Feature Paper: Raymundo LJ, Halford AR, Maypa AP, Kerr AM (2009) Functionally diverse reef-fish communities ameliorate coral disease. Proceedings of the National Academy of Sciences (PNAS), 106(40):17067-17070.
Author Abstract: Coral reefs, the most diverse of marine ecosystems, currently experience unprecedented levels of degradation. Diseases are now recognized as a major cause of mortality in reef-forming corals and are complicit in phase shifts of reef ecosystems to algal-dominated states worldwide. Even so, factors contributing to disease occurrence, spread, and impact remain poorly understood. Ecosystem resilience has been linked to the conservation of functional diversity, whereas overfishing reduces functional diversity through cascading, top-down effects. Hence, we tested the hypothesis that reefs with trophically diverse reef fish communities have less coral disease than overfished reefs. We surveyed reefs across the central Philippines, including well-managed marine protected areas (MPAs), and found that disease prevalence was significantly negatively correlated with fish taxonomic diversity. Further, MPAs had significantly higher fish diversity and less disease than unprotected areas. We subsequently investigated potential links between coral disease and the trophic components of fish diversity, finding that only the density of coral-feeding chaetodontid butterflyfishes, seldom targeted by fishers, was positively associated with disease prevalence. These previously uncharacterized results are supported by a second large-scale dataset from the Great Barrier Reef. We hypothesize that members of the charismatic reef-fish family Chaetodontidae are major vectors of coral disease by virtue of their trophic specialization on hard corals and their ecological release in overfished areas, particularly outside MPAs.
Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.
Review: This paper reports that healthy coral reefs have functionally diverse reef fish communities and low occurrence of coral diseases. This is important because it gives added value to marine protected areas (MPAs) in that they will not only increase fish numbers (and replenish fish stocks on overfished neighboring reefs) but they will stave off coral disease. Healthy corals are better able to combat environmental stressors (e.g., El Niño-induced coral bleaching).
The lead author is a world-renowned coral disease specialist and this study examined the prevalence of six coral diseases: white syndrome, ulcerative white spots, growth anomalies, black band, skeletal eroding band, and brown band. The authors established that differences in disease prevalence between marine protected areas (MPAs) and non-protected areas were "not due to diferences in percent total cover between MPAs and fished sites, percent cover of Porites, the dominant hard-coral genus and disease host, mean total number of coral colonies per transect, or physical damage to colonies." However, there was a strongly significant correlation between high fish diversity and low coral disease prevalence.
The important point that this paper brings up is that you need a diverse reef fish community to stave off disease. While this might be intuitive (healthy reef = many kinds of fish = less disease) the scientists approach their subject in a very interesting way. Many scientists have used butterflyfish abundance and diversity as measures of coral reef health because most butterflyfishes feed on corals. Therefore, only the healthiest reefs with highest coral cover should have the highest numbers of butterflyfishes. What is really interesting about this paper is that they make the observation that butterflyfishes are generally not targeted by fishermen because there isn't much to eat on them. However, as overall fish diversity goes down through overfishing, so too do the numbers of predators to butterflyfishes decrease. And since butterflyfishes move from one coral to another over a large range during their feeding, if there are more butterflyfishes on a reef unhampered in their movements (because of a lack or lower density of predators) then butterflyfishes may transmit coral diseases through their mouth from one coral bite to another.
As a result, you can't just look at butterflyfish numbers and diversity without accounting for overall fish diversity because overfished reefs (without destruction of coral through blast fishing or cyanide fishing) may in fact have relatively high coral cover and therefore, high butterflyfish diversity. But if a researcher only looked at that, they'd miss the big picture and not realize that the reef was actually unhealthy or out of balance.
While not noted in this paper, other important aspects of high reef fish diversity are that:
1) herbivorous fish will keep algal abundance lower, helping to prevent algal overgrowth of reefs and keeping ecological phase-shifts at bay;
2) high reef fish diversity can also increase growth of corals as waste from fish provides a source of nutrition to corals in the low-nutrient waters of coral reefs (and thus is a form of energy recycling or coupling).
Reefs remain under constant threat for their survival and while I, as a coral reef biologist, do not think that coral reefs will be gone entirely in the next 100 years, it is clear from past events that reefs won't be pristine or nearly as extensive. This paper gives added benefit to establishing no-fishing zones and MPAss, but also makes it clear to conservation biologists that they have to be careful about what metrics they study when determining coral reef diversity and making claims about overall reef health.
Because coastal peoples rely on fishing for their livelihood, it is important to present alternatives to fishing all reef areas that make economic sense to local communities. When the value of MPAs is explained, it can only benefit local communities. For those reefs that continue to be fished, proper fishing techniques should still be employed (e.g., size limits, non-destructive fishing techniques or poisons, not fishing at spawning aggregations, protecting certain large predator fishes, rotating fishing times and zones or partial closures during certain times of the year, etc.) to ensure that fished reefs contain higher fish diversity. This paper shows that any fishing on a reef that significantly impacts reef fish diversity will also increase disease prevalence, so it is important as stewards of our local environment to ensure that they continue to provide resources for future generations to enjoy.
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