The benefits from the 24 no-take reserves in the Florida Keys National Marine Sanctuary haven't trickled down from the fish to the coral reefs.
That's the finding of a study published online in April by the peer-reviewed journal Coral Reefs.
"We can't assume that addressing one problem, in this case fishing, will take care of other problems the ecosystem faces," Lauren Toth, the Florida Institute of Technology biologist who was the study's primary author, said in an interview last week. "For the corals themselves, the sort of action that needs to be taken is a combination of local and global changes."
In the study, "Do no-take reserves benefit Florida's corals? Fourteen years of change and stasis in the Florida Keys National Marine Sanctuary," the authors took on a hypothesis in marine science circles that no-take fishing areas, because they provide protections to reef-grazing herbivores, such as parrotfish and surgeonfish, will benefit coral reefs.
According to that hypothesis, as herbivore populations grow, there are more fish to graze on coral algae. Too much algae is known to suffocate corals, reducing their ability to reproduce. Abundant populations of reef grazers can reduce that problem.
The authors note that previous studies having shown mixed results on whether that hypothesis holds up. While corals have flourished in a few areas after no-fishing reserves were established, reserves haven't led to healthier reefs in several other locations.
The Florida Keys sanctuary established its 24 no-take reserves in 1997. Between 1998 and 2011, the Melbourne-based Florida Institute of Technology analyzed how the corals fared within four of those reserves in comparison to how they fared in four areas outside the reserves. For the reserve sites, the scientists selected Western and Eastern Sambo reefs off Key West and Carysfort and Molasses reefs off the Upper Keys. They mainly selected nearby reefs for their analyses outside the reserves.
They found that coral cover actually declined slightly more during the 15-year study period within the reserves than outside of them, though they mainly attributed that difference to the fact that corals in the reserves were healthier at the start of the study and therefore had more room to decline. By the end of the study, coral cover averaged a mere 2.4 percent at all the sites, down more than half from the start of the study.
"The dynamics of corals and macroalgae in the 15 years since the reserves were established in 1997 suggest that although the reserves protected fish, they were of no perceptible benefit to Florida's corals," the study says.
The authors weren't searching for the reasons for the decline. But coral die-offs in Keys waters have long been attributed to a variety of factors, including stress caused by global climate change, the mass die-off in the early 1980s of the keystone reef grazer the Diadema sea urchin, local pollutants, and direct impacts from snorkelers and divers.
The study isn't without critics, however. In an interview last week, Billy Causey, who is now the southeast regional manager for the National Marine Sanctuary Program, and who was the Florida Keys sanctuary manager from 1990 to 2006, questioned the methodology employed by the Florida Institute of Technology team.
Herbivores receive substantial protection throughout the sanctuary due to prohibitions on wire fish traps and spearfishing, he said. In addition, herbivores are not hunted commercially in Florida and are only taken within the sanctuary via permit by tropical fish collectors.
In contrast, herbivores enjoy no such protection in many parts of the Caribbean, where they are also hunted for food.
"They were comparing types of apples," said Causey of the study entirely within the sanctuary. "They weren't comparing apples and watermelons. ... Their results were not surprising to me, but what I would love them to do now is take what they did here and compare it to another Caribbean area."
Toth acknowledged that herbivores receive significant protection throughout the sanctuary, but she said they still enjoy more protection in the reserves.
A comparison between the sanctuary and other parts of the Caribbean, she said, would also be interesting, but difficult to pull off.
"There are so many hydrological and habitat differences between the reefs [of the sanctuary] and the reefs of the wider Caribbean," Toth said. "I think it would be really difficult to make that comparison."