Modern fishing gear, often undetectable and extremely resistant, is very effective in capturing the desired fish species and anything else in its path. An incredible amount of marine life – including turtles, dolphins and juvenile fish – is caught and then discarded dead or dying overboard. Fishing gear is largely non-selective, that is, able to capture all species, including non-target ones. Longlines, bottom trawling and the use of gill nets and traps are the fishing methods that most commonly translate into by-catches. Bycatch occurs because networks also capture everything that is larger than the network, which includes juvenile fish, sharks, seabirds, sea turtles and cetaceans (whales, dolphins, porpoises). Trawling and trammel fishing are among the most dangerous techniques. Tramways, up to 30 meters high, remain suspended just below the water surface or are anchored to the seabed; the gills or fins of fish of a certain size get caught and stuck to them. Furthermore, many whales and dolphins find themselves trapped in nets, fail to rise to the surface and die very painfully. Today, by-catch is the biggest threat to marine mammals all over the world.