Significant tuna fisheries utilise pelagic gillnets. For an overview of gillnet design and function, read the FAO Fisheries description here, as well as the following description published by the Australian Fisheries Research and Development Corporation (FRDC):
Modern nets are typically constructed from synthetic fibres, such as monofilament nylon for gillnets.The top edge of the net is attached to a rope called the headline, floatline or corkline. Floats are attached to the headline to provide buoyancy.
The bottom edge of the net is usually attached by hanging twine to a rope called the footrope or leadline. Weights or sinkers made of lead or other materials are attached to the footrope and spread the net vertically in the water. The type and number of floats and weights used depend on whether the net is to be positively or negatively buoyant (see below).
Gillnets and entanglement nets consist of a panel (or panels) of net held vertically in the water column, either in contact with the seabed or suspended from the sea surface. The size of the mesh in the net determines the size range of the species caught, as smaller fish are able to swim through the mesh. The legal net length and mesh size are set by individual jurisdictions. Gillnets and entanglement nets are used in offshore and inshore waters, and in rivers and estuaries.
Fish are caught in gillnets or entanglement nets in one of three ways:
- gilled—the fish tries to swim through one or more meshes; if it cannot pass through, it becomes caught behind its gill covers as it tries to back out of the net.
- wedged—the fish is tightly held in the net around the body by one or more meshes
- tangled—the fish is caught in the net by some part of its body, such as protruding fins or spines.
Pelagic gillnets (also known as drifting gillnets or drift nets) are made up of individual net panels tied together, allowing easy removal or replacement of damaged sections. They are set in open water and can be set with the headline on the sea surface (positively buoyant) or suspended below the surface (negatively buoyant), with one end of the net often remaining attached to the vessel.
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