Modify a bottom trawl to raise parts of the gear off the seabed during fishing

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

Study locations

Key messages

  • Two studies examined the effects of modifying a bottom trawl to raise parts of the gear off the seabed during fishing on marine fish populations. One study was in the Gulf of Carpentaria (Australia) and one was in the Atlantic Ocean (USA). 





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, randomized, controlled study in 1993 in two areas of seabed in the Gulf of Carpentaria, Australia (Brewer et al. 1996) found that unwanted catch of sharks Carcharinidae, other sharks/rays Elasmobranchii and non-commercial fish species was lower in bottom trawls rigged to fish above the seabed (no groundrope), compared to a conventional trawl with the groundrope in contact with the seabed. Overall, catch rates of unwanted sharks were lower for both raised trawl configurations, 0.4–0.5 m above the seabed (3 kg/h) and 0.8–0.9 m above the seabed (1 kg/h) compared to the conventional trawl (5 kg/h). No other sharks/rays were caught in either modified trawl, but 58 kg/h were caught in the conventional trawl. Catch of unwanted fish species was also lower with the trawl 0.4–0.5 m above the seabed (62 kg/h) and at 0.8–0.9 m above the seabed (12 kg/h) than in the conventional trawl (190 kg/h). In addition, catch rates were similar between both modified trawls and the conventional trawl for all 12 target or commercially valuable species caught (see original paper for data). In November 1993, two 18 × 18 km sites were trawled at 41–58 m depths with three trawl designs with 50 mm mesh codends: two trawls with the groundrope removed, and rigged to fish at either 0.4–0.5 m or 0.8–0.9 m above the seabed by means of floats on the headline floats and weights on the footrope, and one conventional trawl with a 170 kg footrope (see original paper for gear configurations). Sites were trawled alternately for three days at a time (five trawls a day, in separate grids) using a different gear each day, with the gear type randomly re-ordered each time. All catch species were identified and weighed.

    Study and other actions tested
  2. A replicated, controlled study in 2011 of an area of seabed in the Gulf of Maine, North Atlantic Ocean off Portland, USA (He et al. 2015) found that the effect of modifying a shrimp trawl to raise the connecting wires between the doors and net (bridles) off the seabed on unwanted fish catch varied between species, compared to conventional trawls with bridles in contact with the seabed. Of seven unwanted (non-commercial target) fish species/groups monitored, average catch rates were lower in modified trawls for three: long rough dab Hippoglossoides platessoides (3 kg/h), redfish Sebastes fasciatus (2 kg/h) and all flounders combined Pleuronectoidei (4 kg/h), compared to conventional trawls (plaice: 2, redfish: 1, flounders: 3 kg/hr). Catch rates of silver hake Merluccius bilinearis, Atlantic herring Clupea harengus, red hake Urophycis chuss and witch flounder Glyptocephalus cynoglossus were similar between gears (raised bridles: 2–38 kg/h, conventional: 1–35 kg/h). In addition, catch rates of the commercial target Northern shrimp Pandalus borealis was similar between gears (raised bridles: 141 kg/h, conventional: 146 kg/h). Data were collected on a commercial vessel from 30 trawl deployments/gear type between March–May 2011. For the modified gear (raised bridles), the wire bridles were replaced with polypropylene rope, while the conventional gear had steel wires. Codend mesh size was 43 mm. Tows were between 90–155 m, 2.3 knots and 1 h duration. All catch was 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.

Where has this evidence come from?

List of journals searched by synopsis

All the journals searched for all synopses

Marine Fish Conservation

This Action forms part of the Action Synopsis:

Marine Fish Conservation
What Works 2021 cover

What Works in Conservation

What Works in Conservation provides expert assessments of the effectiveness of actions, based on summarised evidence, in synopses. Subjects covered so far include amphibians, birds, mammals, forests, peatland and control of freshwater invasive species. More are in progress.

More about What Works in Conservation

Download free PDF or purchase
The Conservation Evidence Journal

The Conservation Evidence Journal

An online, free to publish in, open-access journal publishing results from research and projects that test the effectiveness of conservation actions.

Read the latest volume: Volume 21

Go to the CE Journal

Discover more on our blog

Our blog contains the latest news and updates from the Conservation Evidence team, the Conservation Evidence Journal, and our global partners in evidence-based conservation.

Who uses Conservation Evidence?

Meet some of the evidence champions

Endangered Landscape ProgrammeRed List Champion - Arc Kent Wildlife Trust The Rufford Foundation Save the Frogs - Ghana Mauritian Wildlife Supporting Conservation Leaders
Sustainability Dashboard National Biodiversity Network Frog Life The international journey of Conservation - Oryx Cool Farm Alliance UNEP AWFA Bat Conservation InternationalPeople trust for endangered species Vincet Wildlife Trust