The Great Barrier Reef is in serious trouble. These fish can help.

The past few decades haven't been kind to Australia's majestic Great Barrier Reef.

Since 1985, half the reef's coral has been ravaged by tropical storms, invasive crown-of-thorns starfish, disease, and coral bleaching driven by rising ocean temperatures. This past March, due to El Niño and global warming, an unprecedented bleaching event in the northern third of the reef turned 95 percent of corals a ghastly bone white.

Usually when disaster befalls coral reefs, they take about 10 or 20 years to bounce back. The problem is that they don't always get that much time to recover, especially now that global warming is making severe bleaching events more common and human activities like fishing and pollution keep expanding.

So that's the grim news. The good news is that the Great Barrier Reef and other reefs worldwide have valuable allies that can help them hang on: rabbitfish, parrotfish, and other brightly colored fish species that can aid recovery. We just have to make sure those fish are free to do their job.

One way to speed up coral reef recovery — local fishing bans

(Shutterstock)

A new study in Ecology Letters, led by Camille Mellin of the University of Adelaide, found that when fishing bans are put in place around the Great Barrier Reef, the local coral recover much more quickly from disaster. The researchers looked at 46 different reefs across 20 years, and this pattern emerged quite clearly.

The key chart is below, showing coral recovery in places within marine protected areas that ban fishing (in green) versus those without protection (in red). The protected areas recover significantly faster after bleaching events, starfish invasions, disease, and major storms:

(Mellin et al., 2016)

Scientists have known for a while that certain fish help maintain the health of reefs by clearing away harmful algae that would otherwise smother the coral. In 2008, researchers recorded the rabbitfish playing this crucial role in the Great Barrier Reef. In the Caribbean, the stoplight parrotfish seems to be a key algae muncher. Other species of parrotfish and surgeonfish appear to be invaluable for protecting reefs around the world.

The problem is that many of these herbivore fish populations have been declining over time, due to heavy fishing. This new study suggests that protecting these fish would help aid reefs that provide billions in benefits for tourism and shoreline protection.

Currently, about half of the Great Barrier Reef Marine Park is off limits to fishing fleets. But that still leaves vast swaths of the park unprotected. Figuring out which species, exactly, are most important to reef health in different areas might make fishing restrictions easier to tailor — the paper mentions that that's the next big research step.

This study on fish and reef health doesn't come in isolation. In 2014, a report from the Global Coral Reef Monitoring Network warned that many Caribbean reefs could disappear "within a few decades." That paper emphasized the importance of protecting the stoplight parrotfish by limiting spearfishing and banning certain traps:

A parrotfish in Leonora's Reef off the Caribbean islet of Klein Bonaire. Parrotfish feed on algae that threaten to overrun the reef. (waywuwei/Flickr)

The paper noted that in places like Bermuda and Belize that have parrotfish protections, reefs have sprung back much more quickly after devastating hurricanes.

Protecting fish will help the reefs — though limiting climate change is still the key step

Last month, when I interviewed Mark Eakin, head of the National Oceanic and Atmospheric Administration's Coral Reef Watch program, he mentioned that ultimately stopping climate change was the key step for salvaging reefs worldwide. Otherwise, the increased frequency of bleaching events plus acidification could prove too much for most reefs to bear.

The world is currently on pace to blow past the 2 degrees Celsius threshold of global warming this century. Somewhere around that point, Eakin said, "we are likely to lose numerous species of coral and well over half of the world's coral reefs."

This year, because of record temperatures and El Niño, the planet is in the throes of the worst global bleaching event ever seen. Some 36 percent of the world's reefs are on "death watch," with those in Kiritimati in the South Pacific and American Samoa hit especially hard:

(NOAA)

That said, halting global warming is a decades-long effort. In the meantime, there are smaller steps we can take to improve reef resiliency. Protecting fish that in turn protect reefs. Restricting sewage and sediment runoff from shores that choke the coral. Limiting dredging in fragile ecosystems. Australia, for its part, just approved a massive coal mine in Queensland that could further cover parts of the Great Barrier Reef in sediment. That certainly won't help.

And why should we care about these reefs? They're essentially the rainforests of the ocean — they cover just 0.1 percent of the world's sea floor, but they're home to 25 percent of marine fish species. They're popular spots for divers and tourists, but they also sustain food for half a billion people and protect shorelines from hurricanes. And they're just plain lovely.

Go deeper:

• I found out about the Great Barrier Reef study from John Upton of Climate Central, whose coverage is worth checking out. (As is Climate Central more generally.)
• The unprecedented coral bleaching disaster at the Great Barrier Reef, explained
• Meet the people trying to rebuild damaged coral reefs.

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