Dr Jonathon Howard, Executive Member NPANSW
The platypus (Ornithorhynchus anatinus) is a unique, semi-aquatic Australian mammal and one of only five living monotreme species on earth. We have constructed dams in over 40% of the sub-catchments in which platypus are present.
Dams serve many purposes. They benefit society by providing flood mitigation, can be used to generate power, and supply water for drinking, agriculture, and recreation. However, there is also an ‘opportunity cost’ associated with any dam: they alter the natural flow of a river, affect water quality, and disrupt critical ecosystem function.
Recently scientists based in New South Wales decided to look at the impact of river regulation on platypus populations. They chose four rivers (Pike Creek–Dumaresq, Gwydir, Severn and Peel) in the northern Murray–Darling Basin. In each of these rivers they conducted environmental DNA (eDNA) analysis over 30 km stretches upstream and downstream of a dam as well as some live trapping. By doing this they could ‘compare-and-contrast’ platypus populations in minimally regulated stream sections upstream of a dam with those populations living in the heavily regulated downstream sections of the same river.
They found dams had significantly changed flow seasonality and created cold-water pollution. The upstream sections of the rivers experienced prolonged periods of ceased flow. The DNA surveys detected platypuses downstream of all dams but failed to find evidence of populations upstream of the Peel River and upstream of the Pike Creek-Dumaresq River. The scientists suggest that this might indicate possible local extinctions.
To assess the health of a platypus, when captured, scientists looked at an individual’s ‘tail-fat’ to give an idea of overall body fat. Platypus captured downstream of the Peel River dam were assessed as being in ‘very poor body condition’ (based on tail-volume, cholesterol and triglyceride indices) and overall trapping success in these sections was low. Indeed, no platypus were caught in two of the rivers. The scientists also found there were significant differences in macroinvertebrate communities downstream of the dams, as a result of cold-water pollution, implying possible cascading impacts on platypus diet.
These findings raise the spectre of population limitation and localised upstream extinction on Peel River and Pike Creek. They show fragmentation of river sections by dams can have a detrimental effect on upstream platypus populations because the animals are denied access to key refuge habitats, which are typically more common in downstream river reaches. Peel River and Pike Creek, where platypus were apparently absent, both have relatively short upstream sections with relatively limited habitat options.
The findings also suggest river regulation alters the seasonality of flows of rivers. When summer flows are significantly higher downstream of a dam, they increase the risk of inundating platypus nesting burrows and drowning juveniles.
It is also worth noting that the taxonomic composition of the macroinvertebrate communities (the food which platypus eat) can also be significantly different above and below a dam.
These findings show how important it is to improve the connectivity of our rivers. This may include translocation of platypus and other species during droughts or fire. As well as avoiding large cold-water releases and unseasonal water releases during breeding periods. Such strategies will grow in importance especially for our inland rivers because the Murray-Darling Basin Authority says it is “virtually certain” that the basin will be hotter and drier by 2050. If there are no changes to water management arrangements, it is predicted that our natural environment and the wildlife it contains could face serious issues.
If you would like to find out more see: https://connectsci.au/mf/article/75/10/MF24037/61180/Impacts-of-river-regulation-and-fragmentation-on