Managing flow

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 and increased exposure to foxes and other predators.

The creeks and rivers of southeastern Australia normally hold the least amount of surface water from mid-summer into early autumn. This also marks the period when juveniles emerge from nesting burrows and learn to feed themselves and adult females are often in poor condition after lactation. A positive relationship has been found to exist between the number of females that breed in a given year and antecedent flow (from weaning to when breeding starts), presumably because females fail to breed if they haven’t accumulated adequate fat reserves (Serena et al. 2014, 2017).

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, summer flows that cause water depth to increase more than about one metre above the level of base flow in spring may reduce platypus breeding success if they inundate nesting burrows or otherwise cause juveniles to drown. The risk that this occurs is believed to be somewhat lower during incubation/early lactation (late August to November in Victoria and New South Wales) than in 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 low- or no-flow 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), minimum water depth 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 rising more than about 1 metre above 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 by no means 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.