The pools found behind small dams and weirs (as shown above) often provide excellent platypus feeding habitat, contribute to regular successful breeding and serve as important drought refuges. Research has also shown that relatively low weir walls (potentially up to as much as 10 metres high depending on their design) may also be quite easy for a platypus to navigate or bypass (Serena et al. 2023; Musser et al. 2024).
In contrast, because a platypus prefers to feed in relatively shallow water (ideally 1-3 metres deep), much of the impounded pool area behind a major dam wall may not be suitable for platypus foraging purposes. Nonetheless, platypus have reportedly been seen since 2000 on 60% of the major (ANCOLD-listed) Victorian weir pools in which a platypus population is known or highly likely to occur upstream (Serena et al. 2023). This presumably reflects the fact that suitable foraging habitat is present in shallow backwaters or inlets around the weir pool perimeter or at the upper end of the weir pool where it grades into an incoming creek or river.
Large dams and weirs may also be problematic to platypus populations in other ways.
First, though the platypus is surprisingly good at scrambling up banks, these animals are generally unable to climb vertically stepped concrete surfaces that are more than about 15 centimetres high (Musser et al. 2024). Many dams and weirs are therefore expected to act as barriers to platypus movement, particularly if a platypus is unable to reach the bank to walk around a stepped surface because the channel is contained within vertically engineered walls (as shown above). This is supported by studies that have found that members of a platypus population located upstream of a substantial weir wall may differ genetically from animals located downstream of the structure, implying that the wall is impeding migration and genetic exchange (Furlan et al. 2013; Mijangos et al. 2022).
Furthermore, leaving the water to bypass a barrier will inevitably create a risk that a platypus is killed by a predator or otherwise gets in trouble. The amount of risk will depend on how far it has to travel across land, how much protective cover exists along the way, and whether a road must be crossed to reach water again (Serena et al. 2023).
Second, regulating river flow downstream of a weir wall fundamentally changes the river’s ecology. For example, water is typically retained in reservoirs during winter and spring in southeastern Australia (when rainfall is usually high) and then released downstream in summer (when irrigation of crops and pastures occurs). The seasonal pattern of high and low flow downstream of a dam is therefore often reshaped to be the reverse of the natural pattern.
Depending on irrigation demand, the amount of outflow from a weir can also fluctuate much more dramatically and erratically than would have been the case historically – or be held unnaturally constant for extended periods of time. Water released from the bottom of a deep reservoir will also be colder than would have been true before the dam was built. In practice, though substantial platypus populations can demonstrably persist downstream of some large dams (Gust and Handasyde 1995; Hawke et al. 2019), altered patterns of river flow and cold-water pollution caused by dam operation may contribute to reduced platypus abundance (Hawke et al. 2019).
Most fish ladders (as shown at left) that are designed to help medium-to-large fish travel around a dam wall are also likely to be used by a platypus. River management is also becoming increasingly sophisticated as more becomes known about the seasonal flow requirements of platypus and other aquatic species. The biggest challenge is to ensure that sympathetic and effective management of shared water resources occurs even when surface water is in short supply during droughts.
Photos: APC
LITERATURE CITED
Furlan EM, Griffiths J, Gust N, Handasyde KA, Grant TR, Gruber B and Weeks AR (2013) Dispersal patterns and population structuring among platypuses, Ornithorhynchus anatinus, throughout south-eastern Australia. Conservation Genetics 14, 837-853.
Gust N and Handasyde K (1995) Seasonal variation in the ranging behaviour of the platypus (Ornithorhynchus anatinus) on the Goulburn River, Victoria. Australian Journal of Zoology 43, 193-208.
Hawke T, Bino G and Kingsford RT (2020) Damming insights: variable impacts and implications of river regulation on platypus populations. Aquatic Conservation: Marine and Freshwater Ecosystems 31, 504-519.
Mijangos JL, Bino G, Hawke T, Kolomyjec SH, Kingsford RT, Sidhu H, Grant T et al. (2022) Fragmentation by major dams and implications for the future viability of platypus populations. Communications Biology 5, 1127.
Musser A, Grant T and Turak E (2024) Movement of platypuses around and through instream structures and natural barriers in the Jenolan Karst Conservation Reserve, New South Wales. Australian Mammalogy in press, doi: 10.1071/AM23031.
Serena M, Crowther D and Kitchingman AM (2023) Managing the impact of large weirs as barriers to platypus dispersal: current knowledge and recommended actions. Arthur Rylah Institute for Environmental Research Technical Report 361. Department of Energy, Environment and Climate Action: Heidelberg VIC.