Spatial variability of coral reef communities: implications for conservation of benthic and herbivorous fish communities across Hawaii
Helyer, Jason S.
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Over the last two decades there has been a proliferation of reports documenting the decline of coral reef systems across the globe (Gardner et al. 2003, Bellwood et al. 2004, Cote et al. 2005, Bruno and Selig 2007). In response, multiple studies have attempted to establish conservation baselines for benthic and fish communities using observations from remote, uninhabited coral reefs (Sandin et al. 2008, Edwards et al. 2011, Williams et al. 2011, Smith et al. 2016). These efforts have highlighted drastic differences in the composition and abundance of benthic and fish communities between remote and populated coral reefs. However, previous studies have focused on island- and archipelago-wide comparisons of reef communities that mask finer-scale spatial variability of coral reef communities within remote, unpopulated reef systems. The focus of my thesis was to describe the spatial variability of benthic and herbivorous fish communities across populated and unpopulated islands in the Hawaiian Archipelago and to examine how such variability affects views about coral reef conservation. I chose to focus on benthic and herbivorous fish communities because: (1) the decline of coral reefs is often associated with persistent shifts in benthic composition from abundant reef building organisms (coral and crustose coralline algae) to abundant fleshy algae (Done 1992, Hughes 1994) and (2) the prevailing view is that herbivorous fishes are the main driver of benthic community composition and thus overfishing of herbivorous fishes contributes to benthic community shifts (Hughes 1994, Bellwood et al. 2004, Mumby et al. 2006, Adam et al. 2015). Furthermore, this topic is highly relevant in Hawaii where herbivore fishing bans are being considered and implemented to conserve and promote healthy benthic habitats. The first chapter of my thesis addressed the view that herbivorous fish are main drivers of benthic community composition in the remote Northwestern Hawaiian Islands (NWHI). I described three benthic community types (reef builder, mixed-turf, and Microdictyon) whose occurrences varied across forereefs in the NWHI. Using these community groups, I found herbivorous fish biomass to be associated with benthic composition which is consistent with prevailing views about the importance of these consumers to benthic community structure. However, herbivorous fish-benthic community relationships varied across islands and patterns suggested that environmental factors may have stronger influences on benthic community composition across some parts of the NWHI. The second chapter of my thesis examined environmental influences on four measures of herbivorous fish community biomass (total, scraper, grazer, and browser) across the Hawaiian Archipelago and their effects on estimates of unfished biomass (Bunfished) and depletion in the Main Hawaiian Islands (MHI). I found environmental factors were strongly associated with herbivorous fish biomass in Hawaii and accounting for environmental differences across islands in the archipelago resulted in variable estimates of unfished biomass. Failure to account for environmental influences on unfished biomass resulted in different conclusions about herbivore depletion in the MHI. Overall, depletion estimates which controlled for environmental differences indicated that biomasses of the four herbivorous fish categories across much of the MHI are near or above 0.5 Bunfished. However, scraper and browser biomass on Oahu, the island with the highest human population density, were near or below 15% of Bunfished-local, highlighting the potential importance of spatial and functional group variability when assessing fishing effects on coral reefs. Together the results of this thesis advance knowledge of natural variability of benthic and herbivorous fish communities across the Hawaiian Archipelago. These results highlight the importance of controlling for environmental variability when assessing anthropogenic influences on coral reef communities. These findings will be useful for resource manages in Hawaii who are developing conservation plans to promote coral reef resilience. Additionally, the approaches outlined in this thesis can be applied to other reef systems to examine the broader relevance of spatial variability in benthic and fish communities to the identification of conservation targets in coral reefs.
- Fisheries