Scientists warn of damage to deep seabed

8 February 2011 by Tom Marshall

Human activities are affecting large areas of the deep seabed, according to a recent study - and trawling the seabed for fish is the most extensive.

Trawler boat

More regulation is needed before the ocean floor suffers even more widespread damage, the scientists say. Better data on human activities - where they're taking place and how much harm they are causing - is also vital to let us manage the whole ecosystem.

'We have to know where the impacts are occurring!' says Angela Benn of the National Oceanography Centre, who led the study. 'It's an urgent need - human activities are expanding into the deep sea at an unprecedented rate, and these ecosystems are under more pressure than ever before with new activities like carbon capture and storage, mining for minerals and deeper drilling for oil and gas.'

Her study makes it clear, though, that deep-sea trawling - a fishing method in which boats drag nets across the seabed to catch bottom-dwelling prey - is the biggest source of harm to the deep ocean floor. Trawlers plough up and disrupt coral reefs and other sensitive habitats, and they could affect as much as 37,160km2 of seabed.

This is ten times greater than the area affected by all the other activities in the study put together. In some areas of the northeast Atlantic, it would mean most sections of seabed get trawled at least once a year.

Benn analysed and combined the available information on the location of different human activities during 2005 on the deep seafloor in the OSPAR (Oslo Paris Commission) area of the northeast Atlantic. The area is heavily exploited and contains important fisheries and oil and gas fields.

Deep-sea dangers

Until recently, many countries just dumped unwanted material into the abyss and forgot about it. That's left an alarming legacy of contamination, ranging from scuttled ships, old ordinance, chemical weaponry and even radioactive waste.

This is on top of disruption to complex seabed ecosystems - like deep-water coral reefs - caused by bottom fishing, drilling for oil and gas, laying communications cables and scientific research.

There's increasing international attention to the problems of pollution and physical damage to the deep seabed, but until now there's been little information on how extensive they are.

Some effects probably won't last long; others may be around for centuries. Low level radioactive waste from power plants, medical facilities and industrial applications was routinely dropped in the ocean until the 1980s, and may continue to leak into the environment for a long time.

On the other hand, there isn't that much of it. The paper, published in PLoS ONE, estimates there's about 113,000 tonnes of low level radioactive waste in deep sea in the OSPAR area, but this covers only around 0.2km2 of the seabed.

Munitions and chemical weapons may be the most dangerous things on the seabed. In coastal waters there have already been cases of fishermen killed by unexploded bombs brought up in their nets - but from what the scientists can tell from the limited information available, they cover just 1.4km2 of the deep seafloor.

The physical effects of the oil and gas industries, cable laying and scientific research also are relatively limited at the moment. Energy exploration and extraction is the most widespread of the three; its pipelines, wells, debris piles and other underwater manifestations cover an estimated 23.2km2. Most of the impact from scientific research is related to bottom trawling by fisheries laboratories.

While carrying out the study, the researchers found reliable information hard to come by. In many cases they had to extrapolate based on the limited data available to the public - the fishing industry is reluctant to reveal where it fishes, for instance. So all scientists can tell about each boat is where it is and how fast it moves, and they have to infer when it was actually fishing.

This meant they had to assume that boats were bottom trawling when they were travelling at an appropriate speed in known trawling areas. For other activities, like military exercises, there's no data available at all.

Benn says this study is a step forward, but that much more information will be needed if appropriate regulations are to be introduced so these deep-sea ecosystems can be managed for the long term.

'Traditionally human activities in the marine environment have only been managed on an industry-by-industry basis. New approaches are now turning towards managing whole ecosystems and multiple impacts from different industries. From the point of view of fisheries, the aim has been to preserve individual fish stocks,' she explains. 'It's only recently that regulations are being developed to protect vulnerable marine ecosystems taken as bycatch in trawls.'

'It's not easy at all to get hold of data in this area, particularly from fisheries,' she continues. 'It is a very serious problem; the deep-water fisheries industry adds only a fraction of one percent to the GDP of Europe. Yet, it has a far larger impact than any other industry. It is almost impossible to get any hard information about how large the impacts are. Without being able to quantify accurately man's impact it will be impossible to introduce adequate environmental management measures.'