Gill nets are fine and transparent nets, that hang vertically in the water column, hold upright by floats on the upper line and weights on the lower line. The netting is almost invisible to fish, so they swim right into it and will get their heads stuck and become snared as they try to reverse. As the fish struggles to free itself, it may become even more entangled.
Gillnetting is a common fishing method used by commercial and artisanal fishermen for marine species, in freshwater and estuary areas. Gill nets can be relatively selective, if the depth of net deployment and the mesh size are set right. Fish smaller than the mesh of the net pass through unhindered, while those too large to push their heads through the meshes are not caught. This gives gillnets the ability to target a specific size of fish, unlike other net gears such as trawls, in which smaller fish pass through the meshes and all larger fish are captured in the net.
However one type of gill net are known as the "curtains of death", because of it's destructive impacts on marine life worldwide: drift nets.
A very common but destructive form of gill nets is the drift net. Drift nets consist of a string of gill nets, that drift freely with the current to catch large schooling pelagic fish, like swordfish and tuna.
The principal negative impact produced by this type of nets is related to the bycatch of non-targeted species like marine mammals, seabirds and turtles. Another problem of drift nets is that they can easily get lost during storms or heavy weather. Hundreds of kilometers of nets and lines get lost every year and will keep fishing as "ghost nets" for multiple decades, possibly even for several centuries.
The use of drift nets longer than 2.5 kilometres on the high seas was banned by the United Nations in 1992. Prior to this ban, drift nets were reaching lengths of more than 60 kilometers. However the implementation of the ban has failed in large areas of the ocean, as multiple NGOs like The Black Fish reported the ongoing use of drift nets longer than 2.5 kilometers in Italy and other countries.
Learn more about the techniques of modern fisheries here.