Given that a platypus normally sleeps in a burrow, it makes sense that these animals will travel through pipes and culverts of considerable length. For example, tagged platypus have been found to routinely traverse a 45-metre-long concrete culvert (diameter = 1.3 metre, grade = 1.1%, water depth generally < 250 mm) carrying creek water through an embankment.
Unlike a fish, a platypus needs to breathe at intervals of less than three minutes when it’s active. The animals can therefore be at risk of drowning in long pipes or culverts that are filled to capacity with water. For example, several platypus are known to have drowned over time in a long piped section of the Upper Canal in the Nepean section of Sydney’s water supply (Tom Grant, pers. comm.).
A platypus’s safety can also be compromised if a culvert’s design forces it to leave the safety of the channel in order to move upstream or downstream. Studies in Tasmania indicate that ease of access is by far the most important factor limiting use of road culverts: platypus consistently bypass culverts that are difficult to enter, e.g. due to a culvert pipe protruding from the bank with a vertical gap (in one case, measuring just 20 cm) between its lip and the water surface below. In theory, these animals may also be deterred from proceeding upstream through a constricted pipe carrying very fast-moving water. In practice, a platypus has been recorded swimming upstream through a culvert in which water was flowing at an estimated velocity of around 2.4 metres/second, i.e. well above the maximum velocity of 0.3 metres/second recommended to facilitate passage by fish with a body length > 100 mm (O’Connor et al. 2017). However, to encourage platypus to continue using culverts during high post-storm flows, consideration can be given to installing a textured or uneven floor surface to improve traction – a platypus has been observed using baffles (fitted inside a culvert to assist fish passage) as handles to help it move upstream against a strong current.
Studies have confirmed that platypus weighing up to one kilogram – i.e. some adult males and a large proportion of adult females – can pass quite easily through a rigid 55-mm grid, with small juveniles presumably capable of squeezing through even narrower gaps (Grant et al. 2004). However, although platypus are known to enter plastic pipes that are as narrow as 10 cm in diameter, they apparently are unable to back up or turn around in such a confined space and are therefore likely to die a slow and horrible death if the far end becomes blocked (for example, by a recently closed valve). Cases are also known where these animals have drowned after becoming wedged in narrow openings in irrigation control gates or overlapping wire mesh panels placed in a channel to catch leaves.
What can be done to protect the platypus?
- Long culverts located along stream or rivers that support a platypus population should be large enough that they are never (or at least very rarely) filled to capacity with water along their entire length.
- To enable a platypus to turn around easily, pipes that are likely to be accessible to this species should have a minimum internal diameter of 250 mm.
- To enable a platypus to safely negotiate grilles or mesh barriers, they should be designed with grid spacings or apertures of 120 mm or more. Conversely, barriers meant to exclude a platypus should have grid spacings or apertures of 30 mm or less.
- To encourage platypus use, culverts located along a natural water course or soil-lined channel should be designed so they don’t protrude with a vertical gap below entry points. Concrete drop structures associated with culverts should incorporate stepped or slanted (ideally < 30o) faces to enable a platypus to scramble up and down without difficulty.
Grant TR, Lowry MB, Pease B, Walford TR and Graham K (2004) Reducing the by-catch of platypuses (Ornithorhynchus anatinus) in commercial and recreational fishing gear in New South Wales. Proceedings of the Linnean Society of New South Wales 125, 259-272.
O’Connor J, Stuart I, and Campbell-Beschorner R (2017) Guidelines for fish passage at small structures. Arthur Rylah Institute for Environmental Research Technical Reports Series No. 276.