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The deep-water coral Lophelia pertusa in Norwegian waters: distribution and fishery impacts - page 11 / 12

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Bottom trawling also increases resuspension of bottom sediment and release of nutrients near the bot- tom (Piskaln et al., 1998; Watling & Norse, 1998). On a world-wide scale siltation is one of the largest sources of degradation of coral reefs (Norse, 1993) and may suppress growth rates of adult colonies (Cortés & Risk, 1985). Siltation or sand deposition due to bottom trawling may have a negative effect on Lophelia corals. Roberts & Anderson (2000), studying Lophelia in an aquarium, have indications that sand deposition can reduce the level of polyp extension. However, Riegl (1995) shows that scleractinian cor- als actively clean sand from the surface and conclude that the corals are capable of coping with consider- able amounts of sand deposition and that declining reef health in sedimented areas may also be due to additional environmental stress.

Ecological consequences

Species diversity is about three times higher on Loph- elia reefs compared to the surrounding soft-bottoms (UK Biodiversity Group, 2000), thus confirming the general positive relation between habitat complexity and species diversity in the marine environment (Hus- ton, 1994). This implies that the reefs on the shelf and fishing banks represent patches of high diversity in an environment of low diversity. Anthropogenic degrad- ation of a significant part of the Lophelia reefs may thus dramatically change the distribution of species diversity along the whole shelf and slope.

It is hypothesised that reefs may function as centres of spreading for associated fauna. Although the fauna associated with the reefs is rich we have no examples of species that are obligate reef dwellers. The reef habitat may nevertheless play an important role for species such as Munidopsis serricornis (Lovén, 1852), Ophiacantha spp. and Eunice spp. which all exhibit high abundances on the reefs, but are seldom found in other Norwegian habitats (Fosså & Mortensen, 2000). If the reefs containing core populations of such species disappear the species may have difficulties in either spreading or sustaining their own populations.

Video inspections showed dense aggregations of redfish (Sebastes spp.) on the reefs, which in May– June, were dominated by gravid females with disten- ded bellies (unpublished information). Furevik et al. (1999) reports that long-line catches of Sebastes spp. may be six times higher, and for ling and tusk two to three times higher, on the reefs compared to non-reefs areas. This give support to fishermen’s reports that the

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reefs are attractive fishing places and that their disap- pearance influences the fish distribution in the area. However, these assertions are still to be confirmed, e.g., we know very little about how important Lophelia is for the fish.

The Lophelia corallites grow 5–10 mm per year (Mortensen & Rapp, 1998) and the growth rate of a Lophelia reef is estimated to be 1.3 mm per year (Mortensen, 2000). Consequently, it will take hun- dreds of years for a colony to reach a diameter of 1.5–2 m while it will take thousands of years to build a reef structure 10–30 m thick. Thus, it will take a long time for the reefs to recover and for the restitution of their ecological function, if at all.

Conclusions

According to verified records Lophelia pertusa is distributed along the Norwegian coast between 59 34.4N, 0511.6E and 7102.0, 2120.0E, mostly between 200 and 400 m depth. Lophelia is particularly abundant on the contin- ental shelf between 6230N and 6530N and on the shelf break between 6230N and 6350 N (locality: Storegga) Damages to Lophelia reefs in the continental shelf and break caused by bottom trawling have been documented for the first time. It is estimated that between 30 and 50% of Lophelia reefs are either impacted or destroyed by trawling Passive gear like long-lines and gillnets anchored on the bottom also impact the coral reefs, but to a considerably lower extent than trawling

Acknowledgements

We are greatly indebted to the fishermen who provided information about the occurrence and status of coral reefs. Without their knowledge, interest and active participation it would have been difficult to accom- plish this project. We are also indebted to the Research Council of Norway for financial support through grant no. 121122/122, to Martin Hovland, Statoil, who made it possible to perform the cruise on SV ‘Seaway Surveyor’ in 1998, and to the Norwegian Director- ate of Fisheries for a grant which made it possible to accomplish the ROV inspections in 1999. Erling Svensen, Egersund, kindly provided us with the photo

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