Risks & Impacts

Impacts on the ecosystem

The scale of the threat to the deep sea’s biodiversity as a result of bottom trawling and other methods of deep-sea fishing is as yet unknown. However, it is potentially comparable to the threat to terrestrial biodiversity associated with the loss of tropical rainforests. Many thousands of species may be at risk, most of which are still unknown to science.

Fragile deep-water ecosystems, coral systems in particular, stand no chance against the ruthlessly effective underwater “bulldozers”. Deep-sea structures are not merely damaged: they are obliterated in a manner akin to clear-cutting a rainforest. After heavy trawling, coral ecosystems on seamounts are reduced to mostly bare rock and coral rubble.

Once destroyed, slow-growing deep-sea species are unlikely to recover for decades or centuries, or even lost forever. Stable living habitats such as coral and sponge communities tend to be both the most heavily damaged and the slowest to regenerate.

Trawlers at anchor in the small port of Ban Phe. Photo: Eleanor Partridge/Marine Photobank.

To make matters worse, the deep sea’s remarkable array of coral, sponge, and other habitat-forming species are, in many cases, likely to harbor undiscovered and endemic species. As noted by the UN’s World Ocean Assessment: There is strong evidence that the richness and diversity of organisms in the deep sea exceeds all other known biomes… and supports the diverse ecosystem processes and functions necessary for the Earth’s natural systems to function”. The risk of extinguishing whole species never before seen is, therefore, particularly high when bottom trawling strips the seamount surfaces of their coral and sponge habitat.

Further, from the UN World Ocean Assessment:

“The documented widespread extent of deep-water trawl fisheries has led to pervasive concern for the conservation of fragile benthic habitats… The time scale for recovery of deep-water reef habitats is unknown but has been estimated to be on the order of centuries to millennia…”

Specifically, considerable damage to deep-water coral communities has been recorded off both coasts of North America, off Europe, from Scandinavia to northern Spain, and on seamounts near Australia and New Zealand. The Institute of Marine Research in Bergen, Norway, estimates that one-third to one-half of the cold-water coral reefs in Norwegian waters have been damaged or destroyed by bottom trawling. Photographs document giant trawl scars along the seabed up to 4km long.

On the high seas south of Australia, in an area known as the South Tasman Rise, observers recorded trawlers bringing up an average of 1.6 tons of coral per hour in their nets in 1997 — the first year of the area’s orange roughy seamount fishery. Up to several thousand tons of coral were estimated to have been brought up in the nets of the 20 or so deep-sea trawlers working in the area. This figure does not include coral that was damaged but not brought up in the nets. By contrast, the catch of orange roughy in the first year of the fishery was reported to be less than 4,000 tons.

In Alaskan waters alone, the US National Marine Fisheries Service estimated that over one million pounds of corals and sponges are removed from the seafloor every year by commercial fishing, roughly 90% of that by bottom trawlers.1


Depletion of high seas fisheries

In addition to the physical impacts of bottom fishing on deep-sea ecosystems, the depletion of deep-sea species is a matter of international concern. Unlike shallow-water species, deep-sea species are often slow-growing, long-lived, “low productivity” species that are highly vulnerable to depletion.

Most high seas bottom fisheries target low productivity species such as orange roughy in the South Pacific and Indian Oceans, grenadiers in the Northeast Atlantic, redfish in the Northwest Atlantic, and deep-sea sharks (often caught as bycatch in deep-sea fisheries in the Atlantic, Pacific and Indian Oceans and marketed to the cosmetics and dietary supplement industry for their liver or “squalene” oil).

In addition, a large number of species have been recorded in the bycatch of many high seas bottom fisheries, in particular bottom trawl fisheries, the majority of which are likely to be low productivity species. The status of target and bycatch species in the deep-sea fisheries on the high seas is largely either unknown or, where information is available, considered overexploited or depleted. Regulations are in place in some high seas fisheries to manage the target catch in deep-sea fisheries. However, few, if any, of the fisheries impacting deep-sea stocks or species on the high seas can currently be considered sustainable.

Plastic in Bottom Trawl Bycatch. Photo: Mike Markovina/Marine Photobank.

The scientific literature often refers to deep-sea fisheries as “serial depletion” fisheries and numerous studies or reports confirm the problematic nature of fisheries for deep-sea species.2 From the UN World Ocean Assessment:

“The vast majority of deep-water fisheries have been carried out unsustainably, or at least without satisfactory assessments of impacts and sustainability. This has led to the serial depletion of dozens of stocks… Severe impacts have been reported for by-catch species, including other fishes…

“The extent of benthic impacts has been described for local fishing grounds but has not been assessed globally; however, if the impacts of these regional studies are generalized, we can extrapolate that fishing, and in particular deep-water trawling, has caused severe, widespread, long-term destruction of these environments globally.”

The International Union for the Conservation of Nature (IUCN) Shark Specialist Group classifies the three main species of deep-sea sharks (leafscale gulper shark, Portuguese dogfish, gulper sharks) in the Northeast Atlantic as endangered or critically endangered. In the South Pacific, New Zealand reports a total of 137 species of fish caught in the deep-sea fisheries on the high seas; the status of these species and the impact of the fishing on these species are unknown. At least 50 species have been recorded in deep-sea trawl surveys along the Emperor seamount chain northwest of Hawaii in the North Pacific, where deep-sea vessels from Japan, Russia and Republic of Korea continue to operate on the high seas.

The International Council for the Exploration of the Seas (ICES), the scientific advisory body to the European Union and the North East Atlantic Fisheries Commission, summed up the problem of deep-sea fishing as follows:

At depths between about 400 and 1500m there may be between 40 and 50 demersal species present depending on gear type. Maximum species diversity occurs between 1,000–1,500m before declining markedly with depth. Deep-water species are typically slow growing, long lived, late maturing and have low fecundity. Fishing has a greater effect on species with such life history trait…making them particularly vulnerable to overexploitation. This applies to both the target and non-target species. A large proportion of deep-water trawl catches (upwards of 50%) can consist of unpalatable species and numerous small species, including juveniles of the target species, which are usually discarded… The survival of these discards is unknown, but believed to be virtually zero due to fragility of these species and the effects of pressure changes during retrieval… Therefore such fisheries tend to deplete the whole fish community biomass.”3

Not only are deep-sea fisheries for the most part unsustainable but the economics of deep-sea fisheries are questionable given the low productivity of deep-sea fish stocks. According to a report published in 2009 by the United Nations Food and Agriculture Organization (FAO), some 285 vessels flagged to 27 countries were estimated to be engaged in high seas bottom fisheries in 2006 (many only fishing part-time on the high seas). Of this number, 80% were flagged to 10 countries: Spain, Republic of Korea, New Zealand, Russian Federation, Australia, Japan, France, Portugal, Belize and Estonia. More than one-third were flagged to European Union countries and the EU fleet took half or more of the high seas bottom catch. The majority of the vessels engage in high seas bottom trawling.4

The UN FAO report estimated that the global catch in high seas bottom fisheries in 2006 was approximately 250,000 tons, representing 0.3% of the marine catch worldwide. The value of the high seas bottom catch in 2006 was estimated at approximately US$ 450 million, a small fraction of the US$ 75 billion overall value of marine catch worldwide. The conclusions of the UN FAO report were similar to the findings of a study on high seas bottom trawl fishing published by IUCN in 2004.

A study released in 2007 by the Fisheries Centre of the University of British Columbia concluded that many deep-sea bottom trawl fisheries on the high seas in recent years would not have been economically viable without state subsidies. A review of deep-sea fisheries in the Northeast Atlantic by the European Commission in 2007 concluded that many deep-sea fish stocks have such low productivity that “sustainable levels of exploitation are probably too low to support an economically viable fishery”.5


  1. NMFS 2001. “Draft Programmatic Groundfish Supplemental EIS.” Jan. 2001 Tables 4.7-4 and 4.7-5.
  2. Cheung WWL, Watson R, Morato T, Pitcher TJ, Pauly D. Intrinsic vulnerability in the global fish catch. Marine Ecology Progress Series 2007;333: 1-12.
  3. From: Report of the Working Group on the Biology and Assessment of Deep-Sea Fisheries Resources (WGDEEP). 3–10 March 2008. Copenhagen, ICES Headquarters. ICES CM 2008/ACOM:14. 531pp. Pages 70–71.
  4. Bensch, A., Gianni, M., Greboval, D., Sanders, J.S. & Hjort A. World Wide Review of Bottom Fisheries in the High Seas. Food and Agriculture Organization of the United Nations. Rome, 2009.
  5. COMMUNICATION FROM THE COMMISSION TO THE COUNCIL AND THE EUROPEAN PARLIAMENT: Review of the management of deep-sea fish stocks. Brussels, 29.1.2007 COM(2007) 30 final.