Dams challenge platypus populations

Posted on March 26, 2022

The construction of dams has many adverse effects on river hydrology and ecology.  Dams limit animal movements and reduce the cover of riparian vegetation, which leads to bank erosion and the filling of stream pools with sediment.  Dam-related impacts have been documented for many types of riverine animals, including macroinvertebrates, amphibians and waterbirds.  However, the effects of dams on platypus (Ornithorhynchus anatinus) populations are poorly understood, partly because platypus are elusive and nocturnally active.  Platypus are semiaquatic mammals endemic to eastern Australia.  They feed mainly on bottom-dwelling invertebrates and favour river habitats with stable streamside vegetation and complex substrates.  In the first critical survey of the impacts of river regulation on platypus, researchers compared platypus abundances in three regions of eastern Australia over a three-year period.  They used fyke nets and unweighted mesh nets to capture platypus at 108 sites, covering a stretch of at least 40 km on unregulated rivers and at least 20 km immediately above and below large dams on regulated rivers.  River flows were minimally regulated upstream of dams, but heavily regulated downstream.  Platypus populations were most obviously impacted on the Mitta Mitta River, downstream of Dartmouth Dam in the Upper Murray region.  In this stretch of river the dam, with a capacity of 3,856 gigalitres and a wall height of 180 metres, has altered the seasonality and volume of flows considerably more than other dams in the study.  Nightly platypus captures were significantly (roughly four times) lower downstream of the dam compared with the upstream section of Mitta Mitta and other unregulated river stretches in the same region.  On the Mitta Mitta River, unusually high downstream flows occur in summer, during the platypus breeding season, and these could cause platypus to be drowned or displaced from burrows.  In addition, due to releases of cold water from Dartmouth Dam, downstream water temperatures can be as much as 12 oC below normal.  The dam has also reduced the diversity and abundance of macroinvertebrates and fish in the downstream section.  In contrast, in the Snowy Rivers region damming had no clear impact on platypus numbers, and downstream captures were high, possibly because of measures taken to restore the river and to mimic natural flow regimes: these have significantly improved river banks and channel depth, and mitigated cold-water pollution.  The results of this study highlight the potential harmful effects of dams on platypus, but also suggest that meaningful steps can be taken to mitigate these impacts.

Reference:   Hawke, T. et al.  2021.  Damming insights: variable impacts and implications of river regulation on platypus populations.  Aquatic Conservation: Marine and Freshwater Ecosystems 31,504–519. https://doi.org/10.1002/aqc.3523