Modify gillnet or entangling (trammel/tangle) net configuration
Overall effectiveness category Awaiting assessment
Number of studies: 4
Background information and definitions
Gillnets are walls of single netting hung vertically on the seabed or in the water column, in which fish get stuck by their gills. Trammel and tangle nets (also known as entangling nets) are variations of the gillnet, used in a similar way, and may be single, double or three-walled nets in which fish or crustaceans will entangle. All of these types of net are commonly used in commercial fisheries and can be set at any depth on a range of bottom-types (Carol & García-Berthou 2007). They have floats on the upper line and weights on the bottom line and can be set anchored to the bottom or left drifting, free or connected to a vessel. Gillnets are considered as relatively selective fishing gears, in terms of fish species and sizes, but entangling nets will catch a wider variety of fish sizes. To help reduce unwanted catch, modifications to the configuration of gillnets and entangling nets may change how they fish in the water, allowing some fish to avoid capture. The behaviour of individual fish species may also influence how effective the net is at allowing escape.
Evidence for a related intervention is summarized under ‘Fishing gear modification – Use a larger mesh size’.
Carol J. & García-Berthou E. (2007) Gillnet selectivity and its relationship with body shape for eight freshwater fish species. Journal of Applied Ichthyology, 23, 654–660.
Supporting evidence from individual studies
A replicated, controlled study in 2003 of an area of seabed in the Gulf of Maine, off New Hampshire, USA (He 2006) found that modifying the configuration of a bottom gillnet (reducing the net height/number of meshes) reduced the unwanted catches of cod Gadus morhua in one of two net designs, compared to two nets of standard height. Cod catch rates were lower in one of two reduced height gillnets (eight meshes deep) compared to the other reduced height net (12 meshes deep) and two types of standard net of 25 meshes height (eight mesh: 8; 12 mesh: 14; standard cod net: 32, tie-down flounder net: 11 fish/five-net fleet). In addition, the eight mesh net had higher catches of the targeted flounder Pleuronectidae species than one of the standard 25 mesh nets (cod net), but lower than the other (flounder net) (eight mesh: 5, standard cod net: 4, tie-down flounder net: 11 fish/five-net fleet). During July and August 2003, forty comparative fishing sets of four types of gillnet (see paper for specifications) were fished on the seabed at depths between 34–76 m, at random locations in the same general area within half a mile apart. The gillnets were left overnight for 18–28 hr.Study and other actions tested
A replicated, paired, controlled study in 2000 of two coastal fished areas in the Atlantic Ocean, off the coast of North Carolina, USA (Thorpe & Frierson 2009) found that modifying the configuration of a gillnet by increasing the tension did not typically reduce the catch rates of four unwanted shark species in a commercial gillnet fishery, compared to unmodified nets. Shark catch rates were reduced in gillnets with increased tension only in nets of the larger mesh size (10.2 cm) and only for two of four species: Atlantic sharpnose Rhizoprionodon terraenovae, (modified: 0.35, unmodified: 0.58 kg/gillnet/hr) and blacknose Carcharhinus acronotus (modified: 0.04, unmodified: 0.13 kg/gillnet/hr); but not blacktip Carcharhinus limbatus (0.09 vs 0.13 kg/gillnet/hr) or bonnethead sharks Sphyrna tiburo (0.23 vs 0.31 kg/gillnet/hr). Catch rates of all four species were not significantly different between nets of 7.6 cm mesh size (modified: 0.05–2.11; unmodified: 0.08–2.46 kg/gillnet/hr). In addition, there was no difference in catch rates of the target fishery species Spanish mackerel Scomberomorus maculatus between modified and unmodified gillnets of the same mesh size (see paper for data). Data was collected from deployments of four gillnets of two mesh sizes (7.6 and 10 cm) by a commercial fishing vessel in May-September 2000. For each mesh size, one gillnet had increased tension (using larger floats on the top-rope and heavier weights on the bottom rope) and was set end to end (15 m apart) with the other, unmodified, net. Between 24–34 sets were made with each mesh size.Study and other actions tested
A replicated study in 2010 of an area of sandy-mud seabed in the Adriatic Sea, Italy (Grati et al. 2015) found that modifying the configuration of a gillnet by increasing the twine diameter reduced the discards of non-commercial, but not commercial, species (fish and invertebrates). The percentage of discarded non-commercial species (fish and invertebrates) in catches decreased with increases in twine diameter (0.30 mm: 27%, 0.25 mm: 30%, 0.22 mm: 33%, 0.20 mm: 41%, 0.18: 39%), but there was no differences between diameters for discarded commercial species (5–7%). The average number of species caught was also lower for the thickest twine diameter compared to the three thinnest (0.30 mm: 7, 0.25 mm: 8, 0.18–0.22 mm: 9). In addition, catch rates of the target fish species, common sole Solea solea, were similar between twine diameters (data reported as statistical results). During July–October 2010, a total of 20 gillnet sets (deployments) were fished for 10–12 h. For each set, 50 single-twine gillnets (10 of each twine diameter: 0.18, 0.20, 0.22, 0.25 and 0.30 mm) were randomly arranged in one group, 1.5 m apart. All species caught were identified and separated into commercially valuable catch and unwanted catch.Study and other actions tested
A replicated, paired, controlled study in 2010–2013 of two estuaries in North Carolina, USA (Rudershausen et al. 2015) found that rectangular mesh gillnets of different mesh sizes and depths reduced catches of unwanted fish including red drum Sciaenops ocellatus and undersized individuals of the commercial target species southern flounder Paralichthys lethostigma compared to conventional diamond mesh nets. Numbers of unwanted fish were lower in rectangular mesh nets than diamond mesh, irrespective of the depth profile (number of meshes) of the net (rectangle: 0.4–0.5 fish/90 m, square: 4.1–4.8 fish/90 m) and mesh size (rectangle: 0.4 fish/30 m, square: 1.2–1.4 fish/30 m). Red drum and undersized southern flounder catches were lower in rectangular than diamond meshes for both net depth profiles (red drum: 0.0 vs 0.2–0.3 fish/90 m; flounder: 0.0–0.1 vs 0.5 fish/90 m). Catches of both species were lower in rectangular nets compared to diamond mesh nets for two of three mesh size comparisons (14.0 and 15.2 cm mesh sizes for red drum and 14.6 and 15.2 cm for undersized flounder – see paper for data). Legal-sized catches of target flounder were similar in rectangular and diamond mesh nets for both low profile (0.9 vs 1.1) and high profile nets (0.4 vs 0.7/90 m), but lower in all three rectangular mesh sizes than corresponding diamond nets (14 cm: 0.2 vs 0.4, 14.6 cm: 0.3 vs 0.5, 15.2 cm: 0.1 vs 0.4/30 m). Experimental gillnet deployments were made in the Neuse River estuary in April–June 2010 and in the Newport River Estuary in April–October in 2011–2013. In 2010, paired deployments (85) of one of two rectangular mesh nets, 20 meshes (‘low profile’) and 33 meshes (‘high profile’) deep, and one diamond mesh gillnet (20 meshes deep), all 14 cm mesh size were made, set parallel to the shore for 12 h. In 2011–2013, a total of 150 paired deployments were made of three rectangular mesh nets and three diamond mesh nets of different mesh sizes (14.0 cm, 14.6 cm or 15.2 cm), set for 12 h parallel to shore.Study and other actions tested