Action

Reduce duration of fishing gear deployments

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

Study locations

Key messages

  • Four studies examined the effects of reducing the duration of time that fishing gear is deployed in the water on marine fish populations. Two studies were in the North Sea. One study was in the Atlantic Ocean (USA) and one was in both the Barents Sea and Atlantic Ocean (Norway/USA). 

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (2 STUDIES) 

  • Survival (2 studies): Two replicated, controlled studies in the North Sea found that survival of unwanted plaice and/or sole released after capture in beam or pulse trawls was higher after shorter duration trawl deployments, but that the opposite was true for plaice captured in otter trawls, over tow durations of between one and two hours. 

BEHAVIOUR (0 STUDIES)

OTHER (2 STUDIES) 

  • Reduction of unwanted catch (1 study): One of two replicated studies (one paired and controlled) in the Barents Sea/Atlantic Ocean and the Atlantic Ocean found that catch rates of unwanted sharks caught in longline gear decreased with decreasing time the gear was deployed in the water, over durations of up to 10 hours. The other study found that shorter tow durations caught similar amounts of small haddock, but more small cod, than longer durations, in bottom trawls fished for between five minutes and one hour.
  • Improved size-selectivity of fishing gear (1 study): One replicated, paired, controlled study in the Barents Sea/Atlantic Ocean found that varying bottom trawl fishing durations between five minutes and two hours had no effect on the size-selectivity of Atlantic cod, haddock or long rough dab.

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, paired, controlled study in 1985–1989 in three areas of seabed in the Barents Sea and Atlantic Ocean off Norway and USA (Godø et al. 1990) found that shorter bottom trawl tow durations did not improve the size-selectivity of Atlantic cod Gadus morhua, haddock Melanogrammus aeglefinus and long rough dab Hippoglossoides platessoides, and more small cod but not haddock were caught, for different tow durations between 5 min to 2 h. Across all three trials, the average fish length was similar between tow durations, for cod (trials 1 & 2, 15 min: 26–32 cm, 30 min: 29–32 cm, 60 min: 32–35 cm; trial 3, 5 min: 50 cm, 30 min: 50 cm), haddock (trials 1 & 2, 15 min: 18–33 cm, 30 min: 17–33 cm, 60 min: 19–32 cm, 120 min 33 cm; trial 3, 5 min: 27 cm, 30 min: 27 cm, 120 min: 33 cm) and long rough dab (trials 1 & 2 only, 15 min: 24–26 cm, 30 min: 24–25 cm, 60 min: 32–35 cm). In addition, in two of two trials there were no differences in catch rates of small haddock between tow durations (5–60 min), however, the catch rates of small cod increased with decreasing shorter tow durations (see original paper for data). Two trials (one and two) were done in the Barents Sea in October 1988 (nine parallel deployments by two vessels: three each of 15, 30 and 60 min) and February 1989 (24 deployments: 16 × 5 mins and 8 × 30 mins). Additional data from a trial on the Georges Bank in January 1985 (trial three)was also analysed (64 deployments: two each of 15, 30, 60 and 120 min at eight stations). Tow durations were based on the standard tow duration for trawl surveys (from 30 min to 2 h).

    Study and other actions tested
  2. A replicated, controlled study in 1972–1982 in an area of seabed in the North Sea (van Beek et al. 1990) found that survival of sole Solea solea and plaice Pleuronectes platessa discards was higher in shorter deployments of beam trawls, but not of plaice in otter trawls, for tow durations between one and two hours. For beam trawls, survival of sole and plaice 84 hours after capture was higher for 60-minute deployments (sole: 21%, plaice: 19%) compared to 120-minute deployments (sole: 7%, plaice: 10%). For otter trawls, survival of plaice 84 hours after capture was lower for 60-minute deployments (11%) compared to 100–105-minute deployments (33%). Commercial fishing vessels carried out 12 × 60-minute and 15 × 120-minute-long beam trawl deployments between November 1979 and December 1982 in the North Sea (location not reported). Gear was towed at 5–5.5 knots. A research vessel carried out 3 × 60-minute and 4 × 100–105-minute-long deployments using an otter trawl between November 1972 and February 1975 towed at 3.5 knots (North Sea, exact location not reported). Sole of 20–28 cm length and plaice of 20–30 cm length were removed from each catch onboard and placed in seawater tanks (40 × 60 × 12 cm). Survival was monitored every 12 hours until all fish had died or the end of the survey.

    Study and other actions tested
  3. A replicated study in 2005–2007 of a fished area of seabed in the Atlantic Ocean off Florida, USA (Morgan & Carlson 2010) found that catch rates of unwanted sharks (Chondrichthyes) on bottom-set longlines were lower at shorter times the gear had been in the water, and varied between species with depth, at fishing durations of up 10 hours. For the four main species, the overall probability of capture (hook being bitten) increased most from 5 hours after the start of gear deployment compared to the first 5 hours of the sets, and for individual species the average amount of time hooks were in the water prior to being bitten was 4 hours for sandbar Carcharhinus plumbeus and blacknose sharks Carcharhinus acronotus, 5 hours for blacktip sharks Carcharhinus limbatus, and 9 hours for bull sharks Carcharhinus leucas, respectively (data reported as statistical model results). Sandbar sharks 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 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). Blacknose sharks and two other shark species were most frequently caught between 41 and 60 m depths: blacknose (<20 m: 15, 21–40 m: 10, 41–60 m: 34, >60 m: 6 sharks/10,000 hook hours), 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). Fifty-five longline deployments were undertaken (8–10 km of longline, 18/0 circle hooks with a 10° offset). Longlines were deployed either overnight for 6–10 h or for 4–6 h during the day. Hook timers on each hook recorded shark capture times.

    Study and other actions tested
  4. A replicated, controlled study in 2014–2015 on an area of seabed in the southern North Sea, Netherlands and UK (van der Reijden et al. 2017) found that reducing the length of the hauls using a pulse trawl increased survival of plaice Pleuronectes platessa compared to standard length hauls, over haul durations of 60–130 minutes. When haul duration was 60–70 minutes, more plaice survived compared to standard 100–130 minute hauls (data not reported). Two fishing vessels were used to carry out three surveys with two short (60–70 minute) hauls and four surveys with standard (100–130 minute) hauls using standard fishing operations with a pulse trawl, between November 2014 and September 2015. After sorting the catch on deck, 40 fish below commercial size from each haul were kept in seawater tanks and fed every 24 hours. Survival was monitored daily for at least 21 days.

    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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