Use a topless (coverless) trawl
Overall effectiveness category Awaiting assessment
Number of studies: 4
Background information and definitions
Trawls may capture significant numbers of unwanted species when they are towed through the water. 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 (Thomsen 1993). ‘Topless’, ‘coverless’ or ‘cutaway’ trawls are trawl nets with a reduced top panel, or longer headlines than footropes, and exploit these behavioural differences to allow escape of unwanted species by swimming upwards away from the footrope and out of the trawl.
Evidence for similar interventions describing the effects of using modified trawl types to reduce unwanted catch is summarized under ‘Fishing gear modification – Use a separator trawl’ and ‘Use an electric (pulse) trawl’.
Thomsen B. (1993) Selective flatfish trawling. ICES Marine Science Symposium, 196, 161–164.
Supporting evidence from individual studies
A replicated, paired, controlled study in 2005 in an area of seabed in the Farne Deeps in the North Sea off northeast England, UK (Revill et al. 2006) found that using a topless prawn bottom trawl net (a ‘cutaway’ trawl) reduced the discarded catches of one of four commercial fish species, compared to a conventional commercial trawl net. Total catches of undersized whiting Merlangius merlangus (<27 cm) were lower with the topless/cutaway trawl (whiting: 10,169 fish; compared to the conventional trawl (cutaway: 10,169 fish, conventional: 4,006 fish). Total catches of undersized Atlantic cod Gadus morhua (<35 cm) and lemon sole Microstomus kitt (<25 cm) were similar between trawl types (cod, cutaway: 110 fish, conventional: 58 fish; lemon sole, cutaway: 1,024 fish, conventional: 895 fish). More undersized plaice Pleuronectes platessa (<25 cm) were caught with the cutaway trawl (358 fish) than the conventional trawl (187 fish) however the authors noted that this may have been due to the cutaway trawl maintaining more consistent contact with the seabed. Commercial target Norway lobster Nephrops norvegicus catch weights were similar in the cutaway (1,613 kg) and conventional trawls (1,536 kg). Data were collected from 26 experimental trawl net deployments on a commercial vessel in March/April 2005 using twin trawls towed parallel. One of the trawls had a new design of trawl (‘cutaway’) with a shortened headline, and the other side was a conventional trawl used in the commercial fishery for Nephrops. All trawls also included a mandatory square mesh panel in the upper panel. Full details of trawl designs are provided in the original study.Study and other actions tested
A replicated, paired, controlled study (year not stated) of multiple fished areas of seabed in the North Sea off Norway and Sweden (Krag et al. 2015) found that use of topless bottom trawls (two designs) reduced the catches of unwanted larger haddock Melanogrammus aeglefinus and of unwanted larger cod Gadus morhua in one of two cases, compared to standard trawls. Total catch numbers of haddock were lower in both large and small topless trawl designs compared to standard trawls (topless: 20–352 fish, standard: 662–2,467 fish). Catches of cod were lower in a larger, high headline topless trawl (topless: 583, standard: 1,755/trawl) but similar in the smaller trawl with low headline (topless: 941, standard: 1,305/trawl) compared to standard trawls. For both species, the effect was significant only for larger individuals (haddock >19–23 cm and cod >34 cm length). Numbers of commercial target Norway lobster Nephrops norvegicus were similar between trawl types (topless: 1,666–1,681 individuals, standard: 1,702–2,057 individuals). Data were collected from 51 comparative trawl deployments in two trials on different commercial vessels (one large, one small), both fishing a topless trawl towed parallel to a similar-size standard trawl. One trial tested a small topless trawl with the upper wings removed and the head rope cut 6.4 m back, and the other a larger topless trawl with the head rope cut 11.3 m back (see paper for full specifications). Catches in each codend were sorted by species and fish lengths measured. Hauls with fewer than 10 individuals of a species were not included for analysis. Study year was not reported.Study and other actions tested
A replicated, paired, controlled study in 2011 in bottom fishing grounds in the Gulf of Maine off Boston, USA (Eayrs et al. 2017) found that using a topless trawl reduced the unwanted catches of cod Gadus morhua in a mixed species bottom fishery, compared to a standard trawl. The catch rates of cod were lower with the topless trawl compared to the standard trawl (topless: 182 kg/hr, standard: 374 kg/h). In addition, catch rates were similar between trawl types for four of five species/species groups of commercial value: yellowtail flounder Limanda ferruginea (topless: 83 kg/h, standard: 82 kg/h), skates Rajidae (topless: 36 kg/h, standard: 32 kg/h), witch flounder Glyptocephalus cynoglossus (topless: 22 kg/h, standard: 25 kg/h) and spiny dogfish Squalus acanthias (topless: 22 kg/h, standard: 27 kg/h). Catches of long rough dab Hippoglossoides platessoides were lower in the topless trawl (topless: 36 kg/h, standard: 49 kg/h), however, the reduction was almost all undersized individuals (data reported as statistical result). Data were collected in May and June 2011 from 30 paired deployments on a commercial fishing vessel using the topless and standard trawl net designs towed in parallel (45 min, 2.6 kt). The headline length of the topless and standard trawls were 46 m and 21 m, respectively, and headline to footrope ratios were 1.7:1 and 0.8:1, respectively. See original paper for full gear specifications.Study and other actions tested
A replicated, controlled study in 2014–2015 in fishing grounds in the North Sea, Skagerrak and Baltic Sea, Northern Europe (Mortensen et al. 2017) found that switching to using a coverless trawl, and various other different trawl gear types/designs, reduced the total discarded catch of seven commercial target species (six fish and one crustacean) compared to conventional trawl types, and the overall effect of using different gear types varied between regions. For one of 12 individual vessels that opted to use a topless trawl (as one of two different gear types used), average total discarded catch of the seven species was lower in the modified trawls (both types combined) compared to the conventional trawl type (modified: 6 kg, conventional: 16 kg). Across all vessels, in two of three regions the average total discarded catch of the seven species was lower in modified trawl types (Skagerrak: 18 kg, Baltic Sea: 256 kg) compared to conventional trawl gears (Skagerrak: 25 kg, Baltic Sea: 328 kg). In the other region (North Sea) discarded catch was higher (modified: 18 kg, conventional: 13 kg). Data were collected from a trial run from December 2014 to July 2015 involving 12 Danish bottom trawlers who were given free choice of trawl gear, in place of the regulatory gear being used, with the challenge of reducing discards. One of the 12 vessels used a 120 mm topless trawl with 1.4–1.6 m vertical opening, as well as a trawl net with a square mesh escape panel (see original paper for other gears types used/vessel). Each vessel switched between using modified and conventional gears between fishing trips.Study and other actions tested