Pascal Lorance, co-chair of the Working Group on the Biology and Assessment of Deep-sea Fisheries Resources (WGDEEP) gets to grips with a definition of 'deep sea'.
'There is no strict global definition of what deep sea means. Whilst nearly everyone agrees that a depth of say 4,000 metres is the deep sea, the problem is what constitutes the upper limit.
At ICES, species that are assessed as deep-sea include those which are found only at great depths off the continental shelf, such as the roundnose grenadier, which lives at depths of between 700 and 2,000 metres in the Atlantic. However, there are some shallow water species like the blackspot seabream, whose juveniles occur near the very coastline and adults migrate seasonally between coastal areas and depths of up to 900 metres, which are also termed deep-sea.
The inclusion of the latter is in line with the FAO definition of deep-sea fisheries, which doesn’t include an actual reference to depth. Instead, deep-sea fisheries are described as those which exploit species that can only sustain low exploitation rates, like the grenadier and the seabream. This definition reflects the complexity of nature, where the roundnouse grenadier occurs in more shallow waters (300-700 metres) in areas influenced by boreal (high northern latitudes) waters such as the Norwegian deep than in the Atlantic Basin.
Ecological zonation (the grading of ecological zones by parameters like altitude or, in this case, depth) of the sea floor is most often delimited as: shelf areas 0-200 metres, upper slope 200-700 metres, mid-slope 700-1500 metres, and lower slope 1,500-2,500 metres. These categories are broadly used and they enable the identification of which ecosystem is looked at on any scale.'