Gillnets are sometimes used by to monitor fish populations.

And when they had done this, they enclosed a large number of fish, and their nets were breaking.
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But the unsparing mesh, with holes smaller than mosquitoes, traps much more life than traditional fishing nets do. Scientists say that could imperil already stressed fish populations, a critical food source for millions of the world’s poorest people.
When they had done so, they caught such a large number of fish that their nets began to tear.
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Divers are all too familiar with this phenomenon, especially in well fished areas. The founders of Ghost Fishing were confronted with ghost nets while diving the many wrecks in the Dutch North Sea. In 2009 they joined a local team of divers who started to clean those wrecks. After some years of local efforts it was time to broaden the horizon and get in touch with like-minded groups all over the world. And so the Ghost Fishing foundation was born. And having done this, they enclosed a great multitude of fishes; and their nets were breaking.
Photo provided by FlickrWhen they had done this, they enclosed a great quantity of fish, and their nets began to break;
Photo provided by FlickrWhen they did this, they caught a great number of fish, and their nets began to tear.
Photo provided by Flickr
Memphis Net & Twine was founded as a fish net company in 1962 and has been a source of fishing netting, fishing nets, rope and twine, and fishing supplies ever since. Beginning in 1988 Memphis Net & Twine began supplying baseball batting cages, baseball backstops, sports backstops, sporting supplies and sporting equipment. Memphis Net & Twine continues to make all types of nets including custom sports nets, industrial nets, aquaculture nets and commercial fishing nets.A bottom trawl is a type of fishing net that's pulled along the seafloor. Fishermen commonly use bottom trawls to catch shrimp and bottom-dwelling fish like halibut and sole. However, in addition to these target fish, the nets also catch a variety of ocean life that's usually thrown back dead or dying. Dragging heavy gear across the seabed can also damage sensitive seafloor habitat. The harmful effects of bottom trawling on bottom-dwelling organisms and their habitat can be reduced by modifying the fishing gear or limiting trawling areas.Ranger Products specializes in a variety of fishing nets that include standard landing nets, hook and tangle-free rubber nets, catch and release trout nets, shrimp, shad, and smelt nets, bait and crab nets, spears, fish baskets and bait traps, along with many other products.Midwater trawlers vary in size—from small ships to large factory vessels. Large industrial ships pull gigantic nets through the open ocean and can catch an entire school of fish—spanning the size of five football fields—at once. These trawls don't impact the seafloor when used in the midwater zone. Setting these trawls on schooling fish using streamer lines to scare away seabirds and avoiding areas with an abundance of marine mammals can help ensure low levels of bycatch in midwater trawl fisheries.Gillnetting uses curtains of netting that are suspended by a system of floats and weights; they can be anchored to the seafloor or allowed to float at the surface. The netting is almost invisible to fish, so they swim right into it. Gill nets are often used to catch sardines, salmon and cod, but can accidentally entangle and kill other animals, including sharks and sea turtles.Drift-net fisheries have been mounted in all of the world's major fishing regions, and unwanted by-catch is always a serious problem. This has proven true for pelagic fisheries for , , , , and other species. One example is the drift-net fishery for swordfish in the Mediterranean, 90% of which is associated with Italian fishers. This industry kills excessive numbers of striped dolphin and sperm whale, and smaller numbers of fin whale, Cuvier's beaked whale, long-finned pilot whale, and Risso's, bottlenose, and common dolphins, along with other non-target marine . As a result of concerns about the excessive by-catch in this swordfish fishery during the early 1990s, the European Union banned the use of drift nets longer than 1.5 mi (2.5 km) (prior to this action, the average set was 26 mi [12 km] in length). However, some fishing nations have objected to this regulation and do not enforce it. It remains to be seen whether this length restriction will prove to be useful in preventing the non-target, drift-net mortality in this fishery.