Gill Netting (image © Seafish)
Gill nets are monofilament nylon nets that are set on the seabed and left to fish. Each end is anchored and the net is held to the seabed by a weighted ‘foot-rope’ and held up by a floating ‘head line.’ The size of mesh and the length of ‘soak time’ is specific to the species of fish being targeted and is governed by legislation for each fishery. Local fisheries bylaws govern mesh sizes within the Cornwall inshore fisheries district, and EU regulations govern mesh sizes outside of the 12 mile limit. Nets cannot be used during periods of strong tidal flows so around Cornwall most netters will stop fishing during spring tides.
Gill nets are designed to be selective. Fish smaller than the mesh size are able to pass through unharmed, and those larger than the mesh are deflected away when they hit the net (except in the case of tangle nets). Fish of the correct size are selected as their heads fit through the mesh and they are then caught by the gills.
Each gill net can be up to 114 metres long and several nets (up to 30) are tied together to make a tier. Large netting boats will deploy 5 or 6 tiers of nets per day, which can be almost 20km of nets. Compared with trawling, gill netting using far less fuel and thus has a lower carbon footprint as a fishing method.
Cornish fishermen use a variety of different netting methods:
Problems associated with gill netting
Because gill nets are set to fish and then left, there is scope for mobile fishing gear to be pulled through set nets even despite the best efforts of fishermen to communicate the net positions. This often results in the loss of part or all of the net. Practices such as wreck netting and netting over rocky reefs can also result in snagged or lost nets. Lost nets can continue to fish and are often termed ‘ghost nets’. They pose a real threat to marine life. The length of time they continue to fish depends on many factors, though fishermen argue that strong currents will roll the nets until they are no longer a threat. More evidence is needed to support this.
Another issue with gill nets is the risk to larger marine animals such as dolphins, porpoises, sharks, rays and seals. Monofilament nets are very difficult to see when they are in the water, and large predators can get accidentally caught in the nets. The scale of this problem has been highlighted by Cornwall Wildlife Trusts Marine Strandings Network in 2015, 96 dead cetaceans were found washed up on Cornish beaches, 12% of these were shown to have evidence of accidental bycatch and drowning in fishing nets. Acoustic deterrent devices known as ‘pingers’ have been developed to significantly reduce the risk of bycatch by emitting a regular sonic noise which scares cetaceans (dolphins, porpoises and whales) away from nets. Use of pingers is now a legal requirement for all netting vessels over 10m in length, however smaller vessels are not yet required to use pingers. This is in part because the large pingers currently used on offshore vessels are so loud there are concerns that if these were to be used inshore it could potentially displace cetaceans from important feeding areas. Cornwall Wildlife Trust has trialled smaller, less problematic pinger devices on inshore nets, which resulted in successfully reducing cetacean activity around nets by around 80% Bannana Pinger Report These 'bannana' pingers are available from British company, Fishtek Marine.
ICES advice 2018 on bycatch of Cetaceans and other marine animals suggests that in our area Celtic sea and wester approaches by catch of Harbour porpoise are above sustainable levels. The total harbour porpoise bycatch in relevant fisheries in Subarea 27.7 (southern part of the Celtic Seas) in 2016 was likely to have been between 620 and 1391 individuals. These figures represent approximately 1.1% and 2.4% of the harbour porpoises present in the subarea. The upper estimate exceeds the threshold of 1.7% of abundance and would be deemed unacceptable by ASCOBANS.
Bycatch of seabirds is also an issue for gill net fisheries in certain applications (notably bass nets in St Ives bay) as is bycatch of rare elasmobranch species such as long nosed skate and angel shark. Tangle nets are particularly effective at catching crawfish, a species that since the 1960’s has become very rare in southwest waters. This along with high catches by scuba divers is thought to explain the rapid reduction in crawfish populations documented at the end of the 20th century.
In 2018 Cornwall Inshore Fisheries and Conservation authority passed a bye law prohibiting the use of nets in Cornish estuaries. This will potentially benefit many species including bass, bream, seatrout and Salmon.