Seaweed May Overtake Coral Reeds in the Absence of 'Lawnmower' Fish

The first global synthesis on the state of the plant-eating fish that live among coral reefs has found that where fishing is prevalent, populations of these fish have declined by more than 50 percent compared to non-fished reef areas.

The impact of these small fish should not be ignored or underestimated, according to the researchers behind the study, who note that the overall health of coral reefs is dependent on its resident plant-eating fish. Aside from being beautiful natural spectacles, coral-reefs are a huge tourist attraction, which generate billions of dollars in revenue worldwide for coastal communities near them. A decline in the health of the reefs, the researchers contend, will consequently lead to an economic decline as well.

These fish, collectively referred to as "lawnmowers" of the reef because they eat the seaweed growing on the reefs, play a vital role in keeping the reefs healthy overall. A variety of commonly-fished species can be considered lawnmower fish. Without these lawnmowers, the health of the reef would be put in jeopardy by being overtaken by seaweeds that can out-compete the corals and drastically alter the reef ecosystem.

"One of the most significant findings from this study is that we show compelling evidence that fishing is impacting some of the most important species on coral reefs," said Jennifer Smith of the Center for Marine Biodiversity and Conservation at Scripps Institution of Oceanography at the University of California, San Diego.

"We generally tend to think of fishing impacting larger pelagic fishes such as tuna but here we see big impacts on smaller reef fish as well and particularly the herbivores," Smith said. "This is particularly important because corals and algae are always actively competing against one another for space and the herbivores actively remove algae and allow the corals to be competitively dominant. Without herbivores, weedy algae can take over the reef landscape. We need to focus more on protecting this key group of fishes around the globe if we hope to have healthy and productive reefs in the future."

Unlike tuna, the fish the live among the reefs are generally not the targets of commercial fishing operations. But the researchers report that rampant small-scale fishing is creating an environmental impact on its own.

The researchers report that fishing the reefs "alters the entire structure of the herbivore fish community" by reducing the number of larger reef fish that play a big role in regulating seaweed growth around reefs, while increasing the presence of smaller fish, some of which can be damaging to reef growth. The algae-farming damselfish is one species that can harm the reefs if their numbers get too great, as their algae is damaging to coral, the researchers said.

"We are shifting the herbivore [fish] community from one that's dominated by large-bodied individuals to one that's dominated by many small fish," Smith said. "The biomass is dramatically altered. If you dive in Jamaica you are going to see lots of tiny herbivores because fishers remove them before they reach adulthood. In contrast, if you go to an unfished location in the central Pacific the herbivore community is dominated by large roving parrotfishes and macroalgal grazers that perform many important ecosystem services for reefs."

Smith and her colleagues said that their study is the first of its kind and that evidence from their assessment should be used in coral reef management and conservation.

"This assessment allows us to set management goals in different regions across the globe," Smith said. "Regional managers can use these data as a baseline to set targets to develop herbivore-specific fisheries management areas. We should be using these important fish as a tool for reef restoration. On reefs where seaweed is actively growing over reefs, what better way to remove that seaweed than to bring back those consumers, those lawnmowers?"

The study is published in the Proceedings of the Royal Society B.