Abstract
Marine predators such as sharks play an important role in coral reef ecosystem functioning. Consequently, understanding how reef sharks utilise reef ecosystems is important for the conservation of both shark species and reefs themselves. Given that climate change is predicted to cause increased fluctuations in environmental variables, understanding how these changes are impacting reef shark habitat and movements will be important for their future conservation and management. Hence, the overall aim of this thesis was to investigate the environmental drivers of reef shark movements in Chagos Archipelago. This project uses five-years of acoustic tracking data from grey reef sharks (Carcharhinus amblyrhynchos) and silvertip sharks (Carcharhinus albimarginatus) from the Chagos Archipelago and environmental data from satellite remote sensing, to provide insights into how animal movement and spatial patterns are influenced by environmental change.Firstly, a systematic literature review was conducted to understand current use of satellite remote sensing for elasmobranch research and to identify gaps and future research opportunities. It revealed limited use of multiple variables in concert when conducting analyses, and limited integration of satellite remote sensing data with acoustic telemetry, an important technique for investigating movement ecology of range-restricted marine species.
Coral reefs are important ecosystems for reef sharks that are experiencing global declines due to anthropogenic driven changes in environmental variables. As such, this thesis developed a satellite-based Reef Environmental Stress Exposure Toolbox (RESET) for remotely monitoring coral reef exposure to environmental stressors. Nine environmental variables from satellite Earth Observation datasets were analysed using Google Earth Engine and used to investigate spatial and temporal variability in environmental stress exposure of important elasmobranch habitat from local to global scales.
Ecological research using acoustic telemetry typically utilises the timings and frequency of detections. In this study, a novel technique was developed for analysing detection gaps in acoustic telemetry data, which are typically ignored, to infer differential space use and movement patterns. Silvertip sharks were found to undertake greater ‘out of range’ movements compared to grey reef sharks, indicating spatial segregation between these two species, and corroborating previous work undertaken using stable isotopes in this region.
Lastly, this thesis integrated RESET with acoustic telemetry data to investigate how changing environmental stress in important elasmobranch habitat alters reef shark movement and connectivity in the Chagos Archipelago. The impact of individual environmental drivers on movement connectivity was also explored. Results showed that grey reef and silvertip sharks reduced movement connectivity and increased residency in response to increasing environmental stress. Thus, this PhD research has advanced understanding of reef shark movement ecology. The new ecological insights and novel methods developed will contribute to the future monitoring, management and conservation of these important and threatened marine predators.
| Date of Award | 1 Jul 2022 |
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| Original language | English |
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| Supervisor | Emma Tebbs (Supervisor) & David Jacoby (Supervisor) |