Change the towing speed of a trawl net
Overall effectiveness category Awaiting assessment
Number of studies: 1
Background information and definitions
Trawling is a method of fishing that involves pulling a fishing net (trawl) through the water behind one or more vessels. Fish entering the net will either be retained by it or may escape through the gaps in the mesh of the trawl netting during fishing. Many factors can influence the likelihood of a fish being caught or escaping from a trawl (termed selectivity or efficiency) including species, size of fish, time of day, trawl configuration and towing speed. The towing speed of a net may affect the size and shape of the meshes through which fish may escape. But it may also affect how likely a fish is to avoid entering the net in the first place. Fish behaviour differs on encountering different trawl nets (Krag et al. 2014) and the towing speed of the net may further change the ability of fish to escape in response to the altered hydrodynamics that may require a different swimming behaviour. Fish have been seen to actively escape trawl nets by accelerating in bursts or changing direction, or, more commonly, escaping in response to contact with the net (Jones et al. 2008). Depending on the species, a faster or slower towing speed may allow more fish to escape.
Evidence for similar interventions is summarized under ‘Deployment of fishing gear and mode of operation - Reduce duration of fishing gear deployments’ and ‘Reduce the hauling speed of a trawl net’. See also ‘Handling of Catch’.
Krag L.A., Herrmann B. & Karlsen J.D. (2014) Inferring Fish Escape Behaviour in Trawls Based on Catch Comparison Data: Model Development and Evaluation Based on Data from Skagerrak, Denmark. PLoS ONE, 9, e88819.
Jones E., Summerbell K. & O’Neill F. (2008) The influence of towing speed and fish density on the behaviour of haddock in a trawl cod-end. Fisheries Research, 98, 166–174.
Supporting evidence from individual studies
A replicated, paired study in 1995 of one area of seabed in the North Sea off southern Norway (Dahm et al. 2002) found that changing the towing speed of a bottom trawl net did not improve the size selectivity of unwanted small cod Gadus morhua and haddock Melanogrammus aeglefinus. Across both vessels and net sizes, the length at which fish had a 50% chance of escaping was similar between two towing speed ranges for both cod (slower: 30–34 cm, faster: 30–34 cm) and haddock (slower: 28–32, faster: 28–33). In April 1995, trawl towing speed was tested on two fishing vessels (20 deployments each), fishing at the same time on the same fishing grounds (exact location not reported). Vessels had different sizes of bottom trawl nets, one a standard size and one a scaled-down size but both had identical codends (see paper for specifications). For the larger net, catches were analysed for towing speeds above and below 3.0 m/s and for the smaller net <3.5 versus >3.5 m/s. Fish retained by the larger net were sampled in twin codends, one with the test net and one with a small mesh to sample all sizes of fish. On the smaller net a cover was attached to the codend to collect fish escaping through the meshes. All codend and cover catches were sorted and weighed by species and total lengths recorded.Study and other actions tested