A gillnet is a wall of netting that hangs in the water column, typically made of monofilament or multifilament nylon.
Mesh sizes are designed to allow fish to get only their head through the netting but not their body. The fish's gills then get caught in the mesh as the fish tries to back out of the net. As the fish struggles to free itself, it becomes more and more entangled. A variety of regulations and factors determine the mesh size, length, and height of commercial gillnets, including area fished and target species. There are two main types of gillnets:
- Set gillnets are attached to poles fixed in the substrate or an anchor system to prevent movement of the net.
- Drift gillnets are kept afloat at the proper depth using a system of weights and buoys attached to the headrope, footrope, or floatline.
Target Species
- Salmon
- Groundfish:
- Cod
- Haddock
- Pollock
- Flounder
- Hake
- Barracuda
- Herring
- Mullet
- Rockfish
- Seabass
- Shad
- Sharks
- Sturgeon
- Swordfish
- Tuna
Fisheries Regions
- Alaska
- Northeast
- Northwest
- Southeast
- Southwest
Risks to Sea Turtles
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.
Risks to Marine Mammals
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. Species most commonly caught in gillnets include:
- Large whales
- Humpback whales
- Fin whales
- Right whales
- Harbor porpoise
- Dolphins
- Bottlenose dolphins
- Common dolphin
- Right whale dolphins
- Steller sea lions
- Gray seals
Bycatch Reduction
Many factors have been studied in trying to minimize incidental capture, including adding breakaway panels and changing net slack, mesh size, and set depth.
Current regulated mitigation measures include:
- Seasonal and regional pinger (acoustic device) used to warn cetaceans are required in California and Maine through New Jersey.
- Prohibitions on large mesh gillnets.
- Closures of certain fishing areas at certain times (known as time/area closures).
- Inserting weak links into gillnets to allow large whales to break free when entangled.
More Information
Pair of bottlenose dolphins. Credit: NOAA Pacific Islands Fisheries Science Center/Lisa Morse.
A humpback mother and calf swimming off the West Coast. Credit: NOAA Fisheries/Ed Lyman. NOAA Fisheries Permit #14682.
Oceanic whitetip shark. Hawaiʻi longline fishermen use leaders to connect weighed branch lines and baited hooks. A new rule prohibits steel wire leaders in the Hawaiʻi deep-set longline fishery, starting May 31, 2022. The rule aims to increase the survival of hooked oceanic whitetip sharks caught as bycatch in the Pacific Islands region. Credit: Andy Mann
Right whale #3853 swimming north offshore of South Carolina on Jan. 20, 2011 with a series of fresh propeller wounds running across its back. The whale was observed 5 days previously offshore of Georgia without propeller wounds. It is unknown whether the whale survived its wounds or not, as it has not been re-sighted since. Vessel collisions are a leading cause of right whale mortality. Credit: EcoHealth Alliance (NOAA permit #594-1759).