No-take marine reserve performance under varying environmental and anthropogenic influences

McClure, Eva Claire (2019) No-take marine reserve performance under varying environmental and anthropogenic influences. PhD thesis, James Cook University.

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Conserving coral reef fish assemblages is important, for ensuring food security for coastal human populations, and for maintaining coral reef ecosystem processes. Sound management of coral reef fish assemblages is a priority, but for management strategies to be truly effective, there is a need to understand what maintains fish assemblages under multiple environmental conditions. No-take marine reserves (NTMRs) are a relatively simple and cost-effective management strategy for conserving marine biodiversity and enhancing food security, particularly in developing island nations with multi-species fisheries. But whether NTMRs can continue to provide social, economic and conservation benefits in the face of increasing environmental threats to coral reefs remains unresolved, and scientific opinion remains divided. Thus, this thesis investigated the relative roles of NTMRs and environmental factors in determining the structure of coral reef fish assemblages. These environmental factors included natural spatial variability in environmental conditions that occurs across continental shelfs, and among and within islands, as well as large-scale environmental disturbance events (cyclones/typhoons, thermal coral bleaching). Specifically, this thesis had four research questions that were addressed in each of four data chapters (Chapters 2-5): 1. How do severe disturbances affect coral reef fish assemblages that develop along prevailing environmental gradients, irrespective of NTMRs? 2. Do NTMRs remain effective as fisheries management tools despite being located in different environments and despite severe environmental disturbances? 3. What are the relative roles of habitat, island geomorphology and NTMRs in driving abundance and species richness of coral reef fishes? 4. Do differences in environmental conditions among coral reef locations result in dietary plasticity of important subsistence fishery species?

Chapter 2 assessed how severe environmental disturbance events affect the structure of coral reef fish assemblages formed along a prevailing environmental gradient, by comparing fish assemblages from the inner-, mid-, and outer-shelf of Australia's Great Barrier Reef (GBR), before and after disturbance impacts, irrespective of NTMR effects. Specifically, changes in benthic cover, and the biomass, taxonomic and trait composition of herbivorous reef fish assemblages were compared at each shelf position, five years before (2008/9), and six months after (2016) two severe cyclones and a thermal bleaching event. Results showed that there was a severe loss of hard coral cover across the shelf following disturbance events (inner-shelf: ~25% to 2%, mid-shelf: ~45% to 5%, outer-shelf: ~60% to 10%), and a substantial loss of fish species trait richness at each shelf position (inner-shelf: 0.23 to 0.06, mid-shelf: 0.3 to 0.16, outer-shelf: 0.24 to 0.07). Taxonomic assemblages of fishes remained distinct at each shelf position before and after disturbance events, with assemblages becoming more distinct both among, and within shelf positions following disturbances. This was attributed to assemblages at each shelf position going from being characterised by multiple species before the disturbances, to being dominated by few or single species following disturbances. These dominant species increased in biomass significantly on mid- and outer-shelf reefs, but not on inner-shelf reefs. Trait-based analyses of the fish assemblages showed increases in trait specialisation and originality on inner-shelf reefs only, indicating a loss of redundancy within the inner-shelf herbivorous reef fish assemblage. Macroalgal cover increased on inner-shelf reefs from 50 to 70% following disturbances. Given the differential response of inner-shelf reef benthic and fish assemblages to environmental disturbances, inner-shelf reefs may have a different recovery rate and trajectory than mid- and outer-shelf reefs considering their exposure to lower water quality.

Chapter 3 compared coral reef fish assemblages among multiple NTMRs and adjacent fished areas in a complex reefscape in the Philippines, to understand whether NTMRs can remain effective as fisheries management tools (i.e. maintain higher fish biomass than fished areas) irrespective of typhoon damage and variability in island types. Coral reef benthic and fish assemblages were compared among 17 habitat matched NTMR-fished control sites across three mainland and four offshore islands that differed in geomorphology and exposure to recent typhoons. NTMRs did not influence the assemblage structure of the benthos, or total hard coral cover relative to fished areas. However, NTMRs had a lower cover of macroalgae than fished areas, particularly on mainland islands. Typhoons had severe negative effects on live hard coral cover, which resulted in a modified fish assemblage structure, and a lower total biomass of fish, irrespective of island type or NTMR protection. There were inherent differences in the assemblage structure of both benthos and fish between mainland and offshore islands, likely attributed to the prevailing environmental conditions of lower water quality on mainland island reefs, compared to the relatively clear water and high wave exposure environments on offshore island reefs. There was also a lower biomass of fish on mainland compared to offshore islands. However, NTMRs consistently had a higher total biomass of fish compared to fished areas, irrespective of island type or typhoon damage. This suggests that NTMRs can provide benefits to fish biomass and thus adjacent fisheries, even among different environmental conditions, and when reefs and habitat for fishes are affected by typhoons.

To understand whether bottom-up (trophic and habitat resources) or top-down (predation pressure) processes have a greater influence on determining fish assemblages, Chapter 4 assessed the relative influence of habitat, island spatial characteristics, and fishing on target and non-target fish groups in a multi-species fishery in the Philippines. Boosted regression trees were used to model the response of the abundance and species richness of four coral reef fish groups targeted by fishing, and the abundance of two groups not targeted by fishing, to 20 predictor variables that included no-take marine reserve (NTMR) presence-absence, and NTMR age and size. Fish groups most strongly responded to bottom-up variables of habitat, followed by spatial characteristics, with top-down control of NTMRs being influential only on fishes targeted by fishing. Encouragingly, NTMRs did not have to be large in size to increase fish density and species richness relative to fished areas. This is an important finding for small-scale, community managed initiatives like those in developing nations. This study has important implications for the development of integrated management strategies that prioritize maintaining habitat (defined here as a bottom-up process) for fishery species.

Given the clear importance of the environment in determining fish assemblages, as seen in Chapters 2-4, Chapter 5 employed stable isotope techniques and manipulative experiments to investigate whether environmental differences among and within islands resulted in dietary plasticity of important subsistence fishery species. Firstly, environmental indicators of sedimentation rates, turf algal productivity, and δ¹⁵N isotopic signatures in the seaweed Sargassum were compared among six sites across three islands in the Philippines to establish whether terrestrially derived sediment (and thus nutrient) inputs onto coral reefs varied among sites. Sediment loads, measured using sediment traps, did vary among sites and appeared to affect nutrient availability, with reefs furthest from terrestrial influence having low sediment loads and δ¹⁵N depleted Sargassum, while the reef closest to a large tourist resort had the highest sediment loads and most δ¹⁵N enriched Sargassum. Turf algal productivity was measured with manipulative cage experiments at each of the six sites. There was generally lower turf algal productivity with increased sedimentation rates, and generally higher productivity under low sedimentation rates, indicating excessive sediment may inhibit turf algal growth. Given environmental differences among the six sites, the diets of three valuable subsistence fishery species common to each site were then investigated using Bayesian stable isotope mixing models to link primary food sources to individual fish. The muscle tissue of three fish species known to have different diets (an algal grazer, an algal browser, and a planktivore) were sampled from each site for analysis of δ¹⁵N and δ¹³C isotope signatures. Potential dietary sources of these fish, of turf algae (from experimental tiles), turf algal detritus, the macroalgae Sargassum, and zooplankton, were sampled from each site for stable isotope (δ¹⁵N and δ¹³C) analysis. The proportion of each dietary source in the muscle tissue of each individual fish from each site was then estimated using the mixing model siar (stable isotope analysis in R). Despite the apparent differences in terrestrial inputs of sediment, and differences in turf algal productivity among sites, there was inconclusive evidence that fish species changed primary dietary sources among sites. However, fish species were feeding on some unexpected sources. The algal grazing fish species (Siganus virgatus) was consuming moderate proportions of plankton, while the planktivorous species (Naso minor) was consuming benthic fleshy macrophytes. With human influences modifying coral reef habitats and fish assemblages throughout the tropics, understanding how ecologically and economically important reef fish species persist remains a question of significance.

Overall, this research increases our understanding of the processes that maintain coral reef fish assemblages under varying environmental conditions and NTMR protection. While NTMRs remain an important and successful tool in subsidising fisheries and maintaining species assemblages for those fishes targeted by fishing, it is clear that environmental conditions are the stronger driver shaping fish assemblages and coral reef ecosystems. This thesis thus has important implications for informing and improving management strategies in a time of climatic uncertainty. While enhancing capabilities of coral reef management is important globally, it is particularly pertinent for developing island nations that rely heavily on marine resources for food security, and therefore rely on the success of small-scale management initiatives, such as NTMRs. Prioritising management goals to incorporate sound land-use practices that reduce sediment and nutrient inputs onto coastal reefs, and that place NTMRs in areas less prone to land-based impacts and typhoon paths, will help ensure habitat for fishes, and thus, fisheries for the future.

Item ID: 61202
Item Type: Thesis (PhD)
Keywords: coral reefs, fish habitat, marine reserves, coral reef fisheries, habitat depletion, conservation
Copyright Information: Copyright © 2019 Eva Claire McClure.
Date Deposited: 16 Dec 2019 02:00
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 35%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 30%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050205 Environmental Management @ 35%
SEO Codes: 96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 50%
96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 50%
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