Gillnets

What are gillnets?

‘Gillnet’ is a collective term for wide fishing nets that resemble a curtain and hang vertically in the water column.

This versatile gear type is used around the world, at depths ranging between 15 - 140 metres. The length of the gillnet can vary from between 50 - 200 meters. Single vessels will deploy a fleet of drifting gillnets, and a fleet can total 300 -2000 metres of netting.

How do gillnets work?

As the name suggests, gillnets are designed to catch fish by their gills. The mesh of the gill net is usually just large enough to allow the head of the target species through but not their bodies. Fish will attempt to swim through the mesh and become trapped by the gills when they attempt to back out.

Unlike other kinds of fishing net, that generally encircle the target catch, gillnets do not require immediate retrieval and are left in open water for periods of time.

Types of gillnet

Drift gillnets

These nets hang below the surface of the water and drift with currents. The net is set adrift, with marker buoys attached to each end and is kept afloat by a series of weights and buoys along its length. 

Drift gillnets are usually used to target migratory open water fish species that swim in shallower depths. They can be deployed by long distance fishing fleets and may be suspended in midwater or closer to the surface. 

Stationary gillnets

Stationary or set gillnets are attached to an anchor system, or to poles imbedded into the seafloor, preventing movement. They are usually used to catch groundfish or demersal fish species that swim near the bottom. They are usually set in more localised areas or near shorelines.

Stationary gillnets can be set and left to fish independently while vessels leave to set or harvest from other nets.

Trammel nets

Trammel nets differ from other gillnets by entangling either the whole or part of the fish, rather than just the head. While gillnets commonly consist of a single layer of netting, trammel or entangling gillnets comprise three layers. The layers form a 'sandwich' of netting, where the middle layer generally has a finer mesh size, in which fish entangle themselves. 

Which species of fish are caught by gillnets?

Gillnets can be used to catch a wide variety of species depending on where in the water column they are set, and the size of the mesh. The mesh size determines which species can be caught, as different fish have different sized gills.

Gillnets can be used to catch schooling pelagic species such as herring, mackerel, squid and sardines. They may also be used to catch lumpfish, cod, hake, salmon or halibut.

Trammel nets are generally considered less selective than gillnets, catching a wider size and species range. 

What are the environmental impacts of gillnets?

All types of fishing gear can have significant impacts on the ocean environment if not sustainably managed.

Bycatch

Trammel nets in particular can be associated with high amounts of bycatch. Bycatch includes undersized fish, non-target species, and Endangered, Threatened and Protected (ETP) species – such as sharks, rays, turtles, marine mammals and birds.

The volume and type of species present in bycatch varies depending on where in the water column the gillnet is suspended and the size of the net’s mesh.

Habitats

As they have minimal interaction with the seabed, gillnets have less impact on marine habitats when in use. However, if netting is lost or abandoned it can act as 'ghost gear’.

Ghost gear or ghost fishing is when fishing gear that is no longer in use, continues to trap marine life. The gear can also beach and become entangled on coral reefs and other sensitive habitats. Discarded gear pollutes the marine environment, with long-term effects on the wider marine ecosystem.

Overfishing

Inappropriately sized gillnet mesh can result in overfishing: when too many fish in a particular stock are caught and there are not enough mature adults left to breed and maintain the fish population’s numbers. For example, mesh that is too small can result in gillnets catching too many juvenile fish, which are then jettisoned as unwanted catch or discards.

Inappropriately sized mesh use may also constitute illegal fishing, if it in contravention of local or international fishing regulations.

How can gillnet fishing reduce its environmental impacts?

All MSC certified fisheries must demonstrate their fishing operations and gear have low impacts on the environment. To be certified as sustainable against the MSC Fisheries Standard, fisheries are often required to make improvements to their fishing operations. 

Improvements include increased onboard monitoring and independent observer coverage, and mitigating interactions with non-target species through gear modifications. Voluntary improvements can include retrieval and recycling of lost gear to prevent ghost fishing.

How do gillnet fisheries reduce bycatch?

Sustainable gillnet fisheries modify their gear with bycatch reduction devices to avoid catching non-target and ETP species such as marine mammals, sharks, rays, turtles and birds.

Modifications include changing the mesh size of the gillnets and some fisheries use 'pingers': acoustic alarms attached to nets that deter marine mammals. 

To minimise interactions with diving sea birds such as eider ducks and guillemots include the use of LED lights or white mesh panels on the gillnets. Both have proved to be effective deterrents and have significantly reduced the amount of seabird bycatch associated with some gillnet fisheries.

How do gillnet fisheries prevent overfishing?

Modified netting with appropriately sized mesh can prevent overfishing. An appropriate mesh size avoids catching juvenile fish and ensures that enough mature adults are left to continue replenishing the population.

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