Effects of Coral Bleaching on Coral Reef Fish Assemblages
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Effects of Coral Bleaching on Coral Reef Fish Assemblages Nicholas A J Graham A Thesis submitted to Newcastle University for the Degree of Doctor of Philosophy School of Marine Science and Technology Supervisors: Professor Nicholas V C Polunin Professor John C Bythell Examiners: Professor Matthew G Bentley Dr Magnus Nyström First submitted: 1st July 2008 Viva-Voce: 1st September 2008 Abstract Coral reefs have emerged as one of the ecosystems most vulnerable to climate variation and change. While the contribution of climate warming to the loss of live coral cover has been well documented, the associated effects on fish have not. Such information is important as coral reef fish assemblages provide critical contributions to ecosystem function and services. This thesis assesses the medium to long term impacts of coral loss on fish assemblages in the western Indian Ocean. Feeding observations of corallivorous butterflyfish demonstrates that considerable feeding plasticity occurs among habitat types, but strong relationships exist between degree of specialisation and declines in abundance following coral loss. Furthermore, obligate corallivores are lost fairly rapidly following decline in coral cover, whereas facultative corallivores are sustained until the structure of the dead coral begins to erode. Surveys of benthic and fish assemblages in Mauritius spanning 11 years highlight small changes in both benthos and fish through time, but strong spatial trends associated with dredging and inter-specific competition. In Seychelles, although there was little change in biomass of fishery target species above size of first capture, size spectra analysis of the entire assemblage revealed a loss of smaller individuals (<30cm) and an increase in the larger individuals (>45cm). This represents a lag effect where fishery production cannot be assured for the long term. A targeted before (mid-1990s) – after (2005) sampling program of coral reef benthos and fish assemblages in 7 countries across the Indian Ocean demonstrated changes in size structure, diversity and trophic composition of the reef fish community have followed coral declines in both fished and protected areas. The thesis highlights the pivotal role that loss of reef structural complexity plays in the effects of bleaching on fish assemblages and that coral reef management needs to radically adapt to address climate change issues. Table of Contents 1 GENERAL INTRODUCTION..………………………………………..1 1.1 CLIMATE WARMING AND IMPACTS ON CORAL REEF BENTHOS……………2 1.2 SHORT-TERM IMPACTS OF BLEACHING ON FISH…………………………..8 2 ECOLOGICAL VERSATILITY AND THE DECLINE OF CORAL FEEDING FISHES FOLLOWING CLIMATE DRIVEN CORAL MORTALITY……………………………………………………………..21 2.1 ABSTRACT………………………………………………………………21 2.2 INTRODUCTION………………………………………………………….22 2.3 METHODS……………………………………………………………….24 2.3.1 Study sites…………………………………………………………….24 2.3.2 Substratum availability……………………………………………….25 2.3.3 Feeding observations………………………….…………...…………26 2.3.4 Chaetodon density surveys……………………....……………………28 2.3.5 Coral food quality……………………………………………………..29 2.3.6 Temporal change in coral feeding fish densities……………………...30 2.4 RESULTS……………………………………………………………...….31 2.4.1 Substratum availability………………………………………………..31 2.4.2 Feeding observations………………………………………………….33 2.4.3 Corallivore density surveys…………………………………………....35 2.4.4 Coral food quality……………………………………………………..35 2.4.5 Temporal change in coral feeding fish densities………………………38 2.5 DISCUSSION………………………………………………………………40 2.6 ACKNOWLEDGEMENTS……………………………………………………46 3 CORAL MORTALITY VERSUS STRUCTURAL COLLAPSE AS DRIVERS OF CORALLIVOROUS BUTTERFLYFISH DECLINE…...47 3.1 ABSTRACT………………………………………………………………...47 3.2 INTRODUCTION……………………………………………………………48 3.3 METHODS…………………………………………………………………50 3.4 RESULTS…………………………………………………………………..53 3.5 DISCUSSION……………………………………………………………… 57 3.6 ACKNOWLEDGEMENTS……………………………………………………60 4 ANTHROPOGENIC STRESSORS, INTER-SPECIFIC COMPETITION AND ENSO EFFECTS ON A MAURITIAN CORAL REEF………………………………………………………………………...61 4.1 ABSTRACT………………………………………………………………...61 4.2 INTRODUCTION……………………………………………………………62 4.3 METHODS…………………………………………………………………64 4.3.1 Study site and sampling techniques…………………………………….64 4.3.2 Data analysis…………………………………………………………...67 4.4 RESULTS…………………………………………………………………..70 4.5 DISCUSSION……………………………………………………………….81 4.6 ACKNOWLEDGEMENTS……………………………………………………86 5 LAG EFFECTS IN THE IMPACTS OF MASS CORAL BLEACHING ON CORAL REEF FISH, FISHERIES, AND ECOSYSTEMS………….87 5.1 ABSTRACT………………………………………………………………...87 5.2 INTRODUCTION……………………………………………………………88 5.3 METHODS…………………………………………………………………90 5.3.1 Study sites………………………………………………………………90 5.3.2 Assessment of fish assemblage and benthic community structure……..91 5.3.3 Establishment of fishery target species and size of first capture………93 5.3.4 Data analysis…………………………………………………………...93 5.4 RESULTS…………………………………………………………………..95 5.5 DISCUSSION……………………………………………………………...104 5.6 ACKNOWLEDGEMENTS…………………………………………………..110 6 CLIMATE WARMING, MARINE PROTECTED AREAS AND THE OCEAN-SCALE INTEGRITY OF CORAL REEF ECOSYSTEMS…..112 6.1 ABSTRACT……………………………………………………………….112 6.2 INTRODUCTION…………………………………………………………..113 6.3 METHODS………………………………………………………………..116 6.3.1 Effect size……………………………………………………………...117 6.3.2 Bayesian meta-analysis……………………………………………….119 6.4 RESULTS…………………………………………………………………122 6.5 DISCUSSION……………………………………………………………...133 6.6 ACKNOWLEDGEMENTS…………………………………………………..139 7 GENERAL DISCUSSION………………………………………………140 7.1 CURRENT STATE OF KNOWLEDGE………………………………………..140 7.2 CONCEPTUAL MODELS…………………………………………………...148 7.3 MANAGEMENT AND RESEARCH DIRECTIONS……………………………..153 REFERENCES………………………………………………………….....157 APPENDIX A: CONTRIBUTIONS DURING PHD……………………..187 APPENDIX B: REPRINTS OF PUBLISHED CHAPTERS…………….193 List of Figures Figure 1.1. Meta-analysis of short-term (<3 years) response of 55 fish species to declines in coral cover. Y-axis metric is change in species abundance divided by change in coral cover. Data from 17 studies are included in the analysis. Species level responses represent mean values, calculated from a minimum of four studies. Only studies reporting a 10% or greater decline in coral were included in the analysis. Data on a species was only included if 5 or more individuals were surveyed in before disturbance counts. Species names are highlighted if mean value with 95% confidence interval fails to intersect zero, indicating consistency of responses among locations. Figure adapted from Wilson et al. (2006)……………………………………………………………………………9 Figure 1.2. Meta-analysis of responses of fish functional groups to two types of disturbance. Y-axis metric is change in fish group abundance divided by change in coral cover. Biological disturbances represent those disturbances that reduce live coral cover, but do not impact the structural complexity of the benthos. Structural disturbances cause a reduction in both live coral cover and structural complexity. Error bars are 95% confidence intervals calculated from the data of ten or more studies. Adapted from Wilson et al. (2006)……………………….17 Figure 2.1. Benthic resource availability at Diego East and Diego Central, Chagos Archipelago. *** P<0.001, ** P<0.01…………………………………………32 Figure 2.2. Feeding selectivity of C. trifascialis, C. trifasciatus and C. auriga at Diego Central and Diego East, Chagos Archipelago. Data only presented for the four coral genera most preferentially consumed. Black bars indicate percent resource availability, open bars indicate percent feeding bites on that resource. + symbol indicates positive selectivity…………………………………………...34 Figure 2.3. Mean density of corallivores present in Diego Central and Diego East. *** P<0.001, ** P<0.01, * P<0.05…………………………………………….36 Figure 2.4. Box and whisker plots of C:N ratios for four coral genera. Box indicates median value, lower and upper quartiles. Whiskers indicate range up to 1.5 times the box. Outliers indicated with an asterix……………………………....37 Figure 2.5. Change in density of C. trifascialis, C. trifasciatus and C. auriga and obligate specialists, obligate generalists and facultative feeders in Seychelles before and after the 1998 bleaching event…………………………………..….39 Figure 3.1. Change in abundance of (a) obligate corallivores and (b) facultative corallivores in response to loss of live coral loss. Solid line represents significant trend…………………………………………………………………………….55 Figure 3.2. Change in abundance of (a) obligate corallivores and (b) facultative corallivores in response to reduced reef structural complexity. Solid line represents significant trend………………………………………….………….56 Figure 4.1. Map of the study site, indicating location in Mauritius, proximity of the ten sampling stations, direction of transects and location of dredged water ski lane. Adapted from Aderoud et al. 1998…………………………………...…..66 Figure 4.2. Correlation-based Principal Components Analysis of log(χ + 1) transformed and normalised environmental data…………………………...….71 Figure 4.3. Percent cover of total live coral in (a) each year by transect and (b) each year by position from the shore.……………………………………………..…72 Figure 4.4. Funnel plots of (a) average taxonomic distinctness and (b) variation in taxonomic distinctness of each sampling station (belt transects) within each year with mean and 95% confidence limits from expected values using master taxonomy aggregation file……………………………………………………..75 Figure 4.5. (a) Non-metric Multi-Dimensional Scaling (MDS) plot of fish assemblages at each of the ten stations (belt transects) within each of the five sample years based on Bray-Curtis similarity