The influence of coral degradation on tropical fish community structure

Coral reefs harbour a spectacular diversity of fishes that derive food and shelter from the habitat provided by living corals. Unfortunately, both natural and anthropogenic disturbances are intensifying the global alteration and loss of live coral cover. The likely impacts of this loss on reef associated fish communities have yet to be fully assessed. The broad aim of this thesis was to examine the processes and mechanisms underlying the effects of live coral reductions on coral-associated reef fish communities. A combination of surveys and experimental manipulations were used to explore this relationship for both the fish community as a whole, and key components that represented species with differing associations with live coral. To examine the role of coral disturbance in structuring diverse coral reef fish communities, I surveyed and compared assemblages associated with 2 coral species (Seriatopora hystrix and Pocillopora damicornis) in 3 health states (live, degraded and dead colonies with algal cover) (Chapter 2). This study showed that despite differences in the structure of assemblages associated with live and degraded colonies between coral species, there were similar patterns in reef fish community change with coral death. Assemblages associated with dead, algal-covered colonies of both coral species held virtually no coral obligates, while coral-specific differences in obligate abundance were apparent between live and degraded colonies. There was a shift in the diversity of reef fish communities between health states, within both coral species. Live and degraded colonies were numerically dominated by coral-associated fishes while dead, algal-covered colonies were numerically dominated by fishes closely associated with non live-coral habitats. The total abundance of species was lowest in algal-covered colonies of both coral species, largely due to the low number of small size classes (new recruits and juveniles) associated with the habitat. Overall, this study has shown that coral health will play a substantial role in structuring reef associated fish assemblages, though the effects of coral loss on fish communities will be dependent on coral species and the taxonomic and functional composition of its associated fish assemblage Within Chapter 2 I found that dead, algal-covered colonies held substantially lower abundances of fishes in small size classes (i.e. new settlers and recruits) than live or degraded colonies. Historically, disparity in coral reef fish community composition has been attributed to ecological factors affecting patterns of reef fish larval settlement. However, the mechanisms that determine how new settlers will respond to coral stress and the repercussions of coral loss in structuring reef fish communities are poorly understood. In Chapter 3, therefore, I examined the role of coral degradation in structuring patterns of coral reef fish settlement. Within aquaria choice experiments, reef associated damselfishes (both coral and non-coral associated) avoided dead, algal-covered colonies, preferentially settling into either live or partially degraded colonies. Using a habitat-limited recruitment model, such avoidance of algal-covered habitats at settlement was predicted to substantially alter patterns of reef fish settlement. I hypothesised that settlement of coral-associated fishes would become habitat-limited within degraded reef landscapes, with new settlers unable to find suitable settlement habitat. Consequently, an extensive field experiment showed that live coral colonies formed important settlement substratum for a range of reef associated fishes. In situ experimental live coral degradation on small coral colonies led to rapid reductions in the abundance of coral-associated fishes settling into the colonies (2 - 4 weeks after disturbance), replaced by extremely low abundances of species closely associated with the algal biomass. From these experiments, I argued that the abundance and diversity of new settlers apparent within coral habitats would reflect the health of the ecosystem; live coral alteration and loss leading to reduced new settler abundance and a phase shift in new settler diversity within the degraded habitat. The response of reef fish communities to coral degradation and algal overgrowth was species and functional group specific (Chapter 2). However, there were higher than expected levels of resistance to coral degradation within a range of coral-associated fishes (Chapter 2). Applying terrestrial-based theory, I hypothesised that such differences in resistance were potentially influenced by species habitat specialisation (Chapter 4). Habitat specialists were more likely to remain within degraded coral colonies than generalists, due to their lowered ability to successfully migrate between habitats. Within Chapter 4, I compared the resistance and migration ability of both habitat generalist and specialist coral-dwelling goby species (Gobiidae). At low levels of coral loss specialists exhibited a higher propensity to remain in colonies than generalists, though there was no difference in resistance at both medium and high levels of coral loss. Over the majority of distances examined, specialists also showed substantially lower levels of successful migration. These results suggest that specialists are more likely to become isolated in degraded habitats than generalists, increasing the probability of their decline with habitat degradation. I argue that if this pattern holds true for the array of coral-associated reef fish species, such disparity in response to live coral disturbance between specialists and generalists may result in disproportionate reductions in the population abundance of habitat specialists in degraded habitats. Although species-specific differences in live coral dependence were likely to influence how fish assemblages responded to live coral loss, within Chapter 2 I showed that the abundance of a diverse range of coral-associated fishes were not substantially lower in degraded than live coral colonies. Recent evidence suggested that even with no observable change in abundance, however, reduced resource availability in degraded habitats may have negative effects on resident fishes physiological condition. Therefore, within Chapter 5, I compared the growth, condition and persistence of 2 planktivorous damselfish species (Pomacentridae) over 29 days, translocated onto coral colonies in 3 experimental treatments: live, partially and fully degraded colonies. This research showed that both species growth rates were directly related to live coral cover; populations associated with fully degraded colonies showed the lowest growth while those associated with live colonies the highest. There was no significant change in the condition or persistence of populations, between treatments. These results suggested that the short-term response of coral-associated reef fishes to low live coral were reductions in growth rather than condition or absolute abundance. I argued that such bottlenecks in the growth rate of fish species associated with degraded habitats may negatively influence their time to maturity, fitness and lifetime fecundity. The results of this thesis have fundamental implications in understanding the response of coral reef fish communities to increasing levels of live coral degradation. Fluctuations in species replenishment, differences in the resistance of individuals dependent on their habitat specialisation, and reductions in the growth of individuals remaining in degraded habitats will significantly affect the structure of the reef associated fish community within degraded reef systems. Ultimately, the alteration and loss of the living coral tissue will play a substantial role in structuring tropical reef fish community structure.