A World Ocean Assessment by the United Nations in 2015 concluded that the deep sea constitutes the largest source of species and ecosystem diversity on Earth. Bottom trawling, it said, has caused widespread, long-term destruction to these environments globally.
At risk are fragile environments associated with seamounts, oceanic ridge systems and similar ‘underwater features’ where cold-water corals and deep-sea sponges provide a rich habitat for many other species. These environments are particularly vulnerable to deep-sea bottom tra
wling. They stand no chance against the ruthlessly effective “bulldozers” dragged by trawlers. After heavy trawling, coral ecosystems on seamounts are reduced mostly to bare rock and coral rubble.
The threat to the deep sea’s biodiversity as a result of deep-sea bottom trawling and other methods of destructive deep-sea fishing is comparable to the loss of tropical rainforests on land. Many thousands of species may be at risk, most of which are still unknown to science and some of which may not exist beyond a very limited location.
Damage to environment
The UN World Ocean Assessment said that it may take “centuries to millennia” for deep-sea ecosystems to recover from the impact of bottom-trawling.
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 in the Northwest Pacific north and west of Hawaii, around New Zealand and Australia, and across the high seas of the Southwest Pacific and Southern Indian Ocean.
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. 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, mainly by bottom trawlers.
Damage to species
Deep-sea bottom trawling is an indiscriminate fishing method. Many species, some endangered, are caught incidentally and thrown back into the sea dead. Bottom trawling is known to have high discard rates compared to other fishing gears.
Notable species targeted by bottom trawl fisheries include orange roughy and roundnose grenadier, which grow and reproduce very slowly, making them especially vulnerable. Orange roughy, for example, do not reach sexual maturity until 22-40 years of age. This makes it slow and difficult for an orange roughy population to replace fish removed by fisheries. Orange roughy fisheries have collapsed in many parts of the world because of overfishing. In the northeast Atlantic, roundnose grenadiers are major target species in the deepwater trawl fishery and are now classified as endangered on the IUCN Red List of species in the region.
The UN World Ocean Assessment reported that the majority of deep-water fisheries have been carried out unsustainably, leading to the serial depletion of dozens of stocks.”