This week an intriguing study came out in Science demonstrating with very conclusive findings that both coral and fish juveniles prefer to be in water that is more typical of a healthy as opposed to a degraded reef (pdf here).
Coral reefs that are degraded have a substantially greater proportion of weedy algae on them than coral themselves. These algae are strong competitors with coral for open space on a reef and herbivorous fishes that eat these algae are critical for helping coral maintain dominance on a healthy reef.
Yet this study shows that there may be a tipping point whereby once a reef becomes dominated by these weedy algae species, the chemical cues that are put off into the water column by the algae may be so intense that they actually repel juvenile fishes and even coral. This study put juvenile fishes and coral in tanks and found that they showed overwhelming preference to water from a healthy (from a marine protected area) vs. a degraded coral reef.
While the study has remarkably strong findings, some caveats should be mentioned. First, they chose extremely healthy and extremely unhealthy reefs to test this question – so that the strength of their findings likely is overestimated when considering more ‘typical’ conditions that would be degraded but likely not nearly as bad. Second, their experimental design was extremely simplistic (while cool) and assesses only “preference”, whereas in reality, the fish or coral may prefer the ideal conditions, but may still settle in less than ideal conditions.