Modify the design of dredges

Overall effectiveness category Likely to be beneficial
Number of studies: 5
Share Tweet Email

View assessment score

How is the evidence assessed?

Effectiveness

40%
Certainty

42%
Harms

19%

Calculating overall effectiveness category

Background information and definitions

Dredging, for instance for bivalves, normally involves towing a heavy steel frame along the seabed, which negatively impacts subtidal benthic invertebrates directly, due to 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). Dredge design can be modified in order to reduce or remove negative impacts on subtidal benthic invertebrates, such as changing the tooth spacing or mesh size, or reducing sediment penetration or bottom-contact (Frandsen et al. 2015). Evidence for other interventions related to the use of dredges are summarised under “Threat: Biological resource use – Cease or prohibit dredging”, “Use lower water pressure during hydraulic dredging”, “Hand harvest instead of using a dredge”, “ Use an otter trawl instead of a dredge”, “Use alternative means of getting mussel seeds rather than dredging from natural mussel beds”, “Use an otter trawl instead of a dredge”, and in “Species management – Cease or prohibit the harvest of scallops”.

_{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.}

_{Frandsen R.P., Eigaard O.R., Poulsen L.K., Tørring D., Stage B., Lisbjerg D. & Dolmer P. (2015) Reducing the impact of blue mussel (Mytilus edulis) dredging on the ecosystem in shallow water soft bottom areas. Aquatic Conservation: Marine and Freshwater Ecosystems, 25, 162–173}

Study locations

Key messages

Six studies examined the effects of modifying the design of dredges on subtidal benthic invertebrate populations. Four were in the North Atlantic Ocean (Portugal) and two were 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 a new design of scallop dredge caught a similar species composition of unwanted catch to a traditional dredge.

POPULATION RESPONSE (3 STUDIES)

Overall abundance (2 studies): One of two controlled studies in the North Atlantic Ocean and in the Irish Sea found that a new dredge design damaged or killed fewer invertebrates left in the sediment tracks following dredging. The other found no difference in total invertebrate abundance or biomass living in or on the sediment tracks following fishing with two dredge designs.
Unwanted catch overall abundance (2 studies): Two controlled studies (one replicated) in the North Atlantic Ocean and the Irish Sea found that a modified or a new design of bivalve dredge caught less unwanted catch compared to traditional unmodified dredges.

Unwanted catch condition (6 studies): Six controlled studies (one replicated and paired, four replicated) in the North Atlantic Ocean and the Irish Sea found that new or modified bivalve dredges damaged or killed similar proportions of unwanted catch (retained and/or escaped) compared to traditional or unmodified designs, three of which also found that they did not reduce the proportion of damaged or dead unwanted crabs (retained and/or escaped).

OTHER (1 study)

Commercial catch abundance (1 study): One replicated, controlled, study in the Irish Sea found that a new dredge design caught a similar amount of commercially targeted queen scallops compared to a traditional dredge.

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

A controlled study in 1999 in one sandy area in the North Atlantic Ocean, off the southwest coast of Portugal (Gaspar et al. 2001) found that a modified bivalve dredge caught less unwanted invertebrate catch, but damaged or killed similar proportions of unwanted invertebrates, compared to a traditional unmodified dredge. The proportion of unwanted individuals/tow was lower for the modified dredge (30–35%), compared to the traditional dredge (42–62%). This pattern was also true when looking at the proportion by weight (modified: 24–26%; traditional: 25–47%). However, the modified dredge did not have statistically lower proportion of damaged invertebrates overall (6–9%) compared to traditional dredges (7–14%), or lower proportion of dead invertebrates (modified: 6–8%; traditional: 6.5–11%), but this effect varied with species (see paper for details). The modified dredge had a metallic grid for retaining bivalves, compared to the traditional dredge which had a net bag. All species other than the commercially targeted smooth clam Callista chione were considered to be unwanted catch. In March, 12 tows/design were investigated at 8–10 m depth. A net bag was fitted to the end of each dredge to retain the caught organisms that would otherwise escape through the dredge mesh. For each dredge design, invertebrates were identified, counted, and given a score of 1–4 according to the amount of damage (1 = perfect condition, 4 = dead).
Study and other actions tested
Referenced paper
Gaspar M.B., Dias M.D., Campos A., Monteiro C.C., Santos M.N., Chícharo A. & Chícharo L. (2001) The influence of dredge design on the catch of Callista chione (Linnaeus, 1758). Hydrobiologia, 153-167.
A replicated, paired, controlled study in 1994–1995 in 13 soft seabed sites in the northern Irish Sea, Isle of Man (Veale et al. 2001) found that scallop dredges with shorter teeth caused similar damage to unwanted invertebrate catch, compared to dredges with longer teeth. The damage sustained by unwanted invertebrates was similar when caught in the dredge with shorter and longer teeth (results not shown). A modified dredge design, with shorter teeth and smaller belly ring, was compared to a traditional design (Newhaven with spring-teeth). In 1994 and 1995, up to 13 fishing grounds were surveyed in June and October (at the start and end of the closed fishing season for great scallops Pecten maximus). In each area, one boat simultaneously towed a group of four modified dredges, and a group of four traditional dredges over 2 nm (one group on either side). Unwanted invertebrate catch (crabs, starfish, urchins, whelks, bivalves, hermit crabs, octopus) was sorted to species level, counted, and given a damage score (1= no visible damage, 4= crushed/dead).
Study and other actions tested
Referenced paper
Veale L.O., Hill A.S., Hawkins S.J. & Brand A.R. (2001) Distribution and damage to the by-catch assemblages of the northern Irish Sea scallop dredge fisheries. Journal of the Marine Biological Association of the United Kingdom, 81, 85-96.
A replicated, controlled study in 1999 in one area of sandy seabed in the North Atlantic Ocean, off northwestern Portugal (Gaspar et al. 2002a) found that modifying dredge tooth spacing did not reduce the proportion of damaged or dead individuals (unwanted catch and escapees) for either the overall combined invertebrate and fish community or for crabs. The proportions of individuals in the overall community that entered the dredge and were damaged or dead were similar using a 2 cm (damaged: 3–5%, dead: 1–1.5%), 4 cm (damaged: 3–4%, dead: 0–1%) or 6 cm (damaged: 4–8%, dead: 1%) tooth spacing design. The proportions of crabs that were damaged or dead were similar using a 2 cm (damaged: 8–25%, dead: 4–11%), 4 cm (damaged: 7–19%, dead: 1–9%) or 6 cm (damaged: 7–29%, dead:1–20%) tooth spacing design. Three tooth-spacing designs (2, 4 and 6 cm) were compared. In July, two bivalve dredges with different designs were towed simultaneously side-by-side at 8–10 m depth (three tows/design; 15 min/tow). A net bag was fitted to the end of each dredge to retain the caught organisms that would otherwise escape through the dredge mesh. For each dredge design, catches were sorted by species group, counted and given a score of 1–5 according to the amount of damage (1 = good condition, 5 = crushed/dead). The effect of tooth spacing was examined for the overall unwanted community (invertebrates and fish) and for crabs.
A replicated, controlled study in 1999 in one area of sandy seabed in the North Atlantic Ocean, off northwestern Portugal (Gaspar et al. 2002b) found that modifying the net mesh size on a dredge did not reduce the proportion of damaged or dead individuals (unwanted catch and escapees) for either the overall combined invertebrate and fish community or for crabs. The proportions of individuals in the overall community that entered the dredge and were damaged or dead were similar using a 35 mm (damaged: 3–8%, dead: 1%), 40 mm (damaged: 4–5%, dead: 1%) or 50 mm (damaged: 3–5%, dead: 0–1.5%) mesh size design. The proportions of crabs that were damaged or dead were similar using a 35 mm (damaged: 8–24%, dead: 4–20%), 40 mm (damaged: 10–29%, dead: 5–8%), or 50 mm mesh size design (damaged: 7–25%, dead: 1–11%). Three mesh sizes (35 mm, 40 mm and 50 mm) were compared. In July, two bivalve dredges with different designs were towed simultaneously side-by-side at 8–10 m depth (three tows/design; 15 min/tow). A net bag was fitted to the end of each dredge to retain the caught organisms that would otherwise escape through the dredge mesh. For each dredge design, catches were sorted by species group, counted and given a score of 1–5 according to the amount of damage (1 = good condition, 5 = crushed/dead). The effect of mesh size was examined for the overall unwanted community (invertebrates and fish) and for crabs.

A replicated, controlled study in 1999 in one area of sandy seabed in the North Atlantic Ocean, off northwestern Portugal (Gaspar et al. 2002c) found that modifying the tooth spacing and net mesh size on dredge did not reduce the proportion of damaged or dead individuals (unwanted catch and escapees) for either the overall combined invertebrate and fish community or for crabs. The proportions of individuals in the overall community that entered the dredge and were damaged or dead were similar for the nine designs tested (damaged: 3–8%, dead: 0–1.5%). The proportions of crabs that were damaged or dead were similar for the nine designs tested (damaged: 7–29%, dead: 1–20%). Nine combinations of three mesh sizes (35, 40 and 50 mm) and three tooth-spacings (2, 4 and 6 cm) were compared. In July, two bivalve dredges with different designs were towed simultaneously side-by-side at 8–10 m depth (three tows/design; 15 min/tow). A net bag was fitted to the end of each dredge to retain the caught organisms that would otherwise escape through the dredge mesh. For each dredge design, catches were sorted by species group, counted and given a score of 1–5 according to the amount of damage (1 = good condition, 5 = crushed/dead). The effect of mesh size was examined for the overall community (invertebrates and fish) and for crabs.
Study and other actions tested
Referenced paper
Gaspar M. (2002) Influence of mesh size and tooth spacing on the proportion of damaged organisms in the catches of the Portuguese clam dredge fishery. ICES Journal of Marine Science, 59, 1228-1236.
A replicated, controlled study in 2001 in one area of sandy seabed in the North Atlantic Ocean, off southwestern Portugal (Gaspar et al. 2003) found that a new dredge design with a shorter mouth did not reduce the proportion of damaged or dead invertebrates caught with the dredge, compared to two traditional dredge designs, but damaged and killed lower proportions of invertebrates left in the tracks following dredging. The proportions of individuals that entered the dredge and were damaged or dead were similar using the new design (damaged: 5%, dead: 5%), a traditional design with a long mouth (damaged: 3%, dead: 3%) and another traditional design with a short mouth (damaged: 7%, dead: 6%). However, the proportion of invertebrates left in the tracks following dredging were lower using the new design (damaged: 17%, dead: 17%), compared to the long-mouthed traditional design (damaged: 42 %, dead: 29%) or the short-mouth traditional design (damaged: 26%, dead: 18%). Three dredge designs were compared: a new design with a shorter mouth and metallic grid instead of a net bag to retain the catch, a traditional design with a long mouth and more teeth, and a traditional design with a short mouth (“north dredge”). A total of 12 tows (4/design; 5 min/tow) were undertaken in June at 8–10 m depth. A net bag was fitted to the end of each dredge to retain the caught organisms that would otherwise escape through the dredge mesh. Divers also sampled the sediment in the dredge tracks after each tow to assess the proportion of invertebrates not caught but left damaged or dead due to dredging (54 quadrats/tow; extracted using a 5 mm mesh sieve). All invertebrates were identified, counted, weighed and given a damage score (1= in good condition, 4= crushed/dead).
Study and other actions tested
Referenced paper
Gaspar M.B., Leitão F., Santos M.N., Chícharo L., Dias M.D., Chícharo A. & Monteiro C.C. (2003) A comparison of direct macrofaunal mortality using three types of clam dredges. ICES Journal of Marine Science, 60, 733-742.
A replicated, controlled, study (date unspecified) in an area of sandy seabed in the north Irish Sea, Isle of Man (Hinz et al. 2012) found that a new design of scallop dredge caught similar species of unwanted invertebrates and fish, but in lower amounts, compared to a traditional scallop dredge. Overall unwanted species composition (invertebrates and fish) was similar between the new and the traditional dredge (composition data presented as graphical analyses). Unwanted catch from both dredges was reported to be dominated by invertebrates. The new dredge design caught fewer unwanted invertebrates and fish (23 individuals/1,000 m²) than the traditional dredge (59). In addition, following fishing impacts, there were no changes in total invertebrate abundance and biomass living in or on the sediments for any of the gears (raw data not presented). The new dredge design caught similar amount of commercially targeted queen scallops Aequipecten opercularis (48 scallops/1,000 m²) compared to the traditional dredge (15). Two queen scallop dredges were compared: a new dredge design with a rubber lip instead of traditional teeth, and a traditional Newhaven dredge. The study site was subdivided into eight 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 (invertebrates and fish) 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).
Study and other actions tested
Referenced paper
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.

Please cite as:

Lemasson, A.J., Pettit, L.R., Smith, R.K. & Sutherland, W.J. (2020) Subtidal Benthic Invertebrate Conservation. Pages 635-732 in: W.J. Sutherland, L.V. Dicks, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2020. Open Book Publishers, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

All the journals searched for all synopses

This Action forms part of the Action Synopsis:

Subtidal Benthic Invertebrate Conservation

Related Actions

Actions	Effectiveness	Studies	Category
Fit one or more soft, semi-rigid, or rigid grids or frames to trawl nets and use square mesh instead of a diamond mesh at the codend Action Link	Unknown effectiveness (limited evidence)	1	Synopsis Link
Use a midwater/semi-pelagic trawl instead of bottom/demersal trawl Action Link	Likely to be beneficial	1	Synopsis Link
Limit the maximum weight and/or size of bobbins on the footrope Action Link	No evidence found (no assessment)	0	Synopsis Link