The Effects of Urbanisation on Oblong Turtles

Main Club November 2020

Our speaker was Anthony Santoro, a PhD candidate at Murdoch University. For his Honours studies on the Oblong Turtle, Chelodina oblonga, Anthony surveyed thirty-five wetlands in the South West, involving about 1500 turtles. These turtles (we previously called them tortoises, but have adopted the American nomenclature) are the snake-necked species that are fairly common in wetlands around Perth, endemic to our SW, and occur in permanent or seasonal water bodies from Jurien Bay to the Fitzgerald River National Park. Globally, 60% of turtle and tortoise species are in danger of extinction because of climate change, land clearing, predation, pollution, and other factors, and these are threats to our local species.

The Oblong Turtle usually lays her eggs in spring, stimulated by rain or low-pressure weather systems. The eggs generally hatch in autumn, and the hatchlings usually emerge around August. The hatchlings are only the size of a 20c piece.

Anthony studied three metropolitan wetlands in close proximity to each other– Bibra Lake, North Lake and Chelodina Wetland at Murdoch University. He caught turtles using modified funnel traps and monitored the turtles using radio telemetry and GPS. Trapped turtles are measured and weighed, their sex determined, placed into age classes (adult and juvenile), and they are fitted with a microchip for identification. Only the females are radio tagged, each tag emitting a different radio frequency so that they can be tracked, their movements mapped and their nesting places determined. Also, turtle corpses are recorded.

Eggs are usually laid 10 to 20cm deep in the soil. When nests are found, the soil moisture and temperature are measured, and conditions such as leaf litter and canopy cover are recorded. This helps to determine optimum conditions for nesting. Wildlife cameras are used for further monitoring.

Anthony tracked 100 turtles – all mature females. Each has its “personality”, he says, but they fall into three categories: “sedentaries”, which stayed around one area, “shifters”, which had several home-ranges and “nomads” which had no real home ranges and showed fairly random movements. One of the nomads even crossed Hope Road three times, moving between two lakes. Habitat preferences were investigated, and vegetated areas, especially beds of Typha (Bulrush) and Baumea (Jointed Rush), were found to be the most preferred

When a lake dries out, the turtles either move to a nearby water body, or else they aestivate – they dig about 20cm into mud and “close down”, often for about six months, until water floods the “sleeping” animal. North Lake, for example, dries up nearly every year, and nearly all the turtles aestivate.

Anthony said that the lakes are drying up more often and for longer, and he is concerned that some turtles may only be active for 3 or 4 months of the year. Turtles may not survive if their active period is reduced beyond a certain limit. As well, they are under threat from predation by foxes and ravens, and also by traffic (mostly nesting females when they are forced to cross our roads). Foxes are destroying hundreds of nests (eggs are dug up and eaten) and this is preventing recruitment.

One solution may be to artificially manage water levels in lakes. Relocation has been tried, but turtles tend to move back to their original home ranges. Tunnels could be a solution to roadkill, if the turtles will use them. Another possibility is to collect eggs, hatch them artificially and relocate the progeny. 

The City of Cockburn is sponsoring Anthony’s research and is taking an active interest in conserving the turtles. This involves fox control, public education, “Slow Down” signs, and constructing mesh cages, with cameras, at nesting sites. Along with DBCA, Murdoch University and Native Arc, they have established a Turtle Trackers Program, in which about 30 people look for turtles in spring and locate nest sites. Good results have been achieved in the first year, with 25 nests being protected. Anthony said that work urgently needs to be done to understand turtle ecology and restore turtle population growth in urban areas. He hopes to assess populations throughout their range from Jurien Bay to Albany as this may help in upgrading their conservation status to Threatened from the Near Threatened category. A higher conservation status may aid in securing sufficient funding to continue research on the turtles.

Mike Gregson