A gillnet is a wall of netting suspended in the water that catches fish by their gills as they try to swim through it. The mesh is sized so that a target fish can push its head partway through but gets stuck when its gill covers flare against the mesh, preventing it from backing out. Gillnets are one of the oldest and most widely used commercial fishing methods in the world, employed in rivers, coastal waters, and the open ocean.
How Fish Get Trapped
The basic principle is simple: a fish swimming into the net doesn’t see it in time and pushes forward into a mesh opening. What happens next depends on the fish’s size relative to the mesh. There are actually four distinct ways a gillnet captures fish. “Gilling” is the classic method, where the mesh catches behind the fish’s gill covers and holds it in place. “Wedging” is similar but involves the mesh gripping tightly around the body just behind the gills. “Entangling” happens when the fish twists or wraps itself in the netting by its fins, tail, or other body parts. And “snagging” catches fish by protruding body parts like spines or fins.
Fish that die in the net typically have mesh wrapped around their gill covers, which prevents them from pumping water over their gills. They essentially suffocate. Fish that are retrieved alive are usually wedged or gilled in a way that still allows some water flow.
What Gillnets Are Made Of
Modern gillnets are constructed from thin monofilament nylon or similar synthetic fibers, chosen specifically because they’re nearly invisible underwater. A gillnet looks like a long, flat curtain. The top edge (the headrope) is strung with floats to keep it upright, while the bottom edge (the footrope) is weighted to hold the net vertical in the water column. The result is a nearly invisible barrier that can stretch hundreds of meters long and several meters deep.
Mesh size is the single most important design choice. Smaller mesh catches smaller fish; larger mesh lets small fish pass through while targeting bigger species. Selection is really about girth, not length. A fish’s maximum body circumference relative to the mesh opening determines whether it gets caught, passes through, or bounces off. Fishery managers use mesh size regulations as a primary tool to protect juvenile fish and non-target species.
Set Gillnets vs. Drift Gillnets
There are two main categories of gillnet, and they work quite differently.
Set gillnets are stationary. They’re anchored to the seafloor or attached to poles fixed into the substrate and left in place, usually for 24 to 48 hours before being hauled in. These are typically used to catch bottom-dwelling fish like California halibut, white seabass, and Pacific herring. In rivers, one end may be anchored on shore while the rest stretches perpendicular to the current to intercept migrating salmon. Many set gillnets now include breakaway panels and anchors designed to release non-target animals.
Drift gillnets, by contrast, are attached to the fishing boat and trail behind it below the surface. A system of weights and buoys positions the net at whatever depth suits the target species. Drift nets are used for fish that swim through open water: chinook salmon, sockeye salmon, swordfish, and others. Because they move with the current, drift gillnets cover more water but are harder to control in terms of what they catch.
Bycatch and Wildlife Impact
Gillnets are effective at catching target fish, but they are also one of the most significant sources of bycatch for marine mammals, sea turtles, and seabirds. Because the netting is nearly invisible, animals that aren’t being targeted swim into it and become entangled.
The numbers are substantial. In European waters alone during 2023, observers recorded 952 marine mammals, 1,238 seabirds, and 186 marine turtles caught as bycatch across all fishing gear types. But observed catches represent only a fraction of total bycatch, since observers are present on a small percentage of fishing trips. Estimated totals are far higher. In the Norwegian Sea, an estimated 2,267 harbor porpoises were killed in set gillnets in a single year. Danish and Swedish gillnet fisheries killed an estimated 2,089 harbor porpoises in 2020, with the true number potentially as high as 6,798. In the Bay of Biscay, strandings data suggested that roughly 11,330 common dolphins died from bycatch during the winter of 2023 alone, the highest level recorded since monitoring began in 1990.
The most devastating example is the vaquita, a small porpoise found only in Mexico’s Gulf of California. Fewer than 19 individuals remained as of 2023, down from thousands a few decades ago. The primary cause of their decline is drowning in gillnets set for totoaba, a large fish whose swim bladder is valuable on the black market. Despite a gillnet ban in the vaquita’s habitat, illegal fishing continues, and the species is on the brink of extinction.
Ghost Fishing
When gillnets are lost or abandoned at sea, they don’t stop catching animals. These “ghost nets” continue to trap fish, marine mammals, and turtles as they drift or sit on the seafloor. In shallower, warmer waters, biofouling (the buildup of algae and marine organisms on the net) gradually makes the mesh visible and heavy, and the net loses its ability to effectively fish within roughly a year. The netting itself begins to weaken at the knots first, becoming structurally unusable after about 240 days in temperate water.
In deeper, colder waters the picture is worse. Biofouling happens much more slowly in the deep ocean, so lost nets can remain functional and dangerous for several years before they degrade enough to stop catching animals. Ghost nets are a major component of marine plastic pollution, and international cleanup efforts now target known accumulation areas.
Reducing the Damage
Several strategies aim to make gillnets less harmful to non-target species. The most widely studied is the acoustic pinger, a small battery-powered device attached to the net that emits sounds designed to warn marine mammals away. In a study of small-scale gillnet fisheries in northern Peru, pingers reduced whale entanglements by about 53% compared to nets without them. Pingers have shown similar results for harbor porpoises in European fisheries, though their effectiveness varies by species and location, and some researchers worry that animals may habituate to the sounds over time.
Other approaches include using shorter soak times (the period the net is left in the water), which reduces the window for bycatch and improves the condition of caught fish. Time and area closures restrict gillnet use during seasons or in locations where vulnerable species are most active. Mesh size regulations help avoid catching juveniles. And breakaway panels built into the net allow larger animals like sea turtles or marine mammals to tear free rather than becoming trapped.
Some regions have moved to ban gillnets entirely. California banned drift gillnets in its waters starting in 2024, and the European Union has debated similar restrictions in areas critical for harbor porpoises and dolphins. The tension between gillnets’ effectiveness as a low-cost fishing tool and their outsized impact on protected species remains one of the central conflicts in fisheries management worldwide.