Lynch, Jason Thomas; (2024) A social-ecological approach to understanding the footprint of fisheries on tropical coral reef biodiversity. Doctoral thesis (Ph.D), UCL (University College London).

Text A social-ecological approach to understanding the footprint of fisheries on tropical coral reef biodiversity.pdf - Accepted Version Access restricted to UCL open access staff until 1 January 2026. Download (8MB)

Abstract

Coral reefs are vital biodiversity centres and provide essential ecosystem services that support millions of people worldwide. However, these ecosystems are in peril, facing declines in health, productivity, and resilience. Effective management is urgently needed to address these issues. Direct human disturbances have been linked to the deterioration of coral reefs on both global and local scales, leading to decreased fishery productivity and eroding crucial ecosystem services. Consequently, human communities that depend on small-scale reef-reliant livelihoods, which comprise 90 % of global fishers, may struggle to meet the nutritional requirements of growing populations. The Great Sea Reef is an exemplary case study as it harbours extraordinary biodiversity, and its coral reefs are vital to much of Fiji’s population due to their ecosystem services. However, these reefs face significant threats from climate-related stressors and human activities, such as overexploitation of fish stocks. Understanding the dynamics and impacts on fish communities and coral reef habitats across the Great Sea Reef is challenging in this traditionally data-deficient seascape. This thesis employs advanced imaging methods to build a comprehensive index of fish populations, habitat complexity, and environmental information. In Chapters 2 and 3, I build context around coral reefs globally and within the Southwest Pacific while highlighting management practices. Chapter 4 details the methods employed in this thesis, including a novel habitat complexity metric. In Chapters 5 and 6, I investigate the impact of human activities on fish biomass and fish wariness while accounting for commonly used habitat complexity metrics. The results highlight that over half of the reefs do not have sufficient biomass for optimal ecosystem functioning. While there is no direct correlation between human population density or proximity to reef sites and fish biomass, higher levels of human pressure negatively affect fish populations. Target fish exhibited significantly greater wariness than non-target fish; however, overall wariness decreased with increasing human pressure, suggesting that more cautious target fish have been fished out. Additionally, larger fish were generally warier, showing a positive correlation as minimum approach distance increased with body length. However, against expectations, there was no significant relationship between habitat complexity and minimum approach distance. Therefore, I call into question the suitability of the available habitat complexity metrics. To respond to this and to further understand the relationship between fish-associated metrics and habitat complexity, I demonstrate the usability and relatability of a novel habitat complexity metric termed ‘HideyHole’ in Chapter 7. This metric identifies and counts holes in reef models, as holes provide critical fish refuge. This metric surpassed traditional measures like rugosity and fractal dimension, showing stronger correlations with fish abundance and size. HideyHole metrics, such as holes per m² and mean hole volume, provided significant insights into habitat availability and fish responses. While this thesis underscores the importance of integrating biodiversity, structural, and environmental metrics to understand human impacts on coral reefs, further work is required to understand the nuances of human-related effects on reefs at local scales. The results are intended to inform local reef fisheries management plans and contribute to the broader understanding of coral reefs in the Southwest Pacific.

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