Weirs & fishways

Platypus generally find it difficult or impossible to climb vertical or near-vertical concrete or metal surfaces, particularly if large volumes of water cascade down the surface. Weir walls can therefore act as barriers that prevent these animals from being able to swim freely along the length of a water course. Platypus can exit the channel and walk around otherwise impassable vertical barriers, with travel documented to occur around sizable natural barriers such as Mackenzie Falls in Grampians National Park in western Victoria (20+ metres high) and concrete weir walls measuring up to at least 8 metres high. However, leaving the water also exposes a platypus to increased mortality risks, e.g. due to predation or being hit by cars.

It’s likely that 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) can also be successfully utilised for the same purpose by platypus. A platypus has been recorded swimming upstream against a river current with a measured surface velocity slightly exceeding 1 metre/second, and also moving upstream through a culvert in which water was flowing at an estimated speed of at least 2.4 metres/second. The species should therefore be able to cope quite easily with a maximum water velocity of 0.3 metre/second (as recommended to enable fish measuring more than 200 mm in length to navigate through fishways: O’Connor et al. 2017).

The presence of an attracting flow at a fishway entrance is predicted to help direct platypus to use the structure, particularly if they are encountering it 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. We predict that a platypus will typically investigate the interior of a lock/lift box only briefly (probably for less than a minute) before it again 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 that is likely to be suitable for platypus use, consider whether it might at least be possible to provide animals with substantial protective cover (in the form of dense shrubby vegetation) when they’re forced to move across land to travel past the barrier.

Photo: APC

Literature cited

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.