English
The trawls are cone-shaped net (made from two, four or more panels) which are towed, by one or two boats, on the bottom or in midwater (pelagic). The cone-shaped body ends in a bag or coded. The horizontal opening of the gear while it is towed is maintained by beams, otter boards or by the distance between the two towing vessels (pair trawling). Floats and weights and/or hydrodynamic devices provide for the vertical opening. Two parallel trawls might be rigged between two otter boards (twin trawls). The mesh size in the codend or special designed devices is used to regulate the size and species to be captured.
Trawl fishing can take place at shallow depths to extreme depths:
- When the net is towed along the bottom of the ocean floor, it is called a “bottom trawl”. Bottom trawls are used to catch fish that live on or near the bottom of the ocean floor, such as cod, haddock, flounder, sole, rockfish, and orange roughy, as well as shrimp, octopus, and squid.
- When towed off the bottom, it is called a “mid-water trawl” or “pelagic trawl”. Mid-water trawls are used to catch species that tend to gather in large groups or “schools” in the top or mid depths of the ocean, like Alaskan pollock, mackerel, herring, and sardines.
Trawls are highly indiscriminate, capturing any and all species in their path. They catch high amounts of species that fishermen are not trying to catch, termed ‘bycatch’. The unintended catch or bycatch may include many species of fish, invertebrates (such as crabs, scallops, starfish or corals), sharks, skates and rays, endangered sea turtles, and sometimes whales and dolphins. Fishermen often throw much of this unintended catch back to sea dead or dying.
Turtle excluder devices and bycatch reduction devices are two common types of gear modifications that are used in trawl fisheries to reduce bycatch. The idea of these devices is to provide a way for large or unwanted animals to escape, while still allowing for the capture of targeted species. Turtle excluder devices are fitted into the neck of the trawl and consist of a grid of bars plus an escape opening/door at either the top or bottom of the net. When a large animal like a turtle hits the bars, it is directed to the escape opening and is able to swim free. Small animals like shrimp and fish, however, pass through the bars to the bag of the net.
Bycatch reduction devices work similar to turtle excluder devices. For trawl fisheries targeting shrimp, these devices are designed to exclude fish, while still allowing shrimp to pass into the bag of the net. In trawl fisheries for fish, bycatch reduction devices are sometimes used to prevent the capture of a particular depleted species. When used properly, turtle excluder devices and bycatch reduction devices can significantly reduce the deaths of sea turtles and other unintended species. Although they don’t reduce bycatch completely, these devices are an important step in the right direction.
Of course, the best way to protect ocean habitats is to limit the areas where trawling is allowed to occur. It is particularly important that fishery managers prohibit trawling in habitats that are especially sensitive to trawl damage, such as coral reefs, sponges, and seagrass beds and in important fish habitats.
Source: http://safinacenter.org/2015/02/fishing-gear-101-trawls-bulldozers-ocean/