Fine-Scale Foraging and Movement Behaviour of Chaetodon Butterflyfish Along a Turbidity Gradient on a Coral Reef in the Solomon Islands

Steele, Emily
Sabetian, Armagan
Lilkendey, Julian
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Master of Science
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Auckland University of Technology

Coral reefs stand as the pinnacle of ecological biodiversity, playing a vital role in supporting a diverse array of organisms. But they are under threat. Anthropogenic influence impacts how coral reefs, and the animals and people they support, function. Conventional methods of assessing coral reef health typically rely on species abundance and biomass estimates, potentially overlooking subtle changes occurring early in the degradation process. This study explores the concept that fine-scale alterations in the movement patterns and foraging behaviour of coral reef fish can serve as indicators of coral reef degradation. Employing stereo-video methodology, footage of coral reef fish behaviour was captured along a turbidity gradient at Vavanga Reef on Kolombangara Island, Solomon Islands. Vavanga Reef receives sedimentation from Vavanga River, creating a turbidity gradient that is exacerbated by historic logging practices. Analysis of foraging substrates, bite rates, bite distances, and three-dimensional tracking data of three species of Chaetodon butterflyfish species revealed insights into their foraging and movement behaviour. The hard-coral specialist foragers, C. baronessa and C. lunulatus, showed significant trends across the turbidity gradient, whereas C. vagabundus, the generalist omnivore, did not. C. baronessa displayed a clear preference for Acropora, with high total bite percentages and preference ratios that depended on substrate availability and turbidity. C. lunulatus consistently preferred Porites across the turbidity gradient; however, this was not seemingly impacted by substrate availability or turbidity, and their foraging substrate choice appeared random. The body mass and length of C. baronessa and C. lunulatus decreased with increasing turbidity, suggesting that turbidity and coral availability influence fish physiology and growth or that these species are distributed along the turbidity gradient according to age. Further investigation of the physiological condition of the species would provide a more definitive answer. The velocity of C. baronessa and C. lunulatus increased with turbidity, indicating adjustments in swimming behaviour to compensate for reduced visibility or prey availability. Furthermore, increased overall dynamic body acceleration values in C. baronessa at sites with lower live coral cover suggest elevated energy expenditure associated with foraging efforts in less favourable conditions. These results may indicate an adaptive response of C. baronessa to changing reef conditions. The discoveries resulting from this research emphasise the importance of identifying nuanced changes in coral reef fish behaviour to understand and address the ongoing degradation of coral reef ecosystems. This research is crucial in the face of escalating pressures that threaten coral reefs and the communities they support.

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