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Until most recently, the great depth of the deep-sea has made it difficult to exploit and the existence of relatively more abundant resources in shallower seas have meant that little incentive existed to fish in such difficult-to-exploit regions. Few deepwater fisheries are of long standing and those that are - the Portuguese (Madeira) line fishery for black scabbardfish (Aphanopus carbo), the Pacific Island fisheries for snake mackerels (Gempylidae) and cutlass fish (Trichiuridae) or the west African fisheries for deep-sea sharks (for extraction of scalene) - were initially artisanal.

With the reduction of opportunities for development of inshore fisheries and the improvement of gear technology and navigation instruments, deep-sea fishing has expanded in the 1990s. A well-known example of recently developed deepwater fisheries is that of the orange roughy, a species that inhabits the slope waters and those of seamounts (as well as the seafloor), particularly around New Zealand and Southeast Australia where this commercial fishery initially began. The fishery later spread to the Walvis Ridge in the Southeast Atlantic (Namibia) and the Southwest Indian Ocean. A small fishery even exists in the Bay of Biscay. This long-living fish reaches about 40cm and 2kg in size though the maximum size varies with region. Specially-aimed trawling techniques were developed after initial massive catches from spawning aggregations were taken in a matter of minutes resulting in split codends.

Orange roughy is particularly sensitive to approaching objects (perhaps an adaptation to avoid predation) so that acoustic assessment using towed bodies containing the transducer have proved futile in some areas. Maximum sustainable levels of exploitation of orange roughy may be as low as 5-10% of unfished biomass, corresponding to natural mortalities (M) of about 0.04 per year. Accumulating evidence about stock declines indicates that none of these fisheries are being exploited sustainably and ongoing yields will likely be around 5% of those initially obtained.

A Trichiurid fishery, which exploits Aphanopus carbo in the Atlantic, is a rare example of a deepwater fishery that, because it has traditionally used hook and line gear, has proved sustainable over a period of about 150 years. Adults of this species are benthopelagic living in the deep range 400-1 600m. The species ranges from Greenland to the Canary Islands and on both sides of the mid-Atlantic ridge. Unusual for a deepwater species, A. carbo grows rapidly and has longevity of around 8 years. However, as with orange roughy, the usual ominous signs are now evident for this fishery. Catch rose from 1 100 tonnes in 1980 to 3 000 tonnes in 1992, gear efficiency has improved through the introduction of monofilament lines and in a large increase in the number of hooks per line set, now at 4 000-5 000 per line.

The Macroudidae are another group whose members are widespread and, in particular locations, abundant. They are typical pelagic 'cruisers' and inhabit the mid-to-upper region of the continental slope. In the North Atlantic, fisheries exist for Macrourus berglax and Coryphaenoides rupestris using bottom trawls initially fishing in depths of 600-800m, and more recently extending down to 1 500m depth. However, experience in these fisheries off Newfoundland shows the all-too-familiar pattern of total allowable catches tracking declining trends in reported landings of this group. Coryphaenoides rupestris have a potential longevity of 70 years, although in the NE Atlantic fish ages are usually in the 20-30 year range. Thus, as for other deepwater species, Macrourids exhibit the characteristics of many deepwater fisheries that render them susceptible to overfishing.

The Pleuronectidae are a highly-evolved group that are not usually associated with deepwater fisheries, but important fisheries for members of this group occur in both the North Atlantic and North Pacific Oceans. In the Atlantic, the best known has been that for Greenland Halibut (Reinhardtius hippoglosoides) on the continental slope depths. This fish had an average size of around 1kg up until the mid-1980s, but has since declined to around 200g in the early 1990s.

The Greenland halibut has been declining in size in the last decade
The Greenland halibut has been declining in size in the last decade
FAO/Fisheries & Aquaculture Department
 
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