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Habitat Degradation

Friends of the Irish Environment

We are concerned about the damage to sensitive marine habitats, particularly seabed habitats, by certain fishing gear and methods.

Towed gears in contact with the seabed such as beam trawls, otter trawls and shellfish dredges are known to have a major impact on the sea bottom by killing and injuring seabed (benthic) organisms, and possibly causing loss of species diversity. Physical disturbance of the seabed can also increase the amount of suspended sediment, and thus increase sediment transport, and alter the chemical equilibrium of the sediments.

Together, the impacts on benthic communities and the alteration and degradation of habitats may result in ecosystem imbalance, such as when long-lived species are replaced by short-lived opportunistic species.

Demersal trawls (i.e. trawl gear towed so that it is close to or in contact with the seabed) constitute one of the most invasive methods of fishing. Nets, often with rollers, chains, and heavy wooden or steel doors (otter boards) to keep the mouth of the net, are dragged across the seabed, scooping-up everything in their path. "Rockhopper" nets, with heavy rollers that allow the trawls to roll or jump over rough terrain, including boulders or coral reef heads, have been employed since the 1980s.

During a fishing trip, a single pass of a trawl removes some 5-20 per cent of the benthic organisms. After 5-20 passes, the seabed is barren of life. Many areas of the world's continental shelves, such as the North Sea, are repeatedly "ploughed" in this way three or four times a year, leaving no opportunity for species and habitat recovery in between. Such habitat alteration has been compared to strip mining or global deforestation through clear-cutting.

Demersal trawling "simplifies" habitat: in other words, it removes the complexities of topographical relief, aquatic plants, invertebrates and other organisms that provide shelter for juvenile fish, including commercially important species. The less shelter that juveniles have, the less their chance of surviving to reproduce, thus exacerbating depletion from overfishing.

Deep-sea Trawling
Regarding the deep-sea west of Ireland (OSPAR Region V - Wider Atlantic), the OSPAR Commission states: "Evidence is beginning to emerge that the sorts of mechanical damage being inflicted upon North Sea benthic habitats and communities by trawling (Lindeboom and de Groot, 1998) is also being inflicted on some of the deeper ecosystems. The initial indications are that these impacts may not only be quite extensive already but may also be more persistent… Several core samples and seabed photographs have shown clear signs of disturbance, including plough marks, the burial of sponges, strong odours of hydrogen sulphide and snagged nets" (OSPAR, 2000).

OSPAR adds that: "There are no data specific to the Atlantic that indicate how long the scars from trawling persist in deeper water, but in the Pacific an experiment was conducted to investigate the impact of possible seabed mining. This experiment showed that plough marks were still clearly visible after seven years (Thiel and Forschungsverbund TUSHE, 1995) and that the macrofaunal populations still showed clear signs of perturbation (Borowski and Thiel, 1998)" (OSPAR, 2000).

Deep-sea fishing vessels are known to trawl at depths down to 1,800m or more on the continental slopes. Such trawling activity has already extensively damaged deep, cold-water coral reefs at the edges of the continental shelf off the coasts of Ireland, Scotland and Norway. Only recently explored, these reefs are formed by just a few species of extremely slow-growing corals, particularly Lophelia pertusa, which is normally found at depths of between 200-1,000m. Some reefs are known to be at least 8,500 years old. They provide a habitat for many hundreds of other species, and are spawning grounds for commercially valuable fishes. For example, mature adults of species such as orange roughy use them as safe spawning areas, and fry and juveniles use them for shelter and protection. Damage to such fragile and important ecosystems is of great concern.

In the deep-sea trawling method used for species such as orange roughy (Hoplostethus atlanticus), vessels tow their trawl gear along the seabed until a seamount begins to show up on the echo-sounder. At this point, the skipper increases the vessel's speed in order to lift the net off the bottom and up over the side of the reef, while continuing to fish the slope of the seamount. The aim is to avoid tearing or snagging the gear on the coral reefs.

In a recent report (ICES, 2002a) to the European Commission, scientists from the International Council for the Exploration of the Sea (ICES) warned that the only way to protect Europe's cold-water coral reefs is to accurately map them and then close them to fishing trawlers (ICES, 2002b).

Research has shown that towed nets, which can break up the reef structure, damage the sensitive coral polyps and swamp the reef with sediment, are seriously damaging cold-water coral in the North-East Atlantic. The most common species is Lophelia pertusa and closed areas for this species have already been used as a protection measure in Norway and Sweden.

David Griffith, General Secretary of ICES, has said: "Towing a heavy trawl net through a cold-water coral reef is a bit like driving a bulldozer through a nature reserve. The only practical way of protecting these reefs is therefore to find out where they are and then prevent boats from trawling over them. We know that most fishing boat skippers would rather steer clear of coral reefs, as the reefs can damage their gear, so producing accurate maps will actually help them to avoid these areas" (ICES, 2002b).

ICES also advised that current bycatch recording schemes for fishing vessels should be widened to include records of Lophelia. This will help identify the main areas where fishing pressure is having an effect on reefs.

ICES. 2002a. Report ( Word file 6.95 Mb)((Section 3 Doc))

ICES. 2002b. Close Europe's cold-water coral reefs to fishing. Press release, September 2002. (( ))

Close Europe's cold-water coral reefs to fishing