Nearly everything in a coral reef ecosystem depends on corals or their accumulated structure in some way. Coral colonies are a source of food and shelter for countless reef-associated organisms and their health is of critical importance to the ecology of the reef community. Besides some of the more obvious impacts of coral bleaching and related mortality (i.e., loss of corals and reef diversity), scientists are discovering that the ecological impacts of coral decline can have detrimental effects on fish communities. Some examples of these impacts are summarized below.
Recent studies have shown that juvenile fish are more likely to recruit to areas that have high coral cover and overall healthy coral communities 1. A team of scientists lead by Geoffrey Jones, conducted an 8-year study in Papua New Guinea looking at the effects of coral degradation on reef fish assemblages. The study showed a parallel decline in fish communities with declines in associated coral communities. Additionally, there were some coral specialists that went locally extinct. One of the key points of the study was fish that were dependent on corals as juvenile recruitment sites, were more likely to be affected by coral degradation over the long term. The study also examined reefs and fish communities both inside and outside of marine reserves and found the trends to be the same for both. Regardless of protected status within a reserve, fish communities cannot thrive without having access to healthy coral communities.
A 12-year study period in the central Great Barrier Reef revealed a marked phase shift from a pre-bleach, or coral associated species such as Pomacentrus moluccensis) to post-bleach, or less habitat specific forms such as Eviota queenslandica) fish assemblage 2. Major, cryptic changes in community composition, which marked a distinct and relatively stable shift from a pre-bleaching to a post-bleaching reef fish assemblage. After 5 years and up to 35 generations after the bleaching event, the assemblage of fish showed little signs of returning to pre-bleached conditions.
At One Tree Lagoon in the southern Great Barrier Reef, extensive coral bleaching and mortality occurred in 1997-1998. Scientists3 monitored the distribution of adults and recruitment of damselfishes before (1993-1995) and after (1999) the bleaching event. Overall decreases in adult damselfish density and recruit diversity occurred at the affected sites. Compared to 1993/1995 densities, adult Pomacenrus wardi, P. moluccensis and Chrysiptera rollandi densities fell at bleached sites. The study demonstrated the indirect community effects of bleaching on reef fish assemblages.
A study at Tutia Reef in Mafia Island Marine Park, Tanzania presents evidence of severe and long-lasting ecological impacts of bleaching events 4. Examining the response of fish assemblages in transplanted coral reef plots before and after the 1998 bleaching event, the study demonstrated significant changes in assemblages in reef fish composition. In particular, the study examined the response of functional groups (i.e., obligate corallivores, facultative corallivores, coral dwellers, benthic invertebrate feeders, etc.) with documented affiliations with coral. The groups were significantly influenced by the habitat alteration. Furthermore, an overall decrease in fish numbers 6 years after the coral mortality event was recorded. Six years after the mass bleaching, herbivores were nearly absent from the plots of damaged corals, and the fish assemblages were significantly altered, with low abundance and diversity. The results of the study also demonstrate how the loss of structure was a more fundamental habitat alteration than loss of live coral tissue.
A study in Seychelles demonstrates that live coral loss from mass bleaching results in local extinctions, substantial reductions in species richness, reduced taxonomic distinctness, and a loss of species within key functional groups of reef fish 5. Surveys of coral and fish communities at 21 sites in Seychelles in 1994 (pre-bleaching), and 2005 (post-bleaching), reveal widespread phase shifts from coral-dominated state to rubble and algal-dominated state. The change in habitat and structural complexity demonstrate dramatic impacts on reef associated fish assemblages, particularly species richness. Corallivorous and planktivorous species were generally lost at the study sites, and species from other trophic groups declined with the decline in habitat complexity. The three reef fish families most affected by bleaching include: monacanthids, chaetodontids, and pomacentrids. Additionally, the study identified possible local extinction of four fish species.
These studies highlight several points, including not only how important health of coral is to fish communities, but also that marine reserves are only one strategy and must be used in concert with a variety of other conservation strategies. Taking proactive steps to reduce threats, other than extraction, both inside and outside reserves, is critical to maintaining healthy coral communities and other associated species assemblages.
1 Jones et al. 2004
2 Bellwood et al. 2006
3 Booth and Beretta 2002
4 Garpe et al. 2006
5 Graham et al. 2006
6 Mumby et al. 2007