Cold-water coral reefs

Coral reefs are not just confined to the tropics - we have our own here in Scotland! Like warm-water corals, cold-water ones have a beautiful hard skeleton, and can form huge reef structures with many associated animals depending on them for shelter and food. Unlike tropical reef-building corals, cold-water corals can grow in the dark, in deep, cold water, catching their own food. Lophelia pertusa is the only reef-forming coral in British waters.

Why are they important?

Cold-water coral reefs form oases of food and shelter on the seabed, and are thought to be breeding grounds and refuges for many commercial fish. Video surveys have revealed the reefs to be home to numerous fish, including redfish, ling, tusk and pollack amongst the living coral, while ling and wolf fish were seen in the dead coral framework. Pregnant redfish may use the reef as a refuge or nursery ground. This abundant fish life makes the reefs a target for fisheries. A wide range of invertebrate animals are also associated with the reefs.

Why do they need protection?

The reefs are slow-growing, fragile and easily damaged - thousands of years of growth can be destroyed in a few minutes by heavy fishing gear. The reefs may take centuries to recover, if at all. Recent increases in fishing for deep-sea species including redfish and grenadiers have devastated some coral reefs. In Norway, an estimated 30-50% of coral was partially or totally destroyed by bottom trawling before laws were introduced in 1999 to protect 970 square kilometres of reefs from destructive fishing activities. In the UK, trawling was banned in 2003 as an emergency measure on the Darwin Mounds, 100 square kilometres of coral reef north-east of Rockall, in response to publicity over the threat to the reefs. The ban was made permanent in 2004, but covers only a small area of reef. Other potential threats to coral reefs include oil and mineral exploitation, and the laying of cables and pipelines.

The OSPAR commission   for the protection of the marine environment of the north-east Atlantic has recognised Lophelia pertusa reefs as a threatened habitat in need of protection. The reefs are a priority habitat for UK Biodiversity Action plans  , now taken forward by the Scottish Government as part of the Scottish Biodiversity Strategy  . Lophelia pertusa is listed under CITES Appendix ll.

Where are the reefs?

Lophelia pertusa has a wide geographic range from 55S to 70N, typically in water temperatures of 4-8ºC, and has been found as deep as 4,000m. Off Britain, reefs form mainly on continental slopes at 200-400m deep, west of Scotland and Ireland. However Lophelia has also been found in shallower water above 150m in several places, notably near Barra and Mingulay, where a complex of reefs covers an area of approximately 100 square kilometres. These reefs are over 4,000 years old, and recently established coral colonies suggest recruitment is still occurring. To the north, reefs occur along the length of the Norwegian coast, including one 13km long and up to 30m high. The shallowest known reef is at 39m (128ft) in Trondheimsfjord.

Where can they grow?

Lophelia larvae initially need hard surfaces to settle on. This can be old reef material, so that once started, the reef is self-sustaining. The reefs can be associated with old iceberg plough-mark zones, where hard glacial deposits have been exposed. The coral grows best on sloping seabeds where there is a strong current. Small coral clumps have been found growing on the legs of oil production platforms in the North Sea, and on submerged drumlins north of Shetland

Nature of the coral

Lophelia pertusa is a beautiful coral, with a pure white, highly branched skeleton. The branches end in a cup 5-10mm across, containing a small polyp with 60 or more translucent tentacles, used to trap food. The living tissue is usually white, but can be pink or orange. The coral grows slowly, only about 6mm per year; the age of older reefs has been estimated at more than 8500 years

In British waters, the bushy colonies grow up to 5-10m across, and may be fixed to the seabed or free. After growing to a diameter of around 2 metres, the colony tends to break under its own weight, but can continue to grow, the older parts of broken pieces being colonised by sponges and other animals. Below the reef is a zone of coral gravel. The reefs have a highly diverse associated fauna, with 614 animal species recorded from Norwegian reefs, including sea fans, sponges, large bivalves, prawns, squat lobsters, basket stars and worms. .