Coral reefs in Norway
Lophelia pertusa

Fishery impact

Bottom trawling in Norwegian waters

Bottom-trawling on the banks of the Barents Sea started in the 1930’s. The activity scaled up in the 1960’s by the introduction of factory and wetfish trawlers. In the mid eighties trawling occurred along the continental break and extended further to the banks on the shelf as a result of lower quotas for the Norwegian Arctic cod.

It was at the end of the 1980’s that rockhopper gear was developed allowing larger vessels to trawl in earlier inaccessible areas due to the roughness of the bottom, e.g by presence of coral reefs.

It was in the early 1990’s that long-line and gill net fishermen contacted the Institute of Marine Research (IMR) to express their concerns about the effects of trawling on coral reefs. They claimed that corals had disappeared from trawling grounds, and that their catches in these areas were lowered. Their worries also concerned the potential function of the reefs as nursery areas for fish.

Documentation of damage

We use ROVs to document effects of bottom trawling on Lophelia coral reefs. Side scan sonar has been used to document trawl tracks.

Dead coral fragments lying on the slopes of reefs are a common sight as it is part of the natural process of decay in coral reefs. Therefore, in order to distinguish natural decay from impacts by human activities, such as bottom trawling, we looked for broken living colonies tilted, turned up down and/or in unexpected/awkward positions on levelled sea bottom.

Remains of fishing gear such as gill nets, anchors, and trawl nets among corals added to the evidence that fishing was taking place. Furrows or scars in the sea bottom are also unmistakably evidence of trawling activity. It has not been possible to perform direct quantitative observations of how much of a reef or reef area has been impacted or destroyed. However, some places such as on the shallowest part of Sørmannsneset one can safely conclude that the colonies have been wiped out. Here we only found fragments of dead Lophelia spread around without evidence of living colonies in the surroundings.

Based on our investigations we have concluded that the damages of coral reefs in Norway amounts to between 30 and 50 % of the total coral area. Because of the inherent limitations of the methodology, the present estimates of total coral areas are to be considered as preliminary and should be confirmed by future surveys.

In general, wherever bottom trawling overlaps with occurrences of corals, there is a possibility of damage.

Damage has also been reported from the north-east Atlantic, e.g. the Darwin Mounds northwest of Scotland, south of the Wyville-Thomson Ridge, and in the Porcupine Sea Bight in Irish waters.

This is documented by a Study Group on Mapping the Occurrence of Cold Water Corals in the Northeast Atlantic [SGCOR] set up by ICES. The group was formed after the European Commission asked ICES "to identify areas where cold-water corals may be affected by fishing." An initial report was delivered to the Commission in October 2001 and a more detailed report was produced in 2002.

Read the ICES report here.

See also: Discovering cold-water coral reefs. ICES CIEM Newsletter. No. 38, December 2001.

Fishing with long-line and gill nets

Lophelia reefs are considered good fishing places for net and long-line. A considerable fishery with these gears are carried out in coral areas. The bottom-nets and lines are anchored to the bottom with anchors weighing 20-120 kg.

During visual inspections of reefs we have observed many lost long-lines and gill nets and other types of fishery related equipment. Lost nets can be seen gost fishing and also cover parts of the coral colonies. One direct effect on the corals is breakage. The effect of a lost net covering coral colonies is not known.

Although these fishing techniques obviously cause breakage and disturbance of corals we assume that the damage is of limited extent compared to the effect of bottom trawling.

Ecological effects of damage

Lophelia reefs represent a complex habitat with a highly diverse associated fauna. The ecological effects of degraded or completely destroyed reefs may thus be substantial.

Impact on coral populations

Increased mortality is the most obvious effect from mechanical impact by bottom trawling. The corals are crushed, buried and wounds in the tissue and possible microbial infection may also reduce the health of the corals. It is not possible to evaluate the impact that wiped out reefs on the shelf has on coral populations. On a scale from intact to extinct there may be a point below which corals won't be able to maintain populations.

The effect will also depend on the reproduction potential, but at present very little is known about the sexual reproduction of the species. It is reasonable to assume that they have a planktonic larvae similar to their tropical counterparts. Evidence of this are found in the North Sea, where corals have colonised submerged components of oil rigs, far away from known location of colonies. A long-lived planktonic larvae may facilitate recolonisation of damaged coral areas.

Bottom trawling also increase resuspension of bottom sediment and releases of nutrients near the bottom. On a worldwide scale siltation is one of the largest sources of degradation of coral reefs and may suppress growth rate of adult colonies. Siltation or sand deposition due to bottom trawling may have a negative effect on Lophelia-corals.

Studies of Lophelia in aquarium indicate that sand deposition can reduce the level of polyp extention. However, other scleractinian corals actively clean sand from the surface and shows that corals are capable of coping with considerable amounts of sand deposition. Declining reef health in sedimented areas may thus also be due to additional environmental stress.

Ecological consequences

Preliminary work indicates that species diversity is about three times higher on Lophelia reefs compared to the surrounding soft-bottoms, thus confirming the general positive relation between habitat compexity and species diversity in the marine environment. This implies that the reefs on the shelf and fishing banks represents patches of high diversity in an environment of low diversity.

Anthropogenic degradation of a significant part of the Lophelia reefs may thus dramatically change the distribution of species diversity along the shelf and slope.

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, Ophiacantha spp. and Eunice spp. which all exhibit high abundances on the reefs, but are seldom found in other Norwegian habitats. If the reefs containing core populations of such species disappear, the species may have difficulties in either spreading or sustaining own populations.

Video inspections have shown dense aggregations of redfish (Sebastes spp.) on the reefs, which in May-June, were dominated by gravid females with distended bellies. Our own experimental fishery shows 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 reefs attract fish and that their disappearance influences the fish distribution in the area. However, these assertions are still to be confirmed, e.g. we now very little about how important Lophelia is for the fish.

The Lophelia corallites grow 5-10 mm per year and the growth rate of a Lophelia reef is estimated to 1.3 mm per year. Consequently, it will take hundred of years for a colony to reach a diametre 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.