IUCN, the International Union for Conservation of Nature

Deep-sea fisheries are generally considered to be fisheries conducted for bottom dwelling species below 200 meters on the continental slope, seamounts, deep-sea ridges and plateaus and associated underwater features. There has been a long history of traditional and artisanal handline fisheries for deep-water species, but industrial deep-sea trawl fisheries only developed in the 1950s and 1960s with the advent of factory trawlers.
Firstly dominated by countries of eastern Europe and former USSR until their decline in the 1980s, deep-water bottom trawl fisheries were then developed by a number of other countries. Kept within countries’ EEZs along the continental slope and on seamounts throughout the 1980s and 1990s, many of these fisheries have progressively expanded into deeper waters and further offshore, including beyond the limit of national jurisdiction.

In just ten years, between 1992 and 2002, the percentage of fish caught in the high seas in relation to the global marine catch rose from 5% to 11%.

Source: Matthew Gianni (2004), high seas bottom trawl fisheries and their impacts on the biodiversity of vulnerable deep-sea ecosystems: options for international action.
 

Deep-sea trawl fishing has taken place over the past several decades in the high seas of the South West Indian Ocean region, with exploratory surveys by vessels from the former USSR beginning in the 1970s. Former USSR vessels conducted periodic deep-sea trawl research cruises on a commercial scale throughout the 1980s and 1990s, and deep-sea trawlers from both New Zealand and Australia were reportedly also fishing in the region during the 1990s. In the period 1999 – 2001 there was a major increase in deep-sea trawling on the high seas with the discovery of orange roughy (Hoplostethus atlanticus) stocks by vessels from New Zealand. The combined catch of all deep-sea species in 2000 was estimated at approximately 40,000 tonnes, involving up to 50 vessels from over a dozen countries, although accurate catch data are unavailable, given the unreported and unregulated nature of the fishery. In 2001, only eight vessels reportedly participated in the fishery and, in 2002, fishing activity declined even further. Very little information is available on the South East Indian Ocean.

Detailed information on the former USSR deep-sea trawl fisheries between 1970s and 1990s indicated that well over 100 species were taken as by-catch, which suggests that the impact on associated and dependent deep-sea species could be significant.
 

The dominant bottom fishery in the high seas of the South West Indian Ocean over the past several years has been the mid-water and bottom trawl fishery on and around seamounts for alfonsino (Beryx splendens) and orange roughy. In addition to the trawl fishery, a deep-sea longline fishery on the high seas developed over the past several years targeting primarily deepwater longtail red snapper (Etelis coruscans). Anecdotal information also suggests that several vessels may be fishing with deep-sea gillnets on the high seas of the south Indian Ocean, primarily for deep-sea sharks.

SIODFA indicated that four deep-sea trawl vessels have regularly fished the high seas of the southern Indian Ocean for orange roughy and alfonsino since 2003, and not all of the vessels fish all year round.

Source: Bensch, A.; Gianni, M.; Gréboval, D.; Sanders, J.S.; Hjort, A. Worldwide review of bottom fisheries in the high seas. FAO Fisheries and Aquaculture Technical Paper. No. 522. Rome, FAO. 2008