Freshwater turtles are one of the most threatened of all vertebrate groups. There is however, a distinct lack of scientific knowledge about the biology of many species. Ecological information regarding the early life-stages is important for freshwater turtle management and conservation, because creating and preserving suitable habitat for embryos, hatchlings and juveniles will increase the number of individuals attaining reproductive age. The overall aim of this thesis was to describe the nesting biology and juvenile ecology of an endangered species of freshwater turtle, which is found only in a single river system in South-East Queensland, Australia.
The Mary River turtle Elusor macrurus is one of the largest freshwater turtles on the Australian continent. The current population is listed as endangered and although hundreds of E. macrurus nests have been recorded since 2004, juvenile turtle are rarely sighted in the wild. This thesis documents three aspects of the biology during the early life-stages of this endangered species of freshwater turtle, from the nest-site selection by the females through incubation to the habitat required by the juvenile turtles once they enter the river. The first aim of the present study was to describe the species nesting biology, the second to understand the nest-site selection by female E. macrurus, the third to reveal the effects of incubation temperature upon hatchlings’ phenotype, the fourth to characterise the habitat selected by the juveniles once they enter the river, the fifth to develop a methodology that would overcome the false-absences inherent to locating small and cryptic animals within low-visibility and refuge-rich environments, and the sixth to use the study findings to provide direction for management and conservation strategies for E. macrurus.
To understand the nesting biology, it was important to determine if the females returned to the same nesting grounds every year. To address the first aim of this study, a nesting bank was monitored by time-lapse infrared cameras over three consecutive years, and a novel methodology was developed to identify individual turtles based upon marks on their carapaces. The results demonstrated that female E. macrurus do exhibit nest-site fidelity. It was unclear however, which features of the river bank were important in the females’ selection. The physical aspects of sandy banks selected by female E. macrurus for nesting were compared against sandy banks where females had never been recorded to nest upon (non-nesting banks). There were no statistical differences in soil characteristics or physical aspects between nesting and non-nesting banks. There was however, a significant difference in the bearing, with the nesting banks facing between 50º (NE) and 300º (NW), whilst the non-nesting banks faced between 90º and 240º. There was difference in the overall mean temperature recorded between nesting and non-nesting banks, as well as the mean daily temperature fluctuation, which was significantly higher at the nesting banks. These thermal conditions are likely to be due to the northerly aspect that these banks face.
Incubation temperature is known to be one of the most important factors influencing the phenotype of the hatchling turtles. The second aim was addressed through empirical studies, to investigate how mean and daily fluctuations in incubation temperature affected the phenotype and exercise performance of the hatchling E. macrurus. The overall mean temperature recorded from the E. macrurus nests ranged from 26 to 31ºC; and the mean daily temperature fluctuation recorded was 5.7ºC (with a variation that was as low as 2ºC and as high as 22ºC). The first study incubated freshly laid E. macrurus eggs at three constant thermal regimes. The treatment groups were similar to the mean nest temperature recorded in the wild (26, 29 and 32ºC). Contrary to what was hypothesised, embryos incubated at the warmest thermal regime had a higher rate of mortality. Those that hatched showed a significantly reduced body size, mass, post-hatch growth rate and poorer performance when swimming and tested for self-righting time compared to those hatchlings incubated at lower temperatures. The second study incubated freshly laid E. macrurus eggs at constant (28ºC) and fluctuating (28 ± 3ºC and 28 ± 6ºC) thermal regimes. These results also refuted my hypothesis and showed that a variation in daily incubation temperature between zero and 6ºC, did not alter the phenotype or performance of hatchling E. macrurus. Daily variations of 12ºC were, however, detrimental as the eggs incubated at 28 ± 6ºC only had a 5% hatching success (n = 1).
Similarly to most freshwater turtle species, the habitat requirements of the juvenile E. macrurus are unknown. The third aim was to identify the characteristics of habitat selected by hatchling and juvenile turtles, and this required the attachment of miniaturised telemetry devices to the turtles before they were released into the river. The turtles were located over the subsequent nine months, and river physical characteristics were recorded and inputted into a presence-only predictive model. The results showed that juvenile E. macrurus occupied a very narrow range of habitat characteristics, which occurred in relatively few areas throughout the main trunk of the river.
The overall aim of this study was to understand the biology of the early life-stages of E. macrurus for the purposes of management and conservation. The study showed that females were nest-site fidelic and favoured north-facing banks. The results were however, inconclusive in identifying the physio-chemical characteristics that differentiated nesting areas from other sandy banks. Moreover, the thermal-manipulation studies did not confirm that the nest-site selection was temperature related. Therefore, the reason why specific banks are recurrently selected over others by nesting female E. macrurus remains a mystery. Importantly, the results illustrated that critical habitat for juvenile E. macrurus are limited throughout the Mary River and were not located in the impounded sections. I argue that the preservation of nesting areas and habitat for juveniles is vital for the long-term conservation of E. macrurus, and recommend such actions to be included in future recovery plans.