platypus foraging habitats by scouring sediment from pools and otherwise promoting healthy geomorphological processes. However, high spring and summer flows that cause water depth to increase substantially above the level of spring base flow (when females select their nesting burrows) can also greatly reduce platypus breeding success if nesting burrows are inundated. The risk to breeding success is believed to be somewhat lower during incubation and early lactation (late August to November in Victoria and New South Wales) as compared to late lactation and the period when juveniles are first learning to swim (December to February) (Serena and Williams 2010; Serena et al. 2014, 2017).

What can be done to protect the platypus?

• The ideal platypus flow regime entails plenty of surface water being consistently available throughout the year. If it is necessary to adopt a reduced watering regime in a managed river system, enough surface water should be released in at least 50% of years (at a minimum frequency of one year in three) to support successful breeding by a substantial proportion of females occupying the system.

• To reduce platypus predation risk, riffles should never be allowed to dry out and ideally should enable a platypus to remain submerged while travelling upstream or downstream. In practice, this will require a minimum depth of 0.15-0.3 metre being maintained across at least part of a riffle’s cross-sectional profile.

• To improve platypus foraging opportunities and reduce predation risk along runs (which are normally much longer than riffles), the minimum water depth in runs should ideally never drop below approximately 0.3 metre (if a channel is less than about 5 metres wide) or 0.5 metre (if a channel is more than 5 metres wide).

• High (though not overbank or near-overbank) flows are presumed to present a relatively low risk to juveniles if peak duration is less than 24 hours (i.e. in line with the duration of most storm events) in late winter or spring.

• The frequency of managed flows that cause water depth to increase more than about 1 metre as compared to typical base flow should be minimised in summer to help protect juveniles from drowning. If it is necessary to schedule a release of greater magnitude in this period, it should be preceded by an equal or larger release in late winter or very early spring to encourage breeding females to place their nesting burrow higher up the bank than might otherwise occur.

• The occurrence of minor to moderate freshes is predicted to assist platypus foraging success by filling marginal aquatic habitats, flushing sediment from the channel, maintaining productive biofilms and otherwise renewing macroinvertebrate food resources. In managed systems, freshes scheduled in late spring through autumn are particularly likely to benefit platypus survival and breeding. It is also possible (though not proven) that freshes may serve to promote travel to relatively isolated populations by breeding males (in late winter and spring) and dispersing juveniles (in autumn).

Photo courtesy of Pete Walsh

LITERATURE CITED

Serena M and Williams G (2010) Factors contributing to platypus mortality in Victoria. The Victorian Naturalist 127, 178-183.

Serena, M, Williams GA, Weeks AR and Griffiths J (2014) Variation in platypus (Ornithorhynchus anatinus) live-history attributes and population trajectories in urban streams. Australian Journal of Zoology 62, 273-234.

Serena M and Grant TR (2017) Effect of flow on platypus (Ornithorhynchus anatinus) reproduction and related population processes in the upper Shoalhaven River. Australian Journal of Zoology 65, 130-139.

Thomas JL, Parrott ML, Handasyde KA and Temple-Smith P (2020) Maternal care of platypus nestlings (Ornithorhynchus anatinus). Australian Mammalogy 42, 283-292.