| Summary: | Rising ocean temperatures caused by anthropogenic climate change are expected to have significant negative impacts on the biodiversity and productivity of shallow tropical oceans, with concomitant effects on fisheries production. Tropical ectotherms are predicted to be particularly vulnerable to ocean warming due to the pervasive effects of temperature on their performance and physiology, combined with adaptation to relatively stable thermal environments. For many coral reef fishes, moderate increases in temperature lead to increases in individual performance and fitness. However, extreme temperatures (outside of those normally or previously experienced) often have marked adverse effects. During initial and short-term exposure to elevated temperatures, fishes may alter their foraging behaviour and increase food intake to compensate for increasing metabolic demands. Alternatively, fishes may conserve energy as temperatures increase. Or, they may move to more optimal habitats. To date, most of the studies on thermal sensitivities of coral reef fishes, have been conducted under controlled laboratory settings. As such, these studies may have overestimated the vulnerability of fishes to temperature because they do not take account of the ability of individuals to mediate the effects of increasing temperature through modification of their behaviour (i.e. moving to more optimal habitats). This is particularly true for large-bodied tropical fisheries species that are typically understudied in the wild. To address this problem, I used in situ observations and high-resolution passive acoustic telemetry to explore temporal and spatial variation in behaviour and movement of coral trout, Plectropomus leopardus on the Great Barrier Reef, Australia. Plectropomus leopardus are an important coral reef mesopredator and are one of the primary species targeted by both commercial and artisanal fishers throughout the Indo-Pacific region. Previous experimental studies have shown that P. leopardus are sensitive to increasing temperature and ocean acidification (Chapter 2) though there have yet to be any field tests to establish their vulnerability to environmental change under natural conditions. To address this knowledge gap, I investigated spatial and temporal variation in foraging behaviour and activity patterns (i.e. amount of time spent resting) of P. leopardus from latitudinally distinct locations. From > 500 hrs of in situ observation, I found that P. leopardus exhibited increased foraging frequency in summer versus winter time, irrespective of latitude, however, foraging frequency substantially declined at water temperatures > 30 °C. In addition, the amount of time spent resting was greatest for P. leopardus during the summer time at both locations, however, the effect was most pronounced at the low-latitude location where individuals spent up to 62% of their time inactive, compared with 43% for the high-latitude population (Chapter 3). These results provided the first indication that P. leopardus moderate their foraging behaviour and activity according to changes in ambient temperature. Using fine-scale acoustic telemetry (< 0.5 km² arrays), I also examined the spatial and temporal variation in home range (Chapter 4), locomotory performance and activity patterns (Chapter 5), and potential use of depth refugia (Chapter 6) for P. leopardus from latitudinally distinct locations. The average home range for P. leopardus was 0.32 km² at the high-latitude location compared to 0.23 km² at the low-latitude location. Seasonal differences were apparent at both locations with P. leopardus showing contraction in home range during summer, especially when temperatures were > 27 °C (Chapter 4). Further, average acceleration for P. leopardus was 0.69 m.s⁻² and acceleration increased with increasing temperature up to 30 °C. However, the impact of ambient water temperature strongly influenced resting patterns for P. leopardus from the low-latitude location, where individuals were twice as likely to be detected at low-activity (i.e. resting) than individuals from the high-latitude location. In contrast, individuals from the high-latitude location were more active and detected more often undertaking routine activity indicating higher rates of activity for P. leopardus from the high-latitude location (Chapter 5). Finally, although P. leopardus altered their depth use on a daily basis, and in response to monthly temperature variation, it was notably small (< 3 m) and did not vary between locations. However, three-dimensional activity space used by P. leopardus differed substantially between locations with P. leopardus from the high-latitude location occupying much greater (> 50%) 3-D activity space than individuals from the low-latitude location (Chapter 6). These results indicate that P. leopardus are more likely to alter their horizontal rather than vertical space use in response to higher ambient temperature. Taken together, the results of this thesis suggest that ever-increasing ocean temperatures may impose significant constraints on the capacity of P. leopardus to meet increasing metabolic costs associated with higher temperatures (Chapters 2 & 3). Given projected increases in ocean temperature, P. leopardus may be increasingly constrained in their ability to obtain sufficient prey resources while forced to conserve energy (Chapters 3 & 4). Reductions in movement and space use will also have ramifications for individual fitness, population viability, and ecological function (Chapters 4, 5 & 6). This will likely have negative demographic consequences for P. leopardus potentially undermining the viability and sustainability of coral reef fisheries, particularly in low-latitude locations.
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