Given that a platypus normally sleeps in a burrow, it makes sense that these animals will often enter pipes and culverts. For example, a study near Melbourne found that animals routinely traversed a 45-metre-long concrete culvert (diameter = 1.3 metre, grade = 1.1%, water depth generally < 250 mm) carrying creek water through an embankment (Serena et al. 1999). This species will also travel through pipes and culverts that are entirely dry (Musser et al. 2024).
An active platypus needs to breathe at intervals of less than three minutes, so there’s a risk that a platypus will drown in a long pipe or culvert that’s filled with water. For example, several platypus have drowned over time in a long piped section of the Upper Canal in the Nepean River section of Sydney’s water supply (Tom Grant, pers. comm.).
A platypus’s safety can also be compromised if it is forced to leave the channel and move across land to bypass a culvert. Studies in Tasmania indicate that ease of access is the most important factor limiting culvert use: platypus consistently walk around structures that are difficult to enter, for example because a culvert pipe protrudes from the bank with a vertical gap (measuring as little as 20 cm) between its bottom lip and the water surface (Otley and le Mar 1998; Mooney and Spencer 2000). Platypus may also find it difficult or impossible to travel upstream through a pipe or culvert carrying very fast-moving water. To promote efficient travel, we recommend that instream structures aim to convey water at a maximum velocity of 0.3 metres/second in most circumstances (as recommended by O’Connor et al. (2017) to promote upstream travel by medium-to-large fish with a body length >100 mm). To further assist platypus to move upstream through culverts during high post-storm flows, consideration should be given to providing a textured or uneven floor surface to improve traction (Magnus et al. 2004).
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 cannot 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 (Taylor et al. 1991). Platypus are also known to have drowned after becoming wedged in narrow openings in irrigation control gates or between overlapping wire mesh panels placed in a channel to catch leaves (Australian Platypus Conservancy, unpub. data).
What can be done to protect the platypus?
- Water flow is likely to be distributed across a wider cross-sectional area and hence less forceful in a box culvert (with a flat bottom) than in a pipe culvert (with a curved bottom), making it easier for a platypus to swim upstream through a box culvert when flow rises after storms, etc. To maintain natural stream velocity and reduce the risk that channel erosion occurs downstream of a culvert, the width of a culvert floor should ideally be at least 75% of adjoining stream bed width, particularly in the case of perennial or nearly perennial water courses (for more details, see https://www.planning.vic.gov.au/__data/assets/pdf_file/0020/720524/Appendix-29-Works-on-Waterways-Guidance-Note-Culverts.pdf).
- To encourage use by aquatic fauna (including the platypus), culvert floors should be located at the same level or a little lower than the level of the adjoining stream bed (ideally allowing a little natural sediment to accumulate inside the culvert). Conversely, culverts should never be designed to protrude so the culvert is separated from the stream bed by a vertical gap.
- Long culverts located along stream or rivers that support a platypus population should be large enough that they never (or at least very rarely) are filled to capacity with water along their entire length.
- Pipes that are accessible to a platypus should have a minimum internal diameter of 250 mm to enable animals to turn around easily after entering.
- To enable a platypus to safely negotiate grilles or mesh barriers, the grid spacings or apertures should be 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, 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.
Photo: APC
LITERATURE CITED
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.
Magnus A, Kriwoken LK, Mooney NJ and Jones ME (2004) Reducing the incidence of roadkill: improving the visitor experience in Tasmania. Report by the CRC for Sustainable Tourism.
Mooney N and Spencer C (2000) Why did the platypus cross the road? Australian Mammalogy 21, 264.
Musser A, Grant T and Turak E (2024) Movements of platypuses around and through instream structures and natural barriers in the Jenolan Karst Conservation Reserve, New South Wales. Australian Mammalogy 46, AM23031.
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.
Otley HM and le Mar K (1998) Observations on the avoidance of culverts by platypus. The Tasmanian Naturalist 120, 48-50.
Serena M, Williams G, Thomas J and Worley M (1999) Effect of a flood retarding basin culvert on movements by platypus Ornithorhynchus anatinus. The Victorian Naturalist 116, 54-57.
Taylor R, Mooney N and Lange K (1991) Observations on platypus. The Tasmanian Naturalist 105, 1-3.