A gillnet is a passive fishing gear designed as a vertical wall of netting, typically constructed from nylon, suspended in the water column. The net is held upright by a buoyant headrope and a weighted footrope, causing the netting to hang like a curtain. The purpose is to intercept fish as they swim, trapping them in the mesh. The name “gillnet” comes from the most common capture method, where the fish’s gills become snagged in the mesh as the animal attempts to swim through or back out.
The Mechanics of Gillnets
The design of a gillnet is engineered to target specific fish sizes based on the net’s mesh size relative to the fish’s physical dimensions. The diamond-shaped mesh is intended to allow a fish’s head to pass through but not the widest part of its body. When the fish tries to retreat, its operculum, or gill cover, catches on the mesh strands, securing the animal.
Fish are captured by a gillnet in three primary ways: gilling, wedging, and tangling. Gilling occurs when the fish is caught precisely behind the operculum, which is the most size-selective method. Wedging happens when a fish is caught further back on the body, generally around the midsection, because it is too large to pass fully through the mesh.
The third method is entanglement, where fins, teeth, or spines catch the netting, often capturing a wider range of sizes and species. A technical design element called the “hanging ratio” controls the net’s slackness and influences the capture method. This ratio, typically between 0.25 and 0.65 in commercial use, dictates how tightly the netting is stretched, with lower ratios resulting in a looser net more prone to tangling.
Primary Types and Deployment
Gillnets are categorized based on their deployment method, determining whether they are stationary or mobile.
Set Nets
Set nets, or anchor nets, are fixed in place, typically anchored to the seabed or a shoreline structure. These nets remain in one location, often used to catch bottom-dwelling fish or species moving through nearshore channels.
Drift Nets
Drift nets are kept afloat by buoys but are not anchored, allowing them to drift freely with ocean currents or tides, often attached to a fishing vessel. This method targets migratory or pelagic species like tuna or shark in the open ocean.
Trammel Nets
A specialized design is the trammel net, which uses three layers of netting. It consists of a loose, fine-meshed inner panel sandwiched between two outer panels of larger mesh. Fish pass through the large outer mesh, hit the slack inner mesh, and push it through the opposite outer mesh, creating a pocket that traps them. Trammel nets are less size-selective than single-panel gillnets, catching a broader range of animals primarily through entanglement.
Ecological Concerns and Bycatch
A significant environmental problem with gillnets is their indiscriminate nature, resulting in substantial bycatch. Bycatch is the unintended capture of non-target species, including marine mammals, sea turtles, and seabirds, that become entangled in the net wall. Marine mammals like dolphins and porpoises, and sea turtles, are particularly vulnerable to gillnet entanglement, which can lead to drowning or severe injury.
The problem is compounded by ghost fishing, which occurs when nets are lost or abandoned in the ocean, becoming derelict fishing gear (DFG). These lost gillnets continue to catch and kill marine life for years, indiscriminately snaring fish, crustaceans, and protected species. Ghost nets are considered a lethal form of marine plastic debris, and the captured animals can attract larger predators, which also become entangled.
Regulatory Measures and Management
Fisheries management agencies employ various strategies to mitigate the environmental impacts of gillnets.
Mesh Size and Closures
One common tool is the imposition of strict mesh size restrictions, designed to allow undersized or juvenile fish to pass through, promoting size selectivity. Regulatory bodies also implement time and area closures, seasonally prohibiting gillnet use where vulnerable species, such as migrating whales or nesting sea turtles, are known to congregate.
Technological Solutions
Technological solutions are also required in some fisheries, such as the mandatory use of pingers, which are small acoustic deterrent devices. These devices emit signals intended to warn cetaceans away from the nets to reduce marine mammal bycatch. Regulations may also mandate specific gear modifications, such as the use of weak links, which allow larger entangled animals to break free.