Deploy fishing gear at selected depths to avoid unwanted species

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

Study locations

Key messages

  • Five studies examined the effect of deploying fishing gear at selected depths to avoid unwanted species on marine fish populations. Three studies were in the Atlantic Ocean (Florida, Brazil, Canary Islands), and one study was in each of the Pacific Ocean (Hawaii) and the Tasman Sea (Australia).





Reduction of unwanted catch (5 studies): Four of five replicated studies (three controlled, one paired and controlled) in the Pacific Ocean, Atlantic Ocean and the Tasman Sea found that deploying fishing gear (longlines, handlines and traps) at selected depths, including above the seabed instead of on it, reduced the unwanted catches of five of 17 fish species, three of eight shark/ray species, non-commercially targeted fish species and Harrison’s dogfish, compared to depths usually fished. The other study found that different shark species were hooked at different depths in the water column during bottom-set longlining deployments.

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 2006 of pelagic waters in the Pacific Ocean around Hawaii (Beverly et al. 2009) found that longlines deployed at selected depths (deeper) reduced catches of five out of 17 unwanted fish species compared to conventional, shallower-set longlines. Catch rates of five out of 17 unwanted species were lower on sets using deeper longlines compared to shallower longlines: wahoo Acanthocybium solandri (0.4 vs 1.2 fish/set), dolphinfish Coryphaena hippurus (1.3 vs 3.8 fish/set), blue marlin Makaira nigricans (0.1 vs 0.4 fish/set), striped marlin Kajikia audax (0.5 vs 1.5 fish/set) and shortbill spearfish Tetrapturus angustirostris (0.1 vs 0.6 fish/set). Catch rates of most other unwanted fish (11 out of 17 species – see paper for species individual data) were similar between longline depths, including the target commercial species bigeye tuna Thunnus obesus (5.7 vs 4.7 fish/set). One unwanted species, sickle pomfret Taractichthys steindachneri, was caught more frequently with deeper longlines (3.5 fish/set) than shallower longlines (2.0 fish/set). A total of 90 longline sets (2,000 hooks/set) were deployed from a vessel fishing out of Honolulu, Hawaii, in June-December 2006. Forty-five sets were modified using 3 kg weighted lines to keep all hooks at depths of 103 m at either end to 248 m in the middle. The other 45 were conventional sets with hooks at depths between 44–211 m. The vessel used deeper and shallower sets on alternate days.

    Study and other actions tested
  2. A replicated study in 2005–2007 of fishing grounds in the North West Atlantic Ocean off Florida, USA (Morgan & Carlson 2010) found that different species of sharks were hooked on bottom-set longline gear at different water depths. Sandbar sharks Carcharhinus plumbeus were only caught at depths >20 m (21–40 m: 43, 41–60 m: 50, >60 m: 12 sharks/10,000 hook hours). Blacktip sharks Carcharhinus limbatus were caught less frequently at depths <60 m (<20 m: 41, 21–40 m: 18, 41–60 m: 15, >60 m: 91 sharks/10,000 hook hours). Three other shark species were most frequently caught between 41 and 60 m depths: tiger shark Galeocerdo cuvier (<20 m: 3, 21–40 m: 8, 41–60 m: 33, >60 m: 28 sharks/10,000 hook hours), Atlantic sharpnose Rhizoprionodon terraenovae (<20 m: 49, 21–40 m: 13, 41–60 m: 43, >60 m: 13 sharks/10,000 hook hours) and blacknose Carcharhinus acronotus (<20 m: 15, 21–40 m: 10, 41–60 m: 34, >60 m: 6 sharks/10,000 hook hours). Fifty-five gear deployments were undertaken, each with 8–10 km of longline with 250 branch lines set with 18/0 circle hooks with a 10° offset, deployed either overnight for 6–10 h or for 4–6 h during the day.

    Study and other actions tested
  3. A replicated, paired, controlled study in 2004-2005 in 608 shallow, coastal water sites in the West Atlantic Ocean off Recife, Brazil (Afonso et al. 2011) found that longline hooks deployed in the water column (pelagic) caught fewer sharks and rays (Elasmobranchii) of three of eight species, compared to hooks set close to the seabed (demersal). Fewer numbers were caught on pelagic hooks than demersal for the three most captured species: southern stingray Dasyatis Americana (pelagic: 0.5 fish/1,000 hooks, demersal: 3.3 fish/1,000 hooks; n=43); blacknose shark Carcharhinus acronotus (pelagic: 0.8 fish/1,000 hooks, demersal: 2.9 fish/1,000 hooks; n=41); and nurse shark Ginglymostoma cirratum (pelagic: 0.1 fish/1,000 hooks, demersal: 1.2 fish/1,000 hooks; n=14). A further five species were caught infrequently on pelagic hooks only (tiger shark, Galeocerdo cuvier, manta ray, Manta birostris, bull shark, Carcharhinus leucas, scalloped hammerhead, Sphyrna lewini, blacktip shark, Carcharhinus limbatus). Catch rates were so low they were not significantly different between pelagic and demersal hooks (pelagic: 0.1–0.4 fish/1,000 hooks, demersal: 0 fish/1,000 hooks; n=11). In September 2004 to August 2005, a total of 384 longline sets with 100 J-shaped hooks each were deployed, half close to the seabed and the other half suspended in mid-water, in depths of 8–14 m, 1–3 km from the coast. Hooks were baited with moray-eel Gymnothorax sp.. Sixty-two sharks and 46 rays were caught in total.

    Study and other actions tested
  4. A replicated, controlled study in 2003–2004 of seabed and near seabed in the Atlantic Ocean off the Canary Islands, Spain (Arrasate-López et al. 2012) reported that shrimp traps fished above the seabed (semi-floating) caught less unwanted fish catch (non-commercially targeted or discarded) catch than traditional bottom traps set on the seabed, and the difference decreased with overall depth. Data were not statistically tested. At 100–400 m depths, semi-floating traps caught 18 unwanted species of fish at catch rates between <0.1–858.9 g/trap/day, and bottom traps caught eight species at <0.1–24.9 g/trap/day. Between 401–800 m depth, semi-floating traps caught eight unwanted fish species (<0.1–2,241.0 g/trap/day) while bottom traps caught four species (0.4–140.6 g/trap/day). At the deepest depths (801–1,130 m), semi-floating traps caught five unwanted fish species (<0.1–186.4 g/trap/day) and four were caught in bottom traps (0.5–41.9 g/trap/day). At all but the deepest depths, conger eels Conger conger accounted for a large proportion of the unwanted catch in bottom traps and in semi-floating traps at the intermediate depths (see paper for species individual data). Target shrimp Plesionika spp. catches between floating and bottom traps varied with species and depth (see paper for data). Four research surveys were done around the Canary Islands in 2003–2004 at depths of 100–1,300 m. Two types of traps were used to target shrimp: semi-floating traps of plastic mesh (20 ×15 mm) covering a conical cylinder (56 × 57 cm), and bottom traps made of wire mesh (19 × 19 mm) and an iron rectangular frame (100 × 100 × 50 cm). Semi-floated traps were set in groups of 75 traps, 15 m apart and 2 m above the seabed (total 1,971). Bottom traps were deployed in lines of 10 traps, 50 m apart (total 487). All traps were deployed in daylight hours and baited with mackerel Scomber colias.

    Study and other actions tested
  5. A replicated, controlled study (year not stated) of two seamount marine reserves in the Tasman Sea, Australia (Williams et al. 2016) found that fishing at specific depths and times reduced unwanted catch of endangered Harrison’s dogfish Centrophorus harrissoni in a restricted commercial blue-eye trevalla Hyperoglyphe antarctica handline fishery. Catch rates were lower (0 fish/100 hooks) on seamounts defined as being ‘non-dogfish habitat’, combining selected depths (280–550 m) and time of day (daytime) compared to ‘dogfish habitat’ (280–550 m, night: 0.1 fish/100 hooks; 550–830 m, day: 0.4 fish/100 hooks). Catches of trevalla were highly variable but appeared slightly lower in the ‘non-dogfish’ habitat’ compared to the ‘dogfish habitat’ (non-dogfish: 8.2–34.3 fish/100 hooks, dogfish: 9.1–48.9 fish/100 hooks; data not tested for significance). Hydraulically powered handlines with 18 hooks each were deployed during 10 vessel trips 4–5 days long during the day in mid-water (280–550 m) and deep-water (550–830 m), and at night in mid-water. In ‘non-dogfish habitat’ 1,036 handline deployments were carried out and 407 deployments were in ‘dogfish habitat’. Handlines were deployed at randomly selected positions and hauled after 5–10 minutes or until it was felt several fish had been hooked. Details of when the study took place were not reported. Fish were identified and counted.

    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.

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Marine Fish Conservation

This Action forms part of the Action Synopsis:

Marine Fish Conservation
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