Ecology of the Macquarie turtle (Emydura macquarii) downstream of a large hypolimnetic-releasing impoundment in Australia’s southern Murray-Darling Basin

Kylie Singh

Research output: ThesisDoctoral Thesis

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Abstract

Worldwide, there are currently 333 recognised extant species of turtle. Their basic body form, complete with shell armour, has remained relatively unchanged throughout nearly 300 million years of natural selective pressures (Buhlmann et al, 2002). However, previously successful survival strategies of turtles have now left them vulnerable to anthropogenic change. The longevity, delayed maturity and high adult survivorship of freshwater turtles has meant that they are incapable of rapidly responding to human-induced habitat changes, and at the same time served to mask the extent of threat to populations. It is most often juvenile turtles that are impacted by habitat alterations, such that throughout recruitment declines a deceptively large, adult-biased population remains (Reese, 1996). Their widespread occurrence creates an illusion of prosperity which is difficult for conservation biologists to overcome when seeking protection measures.

In the Murray-Darling Basin – a heavily regulated inland river system encompassing much of south-eastern Australia – freshwater turtles are experiencing precipitous declines. While still considered widespread and abundant, Macquarie turtle (Emydura macquarii macquarii) populations in the region have declined by 69% since the 1970’s and juvenile recruitment has stagnated (Chessman, 2011). Despite this, the focus of conservation efforts in the Murray-Darling Basin has been on native fishes, and very little is known about the impact of current water management practices in freshwater turtles. This thesis aimed to address some of these knowledge gaps by providing novel insights into the ecology of E. m. macquarii living downstream of a large hypolimnetic-releasing impoundment in Australia’s southern Murray-Darling Basin.

Radiotracking of E. m. macquarii downstream of Hume Dam revealed that turtles often exhibit a pattern of movement characterised by dispersal between multiple small core activity areas in which they reside for extended periods of time. Core activity areas used by E. m. macquarii encompassed a diversity of habitats including the main river channel, permanent and semi-permanent lagoons and anabranches, and a farm dam. Periods of residence in warmer floodplain habitats likely provided E. m. macquarii with the opportunity to mitigate the effects of artificially depressed stream water temperatures caused by the release of hypolimnetic water from Hume Dam – a large impoundment on the upper Murray River.
Radiotracking also revealed that E. m. macquarii respond to structural microhabitat patchiness at the scale of metres. Emydura macquarii macquarii showed a strong preference for microhabitat patches containing woody debris and aquatic macrophytes, and were negatively associated with flow velocity. This raises the possibility that E. m. macquarii may benefit from restoration projects aimed at repairing riparian vegetation and reintroducing woody debris undertaken for fish in the Murray-Darling Basin. However, other large-scale factors contributing to turtle declines such as fox predation and altered flow regimes may override subtle changes resulting from restoration of microhabitat patchiness, and thus it is unlikely to significantly impact turtle conservation.

E. m. macquarii inhabiting an anabranch affected by cold water pollution had different activity and basking patterns to individuals residing in an adjacent floodplain lagoon. In the anabranch, E. m. macquarii were less active in terms of distances moved per day, and basked significantly less frequently during spring compared to those in the lagoon. However, during summer the opposite occurred. Emydura macquarii macquarii may thus have been responding to artificially depressed water temperatures in the anabranch by extending their overwintering period later into spring, and attempting to compensate for this by foraging and basking more during summer when water temperatures become more favourable. Efforts to keep threshold spring water temperatures above approximately 17°C - as has been advocated to improve spawning, egg and larval survival, and growth opportunities for native fish downstream of hypolimnetic-releasing dams in the Murray-Darling Basin – may also benefit E. m. macquarii by maintaining temperatures above their minimum feeding threshold.

Collectively, the findings of this thesis demonstrate that E. m. macquarii has considerable behavioural plasticity habitat use, activity patterns and thermoregulation which have enabled them to persist in a heavily modified area from which native fish have been largely extirpated. However, further research is required to determine the potential impact chronic stressors associated with water resource development are having on the demographics and long-term viability of turtle populations in the region. This is particularly pressing given the continuing anthropogenic pressures placed on the Murray-Darling Basin and its associated degradation, which will increasingly be exacerbated by the effects of climate change.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Charles Sturt University
Supervisors/Advisors
  • Watts, Robyn, Principal Supervisor
  • Robinson, Wayne, Co-Supervisor
Award date15 Nov 2018
Publication statusPublished - 2018

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