Drift nets are a type of net sometimes used in commercial fishing that have weights at the bottom of the net and floats at the top. These nets work by ensnaring fish by their gills, and are often referred to as gill nets. Though the technique is effective for catching large amounts of fish, it has been criticized for indiscriminately killing non-target fish species, and also marine mammals.
Though there are some bans on drift nets, the high seas are unregulated, and therefore they are still often used. Estimates are that as many as 20,000 miles (approximately 32,186 km) of nets are set each night in just the North Pacific Ocean. Though these nets are typically collected each day, the lines are in the water long enough to kill a variety of marine species. Mammals that get entangled in drift nets often die by suffocation because they cannot reach the surface to breathe air.
Typically, drift nets target a certain size of fish, which is determined by the diameter of the mesh. The smaller fish tend to swim right through the nets and the larger fish bounce off and swim away. The nets may also be set at different depths to target various species. The nets are unable to exclusively trap only certain species of fish, but simply catch all species of fish they encounter within a certain size range. Some may have commercial value and others may not. This situation can destroy entire populations of fish in some regions.
Drift nets may be dozens of miles or kilometers long, but concerns about the impact of the nets on marine mammal populations and sea birds caused the United Nations to impose restrictions in 1993. Then, the UN banned all nets longer than 1.5 miles (2.5 km). This has helped reduce the number of unintended fatalities by as much as 66 percent, but has not stopped it completely. Some of the marine mammals and birds killed are threatened or endangered species.
In some cases, especially during times of high seas or storms, drift nets may be lost at sea. These nets, generally made out of monofilament or other synthetic materials, do not break down easily, and therefore can become a big problem. They can drift and kill fish until their weight causes them to eventually sink. These lost nets are called ghost nets.
Though environmentalists criticize the use of the nets, and call for even further restrictions, the fishing industry says they are vital to meet the demand for seafood. Some countries and jurisdictions have gone further than the UN. For example, the European Union banned drift nets for catching tuna in 1998, though the technique can still be used for other species.