Change the size of the main body 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, and trawl configuration. One factor that may affect selectivity is gear size. Fish behaviour can differ on encountering different trawl nets (Krag et al. 2014) and changing the size of trawl gear alters the overall dimensions (such as the width and height at the trawl mouth) and can subsequently affect the numbers of fish able to avoid it. Fish have been observed to actively escape trawl nets by accelerated swimming in bursts or changing direction, or, more commonly, escaping in response to contact with the net (Jones et al. 2008).
For related interventions describing other modifications to the overall design of different types or parts of trawl gear is summarized under ‘Fishing gear modification - Modify the design or configuration of trawl gear (mixed measures)’, ‘Decrease the circumference or diameter of the codend of a trawl net’, ‘Modify the design or configuration of trawl doors’, ‘Modify a bottom trawl to raise parts of the gear off the seabed during fishing’ and ‘Modify design or arrangement of tickler chains/chain mats in a bottom trawl’.
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 study in 1997 of an area of seabed in the North Sea off southern Norway (Dahm et al. 2002) found that reducing the size of a trawl net body did not affect the size-selectivity of Atlantic cod Gadus morhua and haddock Melanogrammus aeglefinus compared to a larger trawl body. The length at which fish had a 50% chance of escaping was similar between two trawl body sizes for both cod (small: 32.8 cm, large: 31.8 cm) and haddock (small: 27.0 cm, large: 27.1 cm). In addition, the size-selection ranges of retained fish of both species were similar between trawl sizes (small: 4.4–8.2 cm, large: 4.9–7.7 cm). In March/April 1997, two bottom trawl nets, one small (‘minihopper’) and one larger (‘codhopper’), of different body dimensions but identical codends, were tested in 23 trawl deployments by a research fishing vessel (see paper for specifications). The small trawl was a specially designed version of a larger trawl which was commonly used for stock assessments. Average measurements for the small trawl were 3.3 m (height) and 20.7 m (spread) and for the larger trawl 3.7 m (height) and 24.9 m (spread). Covers attached to the codend collected fish escaping through the net. All codend and cover catches were sorted and weighed by species and total lengths recorded.Study and other actions tested