Action: Use an otter trawl instead of a dredge
Key messagesRead our guidance on Key messages before continuing
- One study examined the effects of using an otter trawl instead of a dredge on subtidal benthic invertebrates. The study was in the Irish Sea (Isle of Man).
COMMUNITY RESPONSE (1 STUDY)
- Unwanted catch overall composition (1 study): One replicated, controlled, study in the Irish Sea found that an otter trawl caught a different species composition of unwanted invertebrate and fish species (combined) compared to two scallop dredges.
POPULATION RESPONSE (1 STUDY)
- Overall abundance (1 study): One replicated, controlled, study in the Irish Sea found no difference in total invertebrate abundance and biomass living in or on the sediment of the trawl tracks following fishing with either an otter trawl or two scallop dredges.
- Unwanted catch overall abundance (1 study): One replicated, controlled, study in the Irish Sea found that an otter trawl caught fewer unwanted invertebrates and fish (combined) compared to two scallop dredges.
OTHER (1 STUDY)
- Commercial catch abundance (1 study): One replicated, controlled, study in the Irish Sea found that an otter trawl caught similar number of commercially targeted queen scallops compared to two scallop dredges.
Dredging, for instance for bivalves, normally involves towing a heavy steel frame along the seabed, which negatively impacts subtidal benthic invertebrates due to direct physical damage and the retention of unwanted invertebrate catch, and indirectly by changes to the seabed structure and topography, such as creating dredge tracks (Currie & Parry 1996). Otter trawls have a pair of boards or metal plates (otter boards) which attach to the sides of the net and keep the net open as it is pulled through the water (Schwinghamer et al. 1998). Although otter boards can damage the seabed, they may cause less damage than a dredge due to the limited surface area in contact with the seabed in comparison to a dredge. Using an otter trawl instead of a dredge may potentially lessen the negative impact on subtidal benthic invertebrates (Hinz et al. 2012). Evidence for using other fishing gear instead of an otter trawl is summarised under “Threat: Biological resource use – Use an otter trawl instead of a beam trawl”. Evidence for other interventions related to using different fishing gear is summarised under “Threat: Biological resource use”.
Currie D.R. & Parry G.D. (1996) Effects of scallop dredging on a soft sediment community: a large-scale experimental study. Marine Ecology Progress Series, 134, 131–150.
Hinz H., Murray L.G., Malcolm F.R. & Kaiser M.J. (2012) The environmental impacts of three different queen scallop (Aequipecten opercularis) fishing gears. Marine Environmental Research, 73, 85–95.
Schwinghamer P., Gordon Jr D.C., Rowell T.W., Prena J., McKeown D.L., Sonnichsen G. & Guigné J.Y. (1998) Effects of experimental otter trawling on surficial sediment properties of a sandy‐bottom ecosystem on the Grand Banks of Newfoundland. Conservation Biology, 12, 1215–1222.
Supporting evidence from individual studies
A replicated, controlled, study (date of study not reported) in a sandy area in the north Irish Sea, Isle of Man (Hinz et al. 2012) found that an otter trawl caught fewer unwanted invertebrates and fish (combined), and a different unwanted catch species composition, compared to two dredge designs. The otter trawls caught fewer unwanted invertebrates and fish (4 individuals/1,000 m2) than the two dredge types (23–59 individuals/1,000 m2). In addition, overall unwanted catch species composition was different between the otter trawl and the two dredges (species composition data presented as graphical analyses). Unwanted otter trawl catch was reported to be dominated by fish, whereas unwanted dredge catch was dominated by invertebrates. Following fishing with either gear, there were no changes in total invertebrate abundance and biomass living in or on the sediments (raw data not presented). The otter trawl caught similar amount of commercially targeted queen scallops Aequipecten opercularis (45 scallops/1,000 m2) compared to the dredges (15–48 scallops/1,000 m2). Three queen scallop fishing gears were compared: an otter trawl, a new dredge design, and a traditional Newhaven dredge. The study site was subdivided into 12 trawling lanes (40 m wide, 1 nm long) in 20–23 m water depth. Each fishing lane was allocated to one gear design (4 lanes/design). Commercial and unwanted catches were sorted, identified, counted and weighed. Before, and seven days after fishing trials, invertebrates (size unspecified) were sampled in each lane using a 2-m beam trawl (5-min tow; 6 tows/lane) and a sediment grab (0.1 m2; 6 grabs/lane).