Use an alternative commercial fishing method

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages





  • Reduction of unwanted catch (9 studies): Seven of nine replicated studies (two controlled, one randomized, controlled, one paired, controlled) in the Arafura Sea, Greenland/Norwegian Sea, Norwegian Sea, Atlantic Ocean, Mediterranean Sea, Gulf of Maine, Coral Sea, Tyrrhenian Sea and Kattegat and Skagerrak found that using an alternative method of fishing caught fewer discarded fish species and reduced the catches of unwanted (discarded or non-commercial species) fish overall, and of immature halibut, haddock, Atlantic cod, bluefin tuna and over half of the individual fish species. One study found that an alternative fishing method caught larger (and more likely to be mature) unwanted hammerhead sharks. The other study found that sizes of striped sea bream, annular sea bream and red mullet were similar in catches between gear types.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A replicated, controlled study in 1991 of fishing grounds over mud and coral reef in the Arafura Sea, off Northern Australia (Ramm et al. 1993) found that using an alternative method of fishing (semi-pelagic trawl, towed just above the seabed) to target snapper Lutjanus spp. reduced the unwanted fish catch overall, and of just over half of the species individually, compared to traditional demersal (bottom-towed) trawls. Catch rates of unwanted fish were lower in semi-pelagic trawls (195 kg/tow) than traditional demersal trawls (453 kg/tow). Catch rates of 75 unwanted fish species were lower in semi-pelagic trawls, 52 fish species were similar between trawl types, and seven species were caught more frequently in semi-pelagic trawls (see paper for species individual data). In addition, catches of marketable commercial fish were similar between trawl types for 10 of 16 species groups (semi-pelagic trawl: 392 kg/tow, demersal trawl: 320 kg/tow). Fourteen tows were undertaken in March 1991 for each of a semi-pelagic trawl (0.3 m above the seabed) with a buoyed headline, and a traditional demersal trawl. Both trawl nets had a similar codend volume with 112 mm mesh size, 65 m trawl widths and were towed for 3 h. Full details of trawl design are provided in the original study. All catches were weighed, and fish identified.

    Study and other actions tested
  2. A replicated study in 1994 of a wide area of seabed in the Greenland and Norwegian Seas, Norway (Huse et al. 1999) found that fishing using gillnets reduced the capture of unwanted, immature Greenland halibut Reinhardtius hippoglossoides, compared to trawling or longlining. Gillnets caught lower proportions of immature halibut (male: 0%, n=411; female: 6%, n=5,740) than longlines (male: 21%, n=1,485, female: 23%, n=4,661) and trawls (male: 37%, n=3,309; female: 71%, n=2,402). In addition, the average length of halibut caught was higher for gillnets (66 cm) compared to trawl- (50 cm) and longline-caught halibut (60 cm). Data were collected from scientific fishing deployments using three different gears in August–September 1994 at different water depths (400–1,400 m). A total of 130 deployments were made of gillnet fleets with 70–110 mm mesh sizes, 71 sets of longlines totalling 335,310 hooks baited with mackerel and squid (species not given), and 70 deployments by a trawler towing a 136 mm mesh codend. Haul speed and duration were not given.

    Study and other actions tested
  3. A replicated study in 1996 of a seabed area in the Norwegian Sea, off northern Norway (Huse et al. 2000) found that fishing using gillnets reduced the capture of small, unwanted cod Gadus morhua and haddock Melanogrammus aeglefinus compared to trawling or longlining. In two of two trials, gillnets caught larger cod than trawls or longlines (gillnet: 82–86 cm, trawl: 67-69 cm, longline: 68–69 cm). In addition, gillnets caught fewer cod compared to trawls in both cases, and similar amounts in one case and more in one case compared to longlines (gillnet: 1,657–3,391 kg, trawl: 3,238–4,096 kg, longline: 1,773–1,754 kg) (data were not tested for statistical significance). Trawls captured larger haddock than longlines in one case, and smaller in one case (trawl: 53 cm, longline: 51–55 cm,). Fewer haddock were caught in longlines compared to trawls in both cases (longline: 665–1,055 kg, trawl: 1,153–1,973 kg) (data not tested for significance). Three fishing vessels tested a different gear type in one area of 10 × 40 nautical miles in February 1996 over six days. Depths were between 227–259 m. The gillnetter fished nine fleets of 186 mm mesh and two fleets of 200 mm mesh. The longliner used squid and mackerel bait on fleets of 6,300 or 8,230 hooks; 26 fleets were fished. The trawler fishes a standard trawl with a twin codend and 140 mm mesh size.

    Study and other actions tested
  4. A replicated, randomized controlled study in 1996–1997 of an area of pelagic water in the Atlantic Ocean, off Portugal (Erzini et al. 2002) reported that pelagic purse seine nets appeared to discard fewer species of cartilaginous fish (Chondrichthyes) such as sharks and rays, and bony fish (Osteichthyes) compared to other types of fishing gear. Data were not tested for statistical significance. For shark/ray species, of which 85% were usually discarded, 1 species was caught in pelagic purse seines, 2 in demersal purse seines, 7 in trammel nets, 10 in fish trawls and 13 in crustacean trawls. For bony fish, of which 34% were usually discarded and 54% frequently discarded, 34 species were caught in pelagic purse seines, 55 in both demersal purse seines and crustacean trawls, 49 in trammel nets and 66 in fish trawls. In March 1996 to June 1997, a total of 57 fishing trips on 24 vessels were sampled at random by fisheries observers in Algarve waters. Overall, data were collected from 18 pelagic purse seine sets, 33 demersal purse seine sets, 11 trammel net sets, 30 crustacean trawl tows and 36 fish trawl deployments. All fish were identified and counted. Fished depths were 30–500 m, depending on gear type. Due to the variety of vessels and gears sampled, specific gear details were not given in the original paper.

    Study and other actions tested
  5. A replicated, paired, controlled study in 1996–1997 of two coastal areas of the Mediterranean Sea, Italy (Fabi et al. 2002) found that a gillnet, a modified trammel net and a standard trammel net caught similar average sizes of striped sea bream Lithognathus mormyrus, annular sea bream Diplodus annularis and red mullet Mullus barbatus. Across areas and for the same mesh size, average sizes of all species were similar between the gillnet and trammel nets: striped bream (45 mm, gillnet: 16–17 cm, trammel nets: 16–18 cm; 70 mm, gillnet: 25–26 cm, trammel nets: 23–26 cm), annular bream (45 mm only, gillnet: 12–13 cm, trammel nets: 12–14 cm) and red mullet (45 mm only, gillnet: 16 cm, trammel nets: 15–16 cm). Between March 1996 and June 1997, a total of 29 trials were carried out in the Adriatic and 43 in the Ligurian Sea. A gillnet with one monofilament panel, a trammel net with an inner panel of polyamide monofilament and outer panels of twisted polyamide filament, and a standard commercial trammel net with all panels made of twisted polyamide filament, were tied end-to-end and their positions changed for each trial. The nets were lowered into shallow (4–15 m) water at dusk and retrieved the following dawn. All fish were identified, and fish length measured.

    Study and other actions tested
  6. A replicated study in 2005–2006 of four seabed areas in the Gulf of Maine, USA (Ford et al. 2008) found that fishing using longlines reduced the capture of unwanted cod Gadus morhua compared to otter trawls in a haddock Melanogrammus aeglefinus fishery. In all four areas fished, the ratio of cod weight to haddock weight was smallest with a longline compared to an otter trawl (longline: 0.01–0.07 otter trawl: 0.2–0.83 cod/haddock caught). In addition, the ratio of cod weight to all target species (including winter flounder Pseudopleuronectes americanus and yellowtail flounder Limanda ferruginea) was smallest when using a longline (longline: 0.01–0.05, otter trawl: 0.2–0.46). In June 2005 - January 2006 a total of 146 longline hauls and 159 otter trawl hauls were sampled on the Georges Bank. All longliners fished with 12/0 circle hooks. Longliners fished mainly in depths shallower than 50 m whereas otter trawlers fished in deeper waters also. Data were obtained from fishers and fisheries observers. Specifications of the otter trawl gears were not described.

    Study and other actions tested
  7. A replicated study in 1996–2006 in 10 shallow coastal areas in the Coral Sea off the northeast coast of Australia (Noriega et al. 2011) found that drumlines caught larger individuals of scalloped hammerhead shark Sphyrna lewini, compared to surface mesh nets. Average shark length was greater in drumlines (2.1 m) than mesh nets (1.9 m). Size at maturity for the region was reported to be from 2.0 m total length for females and from 1.8 m total length for males. A total of 128 sharks were caught in drumlines and 350 in mesh nets. Data were collected from 344 drumline and 35 mesh net deployments operated by the Queensland Shark Control Program at 10 locations along the east coast of Queensland between 1996 and 2006. Individual drumlines consisted of a single baited hook suspended by chain, two metres below a float anchored to the seabed. Bait was 1–2 kg of mullet Mugil cephalus or shark flesh (species not given). Mesh nets were 186 m long and 6 m deep (mesh 0.5 m). Gears were deployed in 8–10 m depth, 300–1,000 m from, and parallel to, the beach. Both gears were checked by trained fishing contractors and drumlines rebaited, on 15–20 days each month. Full gear specifications are given in the paper.

    Study and other actions tested
  8. A replicated, controlled study in 2003 of a pelagic area in the Tyrrhenian Sea, Italy (Sinopoli et al. 2012) found that using a different type of seine net reduced the capture of Atlantic bluefin tuna Thunnus thynnus compared to a conventional seine net in the dolphinfish Coryphaena hippurus net fishery. Numbers of tuna captured were lower in the modified net compared to the conventional net (modified: 1 fish, conventional: 5–11 fish). Numbers of commercial target dolphinfish captured were similar between gear types (modified: 85–270 fish, conventional: 86–268 fish). In August–November 2003, a modified seine net with a 15 mm mesh coded and 30 mm side meshes, and a conventional purse seine with a of 30 mm mesh codend and two 50 mm side meshes were deployed off Capo d’Orlando, Sicily, in depths of 600–800 m. In total, 48 hauls were carried out, six hauls/gear type/month. Anchored palm leaves Phoenix canariensis were placed 500 m from each other to attract fish. Nets were placed around the batches of palm leaves. Fish were counted and lengths measured.

    Study and other actions tested
  9. A replicated study in 2011–2013 in seabed areas in Kattegat and Skagerrak, Sweden (Hornborg et al. 2017) found that fishing for Norway lobster Nephrops norvegicus with baited creels reduced the capture of unwanted fish compared to bottom trawling using a size-sorting escape grids and mixed bottom trawling. The amount of catch discarded/landed amount of Nephrops was lower in creels (sharks/rays Elasmobranchii: 0.00, plaice Pleuronectes platessa: 0.00, sole Solea solea: 0.00, other flatfish: 0.01, cod Gadus morhua: 0.07, other cod-like fish: 0.02 kg discard/kg landed Nephrops) compared to both trawling types, and highest with mixed trawling (grid trawling, sharks/rays: 0.00, plaice: 0.15, sole: 0.01, other flatfish: 0.86, cod: 0.06, other gadoid fish: 0.10 kg discard/kg landed Nephrops; mixed trawling, sharks/rays: 0.15, plaice: 0.44, sole: 0.02, other flatfish: 0.75, cod: 0.51, other cod-like fish: 0.55 kg discard/kg landed Nephrops). In addition, discarded Norway lobster was reduced in creels (creels: 0.13, grid and mixed trawling: 0.71 kg discard/kg and average size was larger (creel: 46 mm, grid trawl: 38 mm, mixed trawl: 38 mm). Data were obtained from the Swedish fishing fleet logbooks between 2011 and 2013. Typically, mixed trawlers used a minimum mesh size of 90 mm with selective panels of various sizes fitted. Grid trawlers used a 35 mm Swedish grid and a 70–89 mm square mesh codend. Creels were static, baited pots arranged as 40–70 connected pots.

    Study and other actions tested
Please cite as:

Taylor, N., Clarke, L.J., Alliji, K., Barrett, C., McIntyre, R., Smith, R.K., and Sutherland, W.J. (2021) Marine Fish Conservation: Global Evidence for the Effects of Selected Interventions. Synopses of Conservation Evidence Series. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

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Marine Fish Conservation

This Action forms part of the Action Synopsis:

Marine Fish Conservation
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