Population regulation in snake-necked turtles in northern tropical Australia : modelling turtle population dynamics in support of Aboriginal harvests

  • Damien Anthony Fordham

    Student thesis: Doctoral Thesis


    This thesis provides the fundamental science to underpin contemporary harvests of northern snake necked turtles (Chelodina rugosa; Ogilby 1890) and their eggs, and presents fresh scientific insights relevant to ecology, conservation and environmental management. Bawinanga Aboriginal Corporation (BAC) recognises that adverse social disruption results from disconnecting indigenous people and communities from their language, culture and traditions. Through sustainable resource utilisation BAC aims to achieve a measure of self-sufficiency for Aboriginal people in the Maningrida region, a continued connection to traditional values and the conservation of natural resources. Wildlife enterprises draw upon land, culture and its related traditional knowledge, creating economically, socially and culturally suitable long-term employment opportunities. Aboriginal people have developed a wealth of knowledge regarding how turtles can be exploited for subsistence without threatening local persistence. BAC is keen to see this knowledge put to use in developing a local industry, focused initially on producing C. rugosa hatchlings for the domestic pet turtle market, with a view to expanding to harvests of sub-adult and adult turtles. Here I provide optimal conditions for the storage and incubation of C. rugosa eggs and husbandry of subsequent hatchlings, and provide valuable information on the population dynamics of C. rugosa, allowing informed decisions on levels of harvest that are biologically sustainable and the offsets that can be put in place to increase harvests. C. rugosa can nest underwater. My results carry this finding further, by demonstrating that eggs of C. rugosa can survive at least 25 weeks inundation, almost six months, with an optimal inundation duration of 6 weeks. Eggs not held under water suffer the same mortality as eggs inundated for 25 weeks, suggesting that underwater nesting is not a facultative capacity but rather, inundation is essential for the optimal survival of embryos. Inundation duration has a profound effect on incubation period, reducing it by up to 9 weeks over what would be expected at a given temperature. I argue that egg inundation is a developmental response to environmental variability in factors that govern timing of reproduction, duration of development and timing of hatching and emergence. Egg preconditioning can be combined with control over incubation and hatching environment to maximize the development and survival of embryos and subsequent hatchlings. Inundation of eggs for 6 weeks (2-10 weeks is acceptable), incubation of embryos at 28°C and raising hatchlings in water at 28°C will yield the best overall outcomes for a hatchling industry. Many chelonians have low hatchling survival, slow growth, delayed sexual maturity and high sub-adult and adult survival, constraining a quick response to increases in adult mortality from human impacts such as harvesting or habitat alteration. Conversely, C. rugosa is fast growing, early maturing and highly fecund relative to many other turtles. Correlative evidence spanning six study sites and three field seasons indicates that C. rugosa are somewhat resilient to harvest and pig predation. A decline in sub-adult and adult abundance was correlated with increased recruitment and age-specific fecundity, via enhanced juvenile survival, decreased size at onset of maturity, and increased post maturity growth. Experimental manipulation of population density of six replicate wild populations of C. rugosa confirmed that this species is resilient to negative perturbations in density. Remarkably, in some populations, turtle abundance took as little as one year to recover from a strong negative perturbation (> 50% population reduction) in sub-adult and adult density. Model projections indicate that C. rugosa are resistant to chronic increases in mortality. A compensatory decrease in size of female maturity and density dependent hatchling survival are sufficient to allow annual harvests of up to 20 % of sub-adult and adult C. rugosa without substantial population suppression. Taken together these results present an open challenge to a universal generality that freshwater turtles are highly susceptible to any form of off-take and that high sub-adult and adult survival is crucial for achieving long-term population stability. Today pigs (Sus scrofa; Linnaeus 1758), an exotic predator, prey heavily on C. rugosa, providing an unrelenting predation pressure, compromising subsistence harvest rates and threatening local persistence. Even moderate capture-mark-recapture estimates of pig-related turtle mortality (48 %) exceed what can be accommodated by fast growth, reduced size of maturity and density dependent hatchling survival, leading to severe population decline and extirpation within 30 years. My model projections predict that periodic local culling of pigs, fencing of wetlands to exclude predators, and hatchling supplementation to off-set losses to predation, are all viable management strategies to ensure ongoing C. rugosa harvests. Collectively, these findings support an Aboriginal industry that aims to achieve economic self-sufficiency and wildlife conservation through culturally identifiable employment.
    Date of Award2007
    Original languageEnglish
    SupervisorArthur Georges (Supervisor) & Nancy Fitzsimmons (Supervisor)

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