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marine environment

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How Deep is Deep-Water? Deep-water map

Deep-water fisheries are defined as those that occur at depths of more than 400 metres. Scottish deep-water fisheries tend to exploit fish at depths of between 500 and 1,500 metres, mainly in the Rockall Trough and the Faroe-Shetland Channel (map shown right). Vessels working in this fishery regularly land some 12 deep-water species including two deep-water sharks.

Background to the Fishery

Deep-water fisheries in waters to the west of the British Isles are a recent phenomenon, having mainly developed in the last decade. In 1844 it was suggested that, theoretically, animal life should not exist at depths much greater than 300 fathoms (549 metres). However, a series of voyages from the late 1860s into the early 1900s showed that there was a rich diversity of species in deeper waters. In the early 1970s the Ministry of Agriculture, Fisheries and Food (MAFF) and the White Fish Authority carried out extensive surveys of fish populations along the edge of the continental shelf and on the slopes of many offshore banks.

Some of the species discovered on these surveys were tested for marketability, but there was at that time reluctance amongst UK consumers to purchase these curious deep-water species and a UK commercial fishery was not developed. In 1992 a French trawl company set up an operation on the west coast of Scotland to land deep-water species for which they had developed markets in France. Since 1995, a smaller number of Scottish vessels have become involved in the fishery, fishing alongside the French, and marketing most of their catches overseas.

The Deep-Water Environment

The deep-water environment is very unproductive compared to the relatively shallow shelf-seas, conventionally fished (i.e. demersal and pelagic). The food supply is limited, and as a result, most deep-water species have delayed maturity, slow growth, long lifespan and low fecundity.

The waters to the west of Scotland are divided by a major bathymetric feature, the Wyville-Thomson Ridge. The area to the north of this ridge, the Faroe-Shetland Channel, is dominated by cold water originating in the Arctic. Temperatures below 500 metres drop very rapidly to below 0°C, and very few species are able to survive. The area to the south of the ridge, the Rockall Trough, is dominated by much warmer water, originating in the mid-Atlantic, and consequently supports a far greater diversity of life.

Which Species are Found in Deep-Waters?

Of the 130+ species occurring in the deep-waters to the west of the British Isles, approximately 12 are fished commercially. The main commercial fish are benthopelagic but their distribution and abundance change considerably with depth and topography. Most of these fish are quite distinct from those occurring on the continental shelf. Generally they lack a mucous coating so they are not slimy when compared to shelf species; some have large eyes, whilst others have lost their sight completely. Other species show further adaptations to living in deep-water, for example, the male deep-water anglerfish exists as a parasite on the female.

State of the Stocks Blue ray

A comprehensive understanding of the state of an exploited fish stock requires detailed information. In the early years of the fishery, information about the landings and the basic biology of the species was lacking, and as a result it was impossible to make any worthwhile attempt to assess the capacity of the stocks to sustain exploitation. What was known was that many species were slow-growing, late-maturing, and slow to reproduce, making them vulnerable to over-exploitation. On the basis of this, scientists advised great caution in the development of fisheries.

Since then, a time-series of landings data has been built up, and our knowledge of the biology of the species has increased steadily through an on-going programme of research. Although it is not yet possible to conduct the detailed stock assessments used in conventional demersal and pelagic fisheries, the International Council for the Exploration of the Sea (ICES) is now able to provide increasingly robust advice on the state of stocks. Current advice from ICES is that the stocks of most species are below safe biological levels and recent exploitation rates are unsustainable.

On the basis of this, the EU has set in place a new management framework which limits the exploitation of the many deep-water species through a system of quotas.