Significant tuna fisheries utilise pelagic gillnets. For an overview of gillnet design and function, refer to FAO Fisheries descriptions here. Sacchi (2021) provides good illustrations and descriptions of various gillnet setups.
The following description is published by the Australian Fisheries Research and Development Corporation (FRDC):
Modern nets are typically constructed from synthetic fibres, such as monofilament nylon.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.
Marine mammals, seabirds, sea turtles, sharks and rays are all incidentally captured in gillnets. The extent of the problem varies by oceanic region. NOAA describes the issues for sea turtles and marine mammals as follows:
Gillnetting has been a major source of mortality for all sea turtle species.Turtles encountering a gillnet can quickly become entangled around their head or flippers as they try to escape. Entangled turtles will drown if held under the water but have a higher chance of survival if they can reach the surface to breathe. The nylon can tighten around the turtle's soft body parts and cause deep cuts potentially leading to infections, limited movement, or complete loss of the limb. Limited use of appendages can impair a turtle's natural feeding, breathing, and swimming behavior.
Gillnets can entangle a wide variety of marine mammals.Depending on the gillnet mesh size, animals can become entangled around their necks, mouths, and flippers. Entanglement can prevent proper feeding, constrict growth, or cause infection after many months. Marine mammals entangled in set gillnets can drown while those entangled in drift gillnets can drag gear for miles as they migrate and forage, leading to extreme fatigue.
Lokkeborg (2011) describes the gillnet threat for seabirds:
Seabird mortality in gillnet fisheries occurs when diving seabirds encounter gillnets and become entangled in the net. As seabirds may encounter gillnets while they are set and hauled, seabirds may also be caught in nets set deeper than their maximum diving depth. Seabird bycatch has been documented in coastal and high-seas fisheries, as well as in drift and demersal gillnets.
- Sacchi J (2021) Overview of mitigation measures to reduce the incidental catch of vulnerable species in fisheries. General Fisheries Commission for the Mediterranean. Studies and Reviews No. 100. FAO, Rome.
- Løkkeborg S (2011) Best practices to mitigate seabird bycatch in longline, trawl and gillnet fisheries - efficiency and practical applicability. Marine Ecology Progress Series 435:285-303. https://doi.org/10.3354/meps09227.