Depth refuge and the impacts of fishing on coral reef fish communities

Steven Lindfield

    Research output: ThesisDoctoral Thesis

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    Shallow waters easily accessible to SCUBA diving have been the focus for research on the ecology of coral reef ecosystems and impacts of fishing on coral reef fish communities. Deeper waters (>20 m) have received less attention due to the limitations of widespread sampling techniques, yet these areas are thought to provide marine fauna with refuge from fishing impacts and other disturbances. Hence these deeper waters increasingly recognised as an important area for future research and marine resource management. My thesis explores this subject by testing hypotheses about depth refuge, the impacts of fishing and how commonly used survey techniques may bias conclusions when assessing coral reef fish communities.

    The term ‘depth refuge’ has been used to describe the protection afforded by deeper waters from disturbances impacting shallow depths. A good example is the impact of spearfishing on coral reef fishes. Free-dive spearfishing is a frequently practiced method targeting particular species, yet it has obvious depth limitations (typically 15 m). When combined with the use of SCUBA, spearfishing can remove the refuge traditionally provided by deeper waters. Using a combination of historical catch data and fishery-independent sampling with baited remote underwater stereo-video systems, I quantified the negative effects of removing depth refuge in the southern Mariana Islands, Micronesia. This study was the first to investigate the impact of SCUBA spearfishing and has important management implications for Guam and other locations worldwide. The following chapter expanded the depth range into the mesophotic depths (30 m to 90 m) and assessed how fish assemblages change over a depth gradient in the face of various levels of fishing pressure, both at coastal and offshore reefs. I found deepwater coral reefs (to 70 m) were associated with an increase in fish biomass, especially predatory species targeted by fishing. Furthermore, I showed that the majority of fishery-targeted species including herbivores had depth distributions that ranged from shallow inshore depths (~10 m) to the extent of coral reef habitat. With fishing pressure focused on shallowest reef areas, these depth distributions with greater fish biomass on deeper coral reefs highlight the role of mesophotic reefs as critical refuges for a range of fish species.

    Moving away from the relatively high fishing pressure in the southern Mariana Islands, I found deeper waters were important in sustaining the biomass of reef fish across a gradient of habitat and human population at the remote, traditionally fished reefs of Yap’s outer islands in Micronesia. To improve the efficiency of fish surveys in greater depths at these remote locations, I used a novel sampling technique; a diver operated stereo-video system (stereo-DOV) in conjunction with a closed-circuit rebreather (CCR) diving system. Not only do CCRs allow more efficient diving in deeper waters, they also don’t produce bubbles. Due to their silent operation they are thought to minimise diver avoidance of fish; an important but rarely accounted-for issue when surveying fish populations across gradients of fishing pressure. The last data chapter focused on this issue in more detail and investigated how fish surveys are affected by conventional open-circuit SCUBA diving through a direct comparison with the use of a silent CCR diving system. Large differences were detected between diving techniques in heavily fished locations, but not in marine protected areas (MPAs). By producing depressed counts of targeted fish in unprotected areas, the use of OC SCUBA resulted in inflated assessments of the effectiveness of MPAs. While the behaviour of fish towards divers is rarely mentioned in conclusions from studies using underwater visual census, it is an important source of bias that should be acknowledged and minimised where possible. In summary, this thesis provides new insight into existing ecological questions, opening up areas for future research while providing important management recommendations for sustaining coral reef fisheries.

    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2015


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