Fishing has reduced numbers of reef 'lawnmowers' by more than half


Researchers have produced a landmark report on the state of plant-eating fish on coral reefs around the world.

In the first global assessment of its kind, a science team led by researchers at Scripps Institution of Oceanography at UC San Diego, has produced a report on the impact of fishing on herbivorous fish populations. These fish are vital to coral reef health due to their role in consuming seaweed, making them known informally as the "lawnmowers" of the reef. Without the lawnmowers, seaweeds can overgrow and out-compete corals, drastically affecting the reef ecosystem.

Among their findings, the researchers found that populations of plant-eating fish declined by more than half in areas that were fished compared with unfished sites.

"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, one of the co-authors of the study.

"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. 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."

While these reef fish are not generally commercial fisheries targets, there was clear evidence that fishing was impacting their populations globally.  

The researchers also found that fishing alters the entire structure of the herbivore fish community, reducing the numbers of large-bodied feeding groups such as "grazers" and "excavators" while boosting numbers of smaller species such as algae-farming territorial damselfishes that enhance damaging algae growth.

"We are shifting the herbivore community from one that’s dominated by large-bodied individuals to one that’s dominated by many small fish," said Smith. "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."

The report is published in the journal Proceedings of the Royal Society B (Biological Sciences) and offers key data for setting management and conservation targets to protect and preserve fragile coral reefs.

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