Trawling

Bottom trawling is an industrial fishing method in which a large net with heavy weights is dragged across the seafloor, scooping up everything in its path.

Bottom trawling can be disadvantageous because it can stir up significant amounts of sediments that lie on the sea bed [1] and can harm some marine species.

[2] It also causes water pollutants to mix with some plankton, which in turn will move into the food chain, which will then create harmful algae blooms leading to insufficient oxygen.

[citation needed] Trawl nets can also be modified, such as changing mesh size, to help with marine research of ocean bottoms.

For example, researchers conducting a three-year study in the Clarence River found that an estimated 177 tons of by-catch (including 77 different species) were discarded each year.

There are a number of "fixes", such as tying a rope around the "cod-end" to prevent the mesh from opening fully, which have been developed to work around technical regulation of size selectivity.

By-catch reduction grids (typically made of stainless steel or plastic) or square mesh panels of net can be fitted to parts of the trawl, allowing certain species to escape while retaining others.

Depending on the configuration, the footrope may turn over large rocks or boulders, possibly dragging them along with the net, disturb or damage sessile organisms or rework and re-suspend bottom sediments.

[citation needed] For scale, the amount of sediment deposited into the ocean by all rivers in the world is estimated to be 17.8 to 20 gigatons annually.

An article published in New Zealand Journal of Marine and Freshwater Research determined that the resuspended sediment creates anaerobic turbid conditions capable of killing scallop larvae that use the ocean floor as a habitat as they mature.

[13] A 2021 study estimated annual carbon emissions from bottom trawling at almost 1.5 billion tonnes (about 3% of the world total) and recommended that more marine protected areas be established.

The net effect of fishing practices on global coral reef populations is suggested by many scientists to be alarmingly high.

[17] Published research has shown that benthic trawling destroys the cold-water coral Lophelia pertusa, an important habitat for many deep-sea organisms.

[18] Midwater (pelagic) trawling is a much "cleaner" method of fishing, in that the catch usually consists of just one species and does not physically damage the sea bottom.

However, environmental groups have raised concerns that this fishing practice may be responsible for significant volumes of by-catch, particularly cetaceans (dolphins, porpoises, and whales).

[19] The population of Alaska's Bristol Bay red king crab experienced an abrupt collapse during a three-year time span after 1980.

The cause of this crash was controversial with U.S. and Alaskan crab managers and modelers, with some stating the phenomena was a natural occurrence due to Pacific decadal oscillation, a shift in the location of warm and cold waters at an irregular pattern, while other marine biologists questioned the involvement of the new trawling fishery targeting Yellowfin sole in the area.

When the U.S. commercial harvest of the legal male red king crab reached its peak in 1980 after a 10-year increase, a trawl fishery for Yellowfin sole was introduced.

The pot sanctuary was introduced to protect the brood stock of female king crab which congregate in Bristol Bay to lay their fertilized eggs.

[20] Due to the sudden change in the sex ratio, Dew and McConnaughey concluded that sequential, sex-specific sources of fishing mortality were at work.

Analyzing the findings of their study, Dew and McConnaughey determined a strong correlation between trawling activity and the sex ratio change as well as the total population decline.

Dew and McConnaughey hypothesize that since female crabs return to and linger in Bristol Bay to lay pre-fertilized eggs, the trawling in the area disproportionally impacted the female population more than the male population and contributed to the change in sex ratio, as crabs do not die after they spawn.

Dew and McConnaughey noted that dissolving the Bristol Bay Pot Sanctuary exposed a vulnerable time in the red king crab's mating cycle to trawling.

Certain fisheries have bycatch limits that end a vessel's season if exceeded, and anecdotal reports of observers being pressured by crew and captain to lower their estimates have emerged.

These reports center around the financial repercussions that the crew, who get paid a percentage of the total catch profits, would face if their vessel is barred from fishing.

The data gathered from the cameras is used in conjunction with the vessel's logs and dockside monitoring of the catch as it is being unloaded to construct an estimate of the total bycatch.

As the health of the ocean in the future is uncertain due to climate change and other factors, providing biologists with accurate data about a source of fish mortality is essential to preserve the renewable resource that is wild caught seafood.

It is estimated by Oceana that, worldwide, fishermen lose at least $1 billion worth of potential catch annually due to the disposal of bycatch.

A group ex-trawler fishermen founded SeaShare in 1994 after successfully introducing changes to the National Marine Fisheries Service regulations to allow for the retention of bycatch solely for use by hunger-relief agencies.

[citation needed] Besides the aforementioned environmental objections, trawlers also run afoul of international borders and exclusive economic zones.