Fit a size-sorting mesh funnel (a sieve net) to a prawn/shrimp trawl net
Overall effectiveness category Awaiting assessment
Number of studies: 3
Background information and definitions
Trawling is considered a non-selective method of fishing as large proportions of fish that enter the gear may be retained in the net. To help reduce the capture of unwanted fish, a sieve net (a “bycatch reduction device”) can be used to help sort the catch into different species or sizes and allow the escape of some. A sieve net is attached to the full circumference of prawn or shrimp trawl nets and tapers to an apex near the trawl’s codend. An exit opening is made where the sieve net and codend join, allowing fish and other larger animals to pass through the sieve and escape, whereas the shrimp pass through the sieve and into the codend (Revill & Holst, 2004). Mesh sieve or sorting panels work in a similar manner, intending to separate unwanted species from prawn catches by mechanical and behavioural means (Santos et al. 2018).
Evidence for a similar intervention relating to prawn/shrimp trawl nets is summarized under ‘Fishing gear modification - Fit a size-sorting escape grid (rigid or flexible) to a prawn/shrimp trawl net’.
Revill A. & Holst R. (2004) The selective properties of some sieve nets. Fisheries Research, 66, 171–183.
Santos J., Herrmann B., Mieske B., Krag L.A., Haase S. & Stepputtis D. (2018) The efficiency of sieve-panels for bycatch separation in Nephrops trawls. Fisheries Management and Ecology, 25, 464–473.
Supporting evidence from individual studies
A replicated, paired, controlled study in 2000–2001 of a seabed area in the Flemish Banks in the North Sea, Belgium (Polet et al. 2004) reported that shrimp trawls fitted with a sieve net reduced the catches of unwanted non-commercial fish compared to a standard trawl net without a sieve net. For non-commercial fish species, the average percentage reduction in catch with a sieve net compared to without was: 49% for gobies Pomatoschistus spp., 45% for dragonets Callionymus spp., 76% for seasnail Liparis liparis, 35% for pogge Agonus cataphractus, 61% for bullrout Myoxocephalus scorpius, 37% for pipefish Syngnathus spp., 61% for five-bearded rockling Ciliata mustela, 22% for pout Trisopterus luscus and 99% for anchovy Engraulis encrasicholus. In addition, the sieve net showed poor size-selectivity for all commercial fish species with lengths below 10 cm (i.e. lower escape rates), however >10 cm the selection improved with increasing length (see paper for data). Target brown shrimp Crangon crangon catches were reduced by ≤15%. Data were collected from 72 trawl deployments on a commercial fishing vessel between April 2000 and January 2001. Paired deployments (one on each side of the vessel) were done of two standard design shrimp beam trawls (20 mm codend mesh); one fitted with a 70 mm mesh sieve net (116 meshes wide at the front and 16 at the rear, 60 meshes deep) with an escape outlet in the lower trawl body ahead of the codend (see original paper for specifications). A small mesh (11 mm) cover attached over the escape opening collected catch escaping through the sieve net escape. Sub-samples of non-commercial fish catch in the cover and codend was weighed and counted.Study and other actions tested
A replicated, paired, controlled study in 1999–2000 of bottom fishing grounds in the North Sea, England, UK (Revill & Holst 2004) found that shrimp trawl nets fitted with a sieve net (four designs) caught fewer unwanted fish compared to a conventional trawl without a sieve net. Across all four sieve net designs, average catch numbers of unwanted fish were lower in trawls with a sieve net compared to without, for: plaice Pleuronectes platessa (with: 9–15 fish/haul, 12–21 fish/haul), dab Limanda limanda (with: 14–35 fish/haul, 8–55 fish/haul) and whiting Merlangius merlangus (with: 64–133 fish/haul, 73–151 fish/haul). In addition, overall discarded catch (fish and invertebrates combined) was reduced in sieve nets by 56–90% in weight, and losses of target brown shrimp Crangon crangon ranged between 8–21%. Paired deployments were undertaken in The Wash fishing grounds using standard shrimp trawls (20 mm mesh codend) fitted with one of four sieve net designs (see original paper for specifications) and standard shrimp trawls without sieve nets. Trawls were towed at 2–3 knots for 1 h and 480 tows were completed.Study and other actions tested
A replicated, paired, controlled study in 2006–2007 of two inshore areas of seabed in the North Sea, England, UK (Catchpole et al. 2008) found that shrimp trawl nets fitted with a sieve net reduced the catches of unwanted fish compared to trawls without sieve nets. The average weight of unwanted fish catch was lower in trawls with sieve nets than those without (with: 6 kg, without: 11 kg). In addition, unmarketable small brown shrimp Crangon crangon and marketable shrimp catches were reduced with sieve nets by 8% and 14% respectively (with: 22–24 kg, without: 27–30 kg). Sampling was done between January 2006 and 2007 at two coastal sites from five commercial vessels fishing with twin beam trawls. Two beam trawls, were fished simultaneously; one with a sieve net and one without and data collected for 98 valid deployments. The catches from each trawl net was sorted into marketable and non-marketable sizes of shrimp and fish, counted and weighed.Study and other actions tested