It is known from live-trapping studies that a platypus can travel both upstream and downstream past sizable waterfalls as exemplified by Mackenzie Falls in Grampians National Park, which is more than 30 metres high (shown below at left) (Griffiths and Weeks 2010). This indicates that natural waterfalls are unlikely to prevent a platypus from bypassing them if the animal can exit the water and climb up or down at least one adjoining slope. It should therefore not be particularly surprising that 45% of adult males and 23% of adult/subadult females captured within 3 kilometres of the Belgrave Lake weir wall in Melbourne’s outer eastern suburbs – a steep concrete structure measuring just under 6 metres high (shown below at right) – were confirmed to have travelled past the structure on at least one occasion, again presumably by climbing up or down the adjoining vegetated slopes (Serena et al. 2023). Similarly, animals have been repeatedly recorded leaving the water and moving across land to bypass the 10-metre-high Blue Lake Dam wall in Jenolan Karst Conservation Reserve (Musser et al. 2024).
Although platypus are quite good at scrambling up steep banks, they find it difficult or impossible to climb vertical or near-vertical concrete or metal surfaces, particularly if large volumes of water are cascading down. Weir walls and associated structures can therefore be intractable barriers to a platypus seeking to move upstream and/or downstream, particularly if the animal finds it difficult or impossible to leave the channel due to other adjoining vertical walls being present (as illustrated in the two photos below).
Most structures designed to enable medium-to-large fish to travel past weir walls (including rock-ramp, Denil, natural bypass, cone, trapezoidal and vertical-slot fishways) should also be able to be navigated by a platypus. In particular, this species should easily cope with a water velocity of 0.3 metre/second (the recommended maximum to enable fish measuring more than 200 mm in length to navigate through fishways: O’Connor et al. 2017).
Providing an attracting flow at a fishway entrance should help direct a platypus to use the structure, particularly when the structure is encountered for the first time. To accommodate a large platypus comfortably and reduce the risk that hard or sharp edges cause injuries, we recommend that the minimum aperture width in vertical-slot fishways (or in trash racks associated with fishways) should be 150 mm. A minimum water depth of 200-300 mm should ideally be maintained along the length of a fishway at all times (particularly if the structure is unscreened from above) so a platypus using the fishway remains fairly well hidden from the view of predators, including birds of prey.
The only types of fishway that may not be well suited to platypus use are mechanical fish locks and lifts. Based on our knowledge of platypus behaviour, a platypus is likely to investigate the interior of a lock/lift box only briefly (probably for less than a minute) before it leaves. This presumably will mitigate against the animal being inside the box when the door shuts and the lift or lock mechanism starts to operate. Having said that, behavioural studies have never been carried out to describe how platypus interact with mechanical fishways – more information is needed.
If a weir cannot for practical reasons be retrofitted with a fishway or modified structurally to facilitate platypus movement along a spillway, consider whether it might at least be possible to provide animals with substantial protective cover (for example, in the form of dense shrubby vegetation) along the most plausible route that will be used to move past the barrier across land (Serena et al. 2023).
Photos: APC
LITERATURE CITED
Griffiths J and Weeks A (2010) Distribution and status of platypuses in the MacKenzie River, autumn 2010. Report to Wimmera CMA by Cesar Consultants, Parkville VIC.
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 46, in press, doi: 10.1071/AM23031.
O’Connor J, Stuart I and Jones M (2017) Guidelines for the design, approval and construction of fishways. Arthur Rylah Institute for Environmental Research Technical Reports Series No. 274.
Serena M, Crowther K and Kitchingman AM (2023) Managing the impacts of large weirs as barriers to platypus dispersal: current knowledge and recommended actions. Arthur Rylah Institute of Environmental Research Technical Report Series No. 361. Department of Energy, Environment and Climate Action, Heidelberg VIC.