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Development and application of an eco-physiological approach to investigate the impacts of changing environments on elasmobranchs in the field

Lear, Karissa (2020) Development and application of an eco-physiological approach to investigate the impacts of changing environments on elasmobranchs in the field. PhD thesis, Murdoch University.

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Temperature is widely regarded as the most important abiotic factor driving biological processes. This is particularly true for ectotherms, as their body temperature, and therefore metabolism and other physiological processes, are directly regulated by the environment. As such, the increasing temperatures resulting from global climate change will likely have significant effects on the physiology, behaviour, growth, and ultimately survival of ectotherms, and profoundly influence the future health and dynamics of ecosystems. Understanding how the physiology of ectotherms responds to temperature change is therefore essential to accurately forecasting the effects of climate change, and to determining the most suitable conservation actions for many ecosystems. This thesis aims first to develop best practice methods for measuring thermal effects on the metabolic physiologies, activity, and behavioural patterns of ectothermic elasmobranchs in the field, and second to employ these methods to determine the thermal physiological responses of the Critically Endangered freshwater sawfish (Pristis pristis) and bull shark (Carcharhinus leucas) in the Kimberley region of Western Australia. A combination of accelerometry, respirometry, and field monitoring methods were used to determine how variation in temperature and flow regimes drive metabolic rates, movements, activity, and behaviour in both species, and how effectively these animals cope with the extreme conditions imposed by the long dry season in the Fitzroy River. This information was then used to forecast how both species are likely to respond to climate change and water resource development in this region.

The Fitzroy River is a remote, dryland tropical river with substantial intra-annual variation in flow, and supports relatively pristine habitats. As a result, it acts as a natural, intact nursery for both freshwater sawfish and bull sharks, which, in this system, spend the first 4 – 6 years of their life in the freshwater riverine environment. Notably, the Fitzroy River is the last known intact nursery habitat for freshwater sawfish worldwide, and therefore is of global importance to this species’ long-term survival. This thesis aimed to form an understanding of the processes determining survival and fitness of these animals in this nursery habitat. First, the study optimised the calibration of metabolic rate and body acceleration in elasmobranchs, and demonstrated that it is essential to incorporate temperature into such calibrations for accurate estimation of field metabolic rates. Additionally, results showed that static respirometry systems provide more accurate calibrations than do flume respirometers. Second, accelerometers were used to determine the thermal locomotor performance regimes in eight species of elasmobranchs in the wild. These performance data revealed that animals living in enclosed habitats, including many nursery-bound juvenile elasmobranchs such as bull sharks and freshwater sawfish in the Fitzroy River, had more generalist thermal responses than larger animals in more open habitats, but are also more likely to experience extreme temperatures and as a result may be more vulnerable to climate change. Third, an analysis of wet season flow rates and recruitment patterns of freshwater sawfish using a long-term dataset revealed that they used the wet season periods of extreme high flow to recruit into their freshwater nursery, and that recruitment only occurred in years with substantial wet season floods. Fourth, the field metabolic rates of sawfish and bull sharks estimated through accelerometry methods were linked with growth data measured over dry season periods in the Fitzroy River. This showed that a combination of high temperature and limited food resources resulted in significant mass loss during the dry season, potentially threatening survival under increased temperature scenarios. Finally, a comprehensive analysis of body temperature, field metabolic rates, habitat use, and growth of sawfish measured over several years revealed that the volume of wet season flow significantly impacted the body condition of sawfish during the subsequent dry season through governing rates of energy acquisition.

Together, these results strongly suggest that maintaining sufficient water levels and natural flow regimes, including the substantial inter- and intra-annual variation in flow and the periods of extreme high flow, is essential for both the recruitment of sawfish and bull sharks into their Fitzroy River nursery, and their survival during juvenile life stages. This will become particularly important as the climate changes, temperatures increase, and the weather and rainfall patterns become less consistent. Consequently, it is essential that the physiological effects of increased temperatures and variation in flow regimes are carefully considered when making management decisions about water resource development in the Fitzroy River to ensure the best possible conservation of these species and their unique ecosystem. These findings, and the field methods developed in this thesis, will also help to evaluate the physiological challenges faced by elasmobranchs confronted with environmental change in other ecosystems.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Centre for Sustainable Aquatic Ecosystems
United Nations SDGs: Goal 14: Life Below Water
Supervisor(s): Gleiss, Adrian, Morgan, David, Beatty, Stephen and Whitney, Nicholas
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