Originally published 2019-09-04.
Deep-sea corals on parts of the Hawaiian-Emperor seamount chain are starting to grow back after decades of bottom trawling, according to a study published in Science Advances in August 2019. On seamounts that had been closed to fishing for roughly 30 to 40 years, researchers found young corals settling in old trawl scars and coral fragments regrowing, along with more animals overall than on seamounts that are still fished.
It is good news with an important caveat. These are cold, dark, deep-water coral communities hundreds of metres down, not the sunlit tropical reefs most people picture when they hear "Hawaiian coral". And the recovery is early. The scientists describe signs of recovery, not a return to how the seafloor looked before the nets arrived.
What the seamount chain is and what happened to it
The Hawaiian-Emperor chain is a line of mostly underwater volcanic mountains stretching thousands of kilometres across the North Pacific, from the main Hawaiian Islands north-west toward the Aleutians. Formed as the Pacific Plate moved over the Hawaiian hotspot, the seamounts rise from the deep ocean floor and their slopes and summits support rich communities of corals, sponges, fish and crustaceans.
From the 1960s to the 1980s, these seamounts were among the most heavily fished in the world. Foreign fleets dragged heavy trawl nets across the slopes for fish such as pelagic armorhead, and tangle nets were used to harvest precious corals for jewellery. The study cites reported catches of up to 210,000 tonnes of fish and 200,000 tonnes of precious corals a year at the peak. Bottom trawling flattens everything in its path, leaving rubble and bare rock where coral forests stood.
Lost trawl net from on the NW Hancock Seamount at a depth of 400 meters. (Photo by A. Baco-Taylor FSU, E.B. Roark TAMU, NSF, with HURL Pilots T. Kerby and M. Cremer)
A lost trawl net lies draped across the seafloor on the north-west Hancock Seamount at about 400 metres. Abandoned "ghost" gear like this was common across the survey sites, and some of it had become a surface for new coral growth.
How the protection came about
The turning point was 1977, when the United States established its 200-nautical-mile Exclusive Economic Zone. Seamounts inside the zone around the north-western Hawaiian Islands were closed to foreign trawlers, while those further north along the Emperor chain, in international waters, could still be fished.
Protection later expanded. In 2006 President George W. Bush created what is now Papahānaumokuākea Marine National Monument around the north-western Hawaiian Islands, and in 2016 President Barack Obama extended it to the edge of the EEZ, making it for a time the largest marine protected area in the world. It has been a UNESCO World Heritage Site since 2010.
That history set up a rare natural experiment: neighbouring seamounts with similar geology and the same fishing past, but very different decades since the nets stopped.
What the scientists did
The work was led by Amy Baco-Taylor of Florida State University with colleagues including E.B. Roark of Texas A&M University, and funded by the US National Science Foundation. Over several research cruises the team surveyed seven seamounts. Four had been protected for decades, including the north-west and south-east Hancock seamounts, and three in international waters were still being trawled.
In 2014 and 2015 the autonomous underwater vehicle Sentry photographed the seafloor at depths between about 200 and 700 metres, capturing more than 536,000 images. Later dives with the Hawaii Undersea Research Laboratory's crewed submersibles in 2016 and 2017 let scientists look closely and collect samples. Tens of thousands of images were screened and more than 22,000 were used to count animals at fixed depths.
A young colony of H. laauense amid a bed of other octocorals. (Photo by A. Baco-Taylor FSU, E.B. Roark TAMU, NSF, with HURL Pilots T. Kerby and M. Cremer)
A young colony of the precious coral Hemicorallium laauense grows among other octocorals, with a crab in the foreground. Precious corals like this one were the target of the old coral fisheries, so new colonies are one of the clearest signs that protected sites are being recolonised.
The signs of recovery they found
The damage was still obvious. Trawl scars, coral rubble, broken stumps and lost fishing gear showed up across the surveys. But on the protected seamounts the team also found:
- Corals regrowing from fragments, including the precious coral H. laauense and the reef-building coral Enalopsammia, on the Hancock seamounts.
- Soft corals growing over trawl scars on north-west Hancock, places that would once have been bare.
- Healthy patches that escaped the nets, such as octocoral beds and Solenosmilia stony coral reefs on south-east Hancock. These survivors matter because they can supply larvae to repopulate damaged areas.
- More life overall. Protected seamounts had significantly more animals, more corals and more kinds of animals per image than the still-trawled ones, with median abundance roughly 225 to 250 percent higher at some depths.
A patch of recovering scleractinian reef on the SE Hancock Seamount at 650m. (Photo by A. Baco-Taylor FSU, E.B. Roark TAMU, NSF, with HURL Pilots T. Kerby and M. Cremer)
A patch of stony (scleractinian) reef regrowing on the south-east Hancock Seamount at around 650 metres. Deep-water reef-building corals grow very slowly, so even small patches like this represent many years of growth.
Some young corals appeared even on the seamounts that are still fished, which suggests these communities can bounce back if given the chance. Continued trawling, however, keeps knocking them down.
Why recovery takes so long in the deep sea
Deep-sea corals live in cold, dark water with little food, so they grow and reproduce slowly. Some precious corals and black corals can live for centuries or even thousands of years. When a trawl net removes a colony that took hundreds of years to form, a few decades of protection is a short time in coral terms.
That is why the authors were cautious. They could not yet say exactly how long it took for new corals to settle, or whether the seamounts will ever regain their former diversity. Ageing the colonies and identifying species from samples was still under way when the paper was published.
What it means for ocean protection
The study speaks directly to fisheries managers. Its main message is that long-term closures can work for deep-sea ecosystems, but short-term ones may not be enough, because the recovery being seen took 30 to 40 years to become visible. The authors also warn that continued bottom-contact fishing on heavily trawled seamounts can destroy the remnant patches that might otherwise seed recovery.
A few points are worth keeping in mind when reading headlines about this research:
- It concerns deep-sea coral communities, not the shallow reefs around Hawaii's beaches, which face different threats such as heat-driven bleaching, runoff and invasive algae.
- "Recovery" here means early regrowth and higher abundance than fished sites, not a return to pristine conditions.
- Protection only helps where it applies. Seamounts further along the Emperor chain, outside US waters, remain open to fishing.
Even with those caveats, the findings are hopeful. They show that the deep sea is not beyond repair, and that when people stop dragging nets across fragile habitats, life starts to return, slowly and on its own timetable.
Sources: Baco et al., "Amid fields of rubble, scars, and lost gear, signs of recovery observed on seamounts on 30- to 40-year time scales", Science Advances 5(8), 2019; Wikipedia: Papahānaumokuākea Marine National Monument. Photos: A. Baco-Taylor (FSU), E.B. Roark (TAMU), NSF, with HURL pilots T. Kerby and M. Cremer.
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