Use a separator trawl
Overall effectiveness category Awaiting assessment
Number of studies: 2
Background information and definitions
Capture of fish in trawls depends on the behavioural reaction of fish to the presence of the net. When a trawl approaches, fish turn and swim in the same direction as the net until exhausted, at which point they enter the net, albeit at different vertical positions. Some species (e.g. haddock Melanogrammus aeglefinus, whiting Merlangius merlangus) swim high in the net and others (e.g. cod Gadus morhua, flatfish) stay low in the trawl. In separator trawls, a horizontal net panel is fitted into the trawl net that splits the trawl into upper and lower sections and into separate upper and lower codends. Catch is then separated into each codend, which can be made of different mesh sizes to allow more or less of the catch in each to be retained. For example, in mixed flatfish and haddock fisheries, separator trawls with larger mesh upper codends may allow less unwanted cod to be caught whilst retaining target catches (Ferro et al. 2007).
Evidence for similar interventions describing the effects of using modified trawl types to reduce unwanted catch is summarized under ‘Fishing gear modification - Use a topless (coverless) trawl’ and ‘Use an electric (pulse) trawl’.
Ferro R.S.T., Jones E.G., Kynoch R.J., Fryer R.J. & Buckett, B. E. (2007). Separating species using a horizontal panel in the Scottish North Sea whitefish trawl fishery. ICES Journal of Marine Science, 64, 1543–1550.
Supporting evidence from individual studies
A replicated, randomized study in 1994 in two areas of seabed in the North Sea, UK (Cotter et al. 1997) found that using a separator trawl separated the majority of unwanted Atlantic cod Gadus morhua into a lower trawl net from other target fish species that were retained in an upper trawl net, and size-selectivity in the lower trawl net increased with increasing mesh size. Across five trawl deployments, the probability of cod entering the lower codend was 0.87 (min: 0.74, max: 0.96). The length at which cod had a 50% chance of escape was 33.8 cm in the lower codend with a 100 mm mesh size (min: 26.2, max: 53.1 cm) and 57.6 cm in the lower codend with a 140 mm mesh size (min: 42.5, max: 80.1 cm). Results were not tested for statistical significance. Five experimental trawl deployments were undertaken with a separator trawl off Whitby, northeast England in March 1994 and in the Moray Firth, Scotland in November 1994. The trawl was a standard trawl divided horizontally by a panel behind the footrope into two codends. The upper codend had 100 mm mesh size and two randomly assigned lower codends had 100 mm or 140 mm mesh size. Full details of trawl design are provided in the original study.Study and other actions tested
A replicated study in 1993–1994 of an area of seabed in the Atlantic Ocean off the coast of Portugal (Campos & Fonseca 2004) found that shrimp trawl nets combining a separator panel with a square mesh escape window reduced the catch of one of two unwanted fish species, compared to a square mesh window alone. Overall, the percentage escape of boarfish Capros aper was higher by 10–44% (105–1,430 kg) from nets with a separator panel compared to 17% (896 kg) with no panel and amounts of escaped blue whiting Micromesistius poutassou were similar (67–81%; panel: 58–187 kg, no panel: 107 kg). In addition, the escape rates of boarfish increased with increasing mesh size of the square mesh panel (70 mm: 10%, 100 mm: 44%). Escaped catch of the target species rose shrimp Parapenaeus longirostris and Norway lobster Nephrops norvegicus was <1–28% in nets with panels compared to 1–24% with no panel. Four fishing trials were undertaken, each testing one of four trawl nets: three with different separator panel/escape window mesh size combinations and one with the window alone (see paper for specifications). For each net, six or seven experimental hauls were conducted in July 1993 to May 1994 off the Algarve coast. Fish and crustaceans that escaped through the square mesh window were collected in a small mesh cover mounted over the escape window. Codend (top and bottom) and cover catches were sorted by species, weighed and lengths recorded.Study and other actions tested