Deep-sea fishing, acidification threaten cold-water coral reefs

By Kate Reynertson

The branching, ivory thickets of the Oculina Banks may not be as colorful as corals in shallower, tropical waters, but they are home to more than 70 reef-dwelling species in the cool deep waters off the Florida coast.

Scientists discovered the corals in the mid-1980s, 100 meters below the ocean surface on the edge of the continental shelf. Illegal bottom fishing has killed off over 90 percent of the fragile Oculina ecosystem, said Sandra Brooke, a researcher who wrote her doctoral dissertation about deep-water corals.

Attempts at conservation have been largely unsuccessful, but scientists are pushing for better protection so that these diverse communities can be studied.

Deep-water fishing, especially bottom trawling, and ocean acidification from increasing atmospheric carbon-dioxide each pose a significant threat to cold-water coral ecosystems, according a recent study.

Trawling, a fishing technique for which boats pull weighted nets along the ocean floor, plowing through everything in their path, is certainly the most immediate threat. The U.S. government placed the Oculina banks under protection in 1984 but illegal bottom fishing continues to injure the ecosystem, Brooke said.

“These banks have been protected for 20 years and no one cares,” she said. ”You can draw lines on the ocean forever—but if you don’t enforce the law, it doesn’t do a damn thing.”

Cold-water coral habitats worldwide have been similarly damaged. There has been no comprehensive study of the ocean floor to locate and catalog existing coral ecosystems, so the extent is not fully known. But almost every study of known cold-water corals shows trawl damage, said J. Murray Roberts of the Scottish Association for Marine Science.

Basic trawling gear alone can weigh several tons and is often weighted further so that it will stay open as it scrapes along the sea floor.

Trawl fisherman do not like to trawl through corals because it can break their equipment, but trawling gear is being toughened up and modified so that it can crash through rocks and corals without snagging, Roberts said.

Even with the strengthened gear, corals can still ruin a trawl fisherman’s catch. “Some skippers will tow chains along the ocean floor first to make a trawling pass more successful,” Roberts said.

In one hour of trawling for orange roughy off the coast of New Zealand 1.6 tons of corals were removed from the sea floor, according to a recent study.

Such trawling practices have reduced the coral coverage of the Tasmanian seamount from 90 percent to 5 percent, according to a 2000 study. Such significant losses are nearly impossible to recover from.

Corals need to reach a certain size before they can reproduce. When large portions of corals are destroyed their energy is put into growth rather than reproduction and the reefs stop expanding, said Rhian Waller of the Woods Hole Oceanographic Institute.

Some deep water reefs have been developing for tens of thousands of years, Roberts said, but it is not known exactly how long it will take for reefs to recover.

Few studies have been carried out because it is difficult to know precisely when a given area was last damaged by trawling. But some have been identified and are now being watched to see how long it takes them to heal, Waller said.

Ocean acidification is a relatively new phenomenon that scientists have just become aware of in the last few years, but it could cause deep-water corals to literally disintegrate.

As corals grow they create calcium carbonate shells through a process known as calcification. Calcification creates the supportive skeleton for the coral tissues serving as a protection from predators or an additional source of carbon dioxide for plants living in symbiosis with the corals. The process of calcification is important to the corals’ survival, said Richard Feely of the National Oceanic and Atmospheric Administration.

Corals depend on a particular ratio of carbon dioxide to carbonate in the ocean waters to carry out calcification. Near the ocean’s surface there is a natural excess of carbonate. In deeper waters there is a natural deficit. Most cold-water corals live just above the transition zone.

The same carbon dioxide that is causing global warming in the atmosphere is altering the chemical make-up of the oceans so that the transition zone from carbonate excess to deficit is becoming shallower. Studies have indicated that corals calcification decreases dramatically as this transition zone is approached. Using the best estimates for global carbon dioxide emissions over this century, scientists estimate that as much as 70 percent of the world’s cold-water corals will be exposed to the corrosive waters below the transition zone by 2100, which will dissolve their skeletons, killing the corals.

Acidification studies are more conclusive than climate change studies because there are fewer factors involved. The effect of carbon dioxide on the oceans is a very straightforward calculation, said Christopher Sabine of the National Oceanic and Atmospheric Administration.

Gases naturally move from areas of higher concentration to areas of lower concentration. With the concentration of carbon dioxide in the air increasing, a large amount of the chemical necessarily diffuses into the ocean.

Every year, 8 gigatons of carbon dioxide is absorbed by the ocean. That is 22 million tons of carbon dioxide every day.

“A small car like a VW bug weighs about 1 ton,” Sabine said. “So the oceans are absorbing the equivalent of 22 million VW bugs everyday.

“People think the oceans are this vast reservoir and we cannot really impact the oceans, but we are, and most people can’t conceive of the scale of what we’re doing.”

There have not been any definitive experiments on cold-water corals to confirm these predictions, but scientists have proved that increased carbon dioxide levels are harming tropical corals and agree that there is no evidence to think that cold-water corals will react any differently.

Chris Langdon of the University of Miami agrees that there is no evidence to suggest they will react differently, but he hopes that the deep-water corals might be different because they live in a more extreme environment than the tropical corals.

“I’m hoping they’ve adapted and that we can learn from them how to help warm water corals survive,” he said.

The real obstacle in learning more about cold-water corals is funding. Deeper corals can be studied only with submersibles or towed cameras. The types of submersibles needed cost about $20,000 a day to operate, Langdon said.

The is no funding currently allocated to complete a comprehensive survey of the world’s oceans to locate all of these cold-water systems, but it is a project that is being tackled on a smaller scale, one step at a time. Knowing where to find these corals is the first step in learning how they impact the ocean ecosystem and identifying any potential uses they may have in research and medicine.

In order to alleviate the threat of trawling a proposal has been taken before the United Nations to put a moratorium on bottom trawling.

But this moratorium would be in place only until more research can be completed and participation in the moratorium would be voluntary, Brooke said.

Preventing ocean acidification relies solely on restricting carbon dioxide emissions. The Earth has experienced higher carbon dioxide levels in the distant past, but since humans evolved there has been relative consistency of climate and consistent levels of carbon dioxide. There have been carbon dioxide increases before—they can be observed in the corals themselves—but increases have never occurred at this rate. In the past, species have had tens of thousands of years to adapt to this kind of change, but now its happening in only hundreds of years, Sabine said.

Even if carbon dioxide emissions were stopped immediately there would still be a lag time where they would continue to be incorporated into the water. “The train has left the station and we’re just expecting these things to happen,” said Lisa Robbins of the United States Geological Survey.

Photographs courtesy of Sandra Brooke.