Cease or prohibit dredging

Overall effectiveness category Likely to be beneficial
Number of studies: 4
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How is the evidence assessed?

Effectiveness

75%
Certainty

55%
Harms

0%

Calculating overall effectiveness category

Background information and definitions

Fishing can impact subtidal benthic invertebrates through species removal or habitat damage from fishing gear coming into contact with the seabed (Collie et al. 2000). Mobile fishing gear such as towed dredges, for instance used in the harvest of bivalves such as clams and scallops, involves towing a heavy steel frame along the seabed, and are known to be particularly damaging to benthic biota. Ceasing or prohibiting dredges in an area, for instance through bylaws or voluntary agreements (Blyth et al. 2002; Bull 1989; Schejter et al. 2008), can remove this direct pressure to subtidal benthic invertebrates and allow them to potentially recolonise and recover naturally over time (Blyth et al. 2004). Recreational and artisanal bivalve fishing may cause less impact compared to dredging due to the smaller scale of the operations and the harvesting methods used (for instance hand-harvest). Evidence for related interventions is summarised under “Threat: Biological resource use – Cease or prohibit all towed (mobile) fishing gear”. When this intervention occurs within a protected area, evidence has been summarised under “Habitat protection – Designate a Marine Protected Area and prohibit dredging” and “Designate a Marine Protected Area and prohibit the harvest of scallop”.

_{Blyth R.E., Kaiser M.J., Edwards-Jones G. & Hart P.J.B. (2004) Implications of a zoned fishery management system for marine benthic communities. Journal of Applied Ecology, 41, 951-961.}

_{Blyth R.E., Kaiser M. J., Edwards-Jones G. & Hart P.J.B. (2002) Voluntary management in an inshore fishery has conservation benefits. Environmental Conservation, 29, 493–508.}

_{Bull M.F. (1989) The New Zealand scallop fishery: a brief review of the fishery and its management. Edited by: MLC Dredge, WF Zacharin and LM Joli, 42.}

_{Collie J.S., Hall S.J., Kaiser M.J. & Poiner I.R. (2000) A quantitative analysis of fishing impacts on shelf‐sea benthos. Journal of Animal Ecology, 69, 785–798.}

_{Schejter, L., Bremec, C.S. & Hernández, D. (2008) Comparison between disturbed and undisturbed areas of the Patagonian scallop (Zygochlamys patagonica) fishing ground “Reclutas” in the Argentine Sea. Journal of Sea Research, 60, 193–200.}

Study locations

Key messages

Four studies examined the effects of ceasing or prohibiting dredging on subtidal benthic invertebrate populations. One study was in the North Atlantic Ocean (Portugal), one in the South Atlantic Ocean (Argentina), one in the English Channel and one in the Irish Sea (UK).

COMMUNITY RESPONSE (3 STUDIES)

Overall community composition (3 studies): One of three site comparison studies (one replicated, one before-and-after) in Atlantic Ocean and the Irish Sea found that after ceasing dredging, overall invertebrate community composition was different to that in dredged areas. The other two found that communities remained similar in dredged and non-dredged areas.
Overall richness/diversity (3 studies): One of three site comparison studies (one replicated, one before-and-after) in Atlantic Ocean and the Irish Sea found that after ceasing dredging, large (macro-) invertebrate diversity was higher but small (meio-) invertebrate diversity was lower compared to dredged areas. The other two found that overall diversity remained similar in dredged and non-dredged areas.

POPULATION RESPONSE (3 STUDIES)

Overall abundance (3 studies): One of three site comparison studies (one replicated, one before-and-after) in Atlantic Ocean and the Irish Sea found that four years after ceasing dredging, large (macro-) and small (meio-) invertebrate abundance and/or biomass appeared higher to that in dredged areas. The other two found that abundance and/or biomass remained similar in dredged and non-dredged areas after either two or six years.
Tunicate abundance (1 study): One replicated, site comparison study in the English Channel found that a year after ceasing dredging in three areas, abundance of ascidians/sea squirts (tunicates) was similar to that in dredged areas.
Bryozoan abundance (1 study): One replicated, site comparison study in the English Channel found that a year after ceasing dredging in three areas, abundance of bryozoan was higher than in dredged areas.
Crustacean abundance (1 study): One replicated, site comparison study in the English Channel found that a year after ceasing dredging in three areas, abundance of spider crabs was higher than in dredged areas, but abundance of edible crab was similar.
Cnidarian abundance (1 study): One replicated, site comparison study in the English Channel found that a year after ceasing dredging in three areas, abundance of sea fans was higher than in dredged areas.
Sponge abundance (1 study): One replicated, site comparison study in the English Channel found that a year after ceasing dredging in three areas, abundance of sponges was higher than in dredged areas.

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 replicated, site comparison study in 1999 of six sandy seabed sites off the Algarve coast, North Atlantic Ocean, southwestern Portugal (Chícharo et al. 2002) found that sites closed to dredging had different invertebrate community composition, higher macro-invertebrate (>1 mm) diversity, but lower meio-invertebrate (150 µm–1mm) diversity after four years, than sites where dredging continued. Communities in the closed and fished areas were 88% dissimilar (data presented as statistical model result). Macro-invertebrate diversity was higher, but meio-invertebrate diversity was lower, inside the closed area compared to the fished areas (reported as diversity indices). Macro-invertebrate abundance averaged 12 individuals/m² in the closed area, and 4 individuals/m² in the fished area. Macro-invertebrate biomass averaged 0.61 g/m² in the closed area, and 0.65 g/m² in the fished area. Meio-invertebrate abundance averaged 49 individuals/m² in the closed area, and 42 individuals/m² in the fished area. Meio-invertebrate biomass averaged 5 g/m² in the closed area, and 0.1 g/m² in the fished area. Abundance and biomass data were not statistically tested. In 1995, an area was closed to dredge fishing (whether other fishing activities continued is unclear). Invertebrates were surveyed at three 50 x 50 m sites in the closed area and three in a nearby area where dredging continued (7–9 m depth) using quadrats and cores. Macro- and meio-invertebrates were identified, counted, and dry-weighed.
Study and other actions tested
Referenced paper
Chícharo L., Chícharo A., Gaspar M., Alves F. & Regala J. (2002) Ecological characterization of dredged and non-dredged bivalve fishing areas off south Portugal. Journal of the Marine Biological Association of the United Kingdom, 82, 41-50.
A site comparison study in 1998–2002 in two areas of soft seabed in the South Atlantic Ocean, Argentina (Schejter et al. 2008) found that an area prohibiting the commercial dredging of scallops for six years did not have different invertebrate community composition, species richness, or biomass, compared to adjacent fished areas. Community data were presented as graphical analyses. Species richness was similar in closed areas (11–24 species groups/site) and fished area (6–25 species groups/site) throughout the study. Six years after closure, biomass of invertebrates was similar in the closed (2–13 kg/100 m²) and fished areas (2–16 kg/100 m²). The area was closed to commercial dredging of scallops in 1996. Samples were collected at 100 m depth once a year between 1998 and 2002 using a dredge (which does not catch scallops; 10 mm mesh) at 23 sites in the closed area and at 71 adjacent sites outside. Invertebrates were identified to species level when possible, counted and weighed. Information was updated using an erratum (Schejter et al. 2009).
_{Schejter L., Bremec C.S. & Hernández D. (2009) Erratum to “Comparison between disturbed and undisturbed areas of the Patagonian scallop (Zygochlamys patagonica) fishing ground “Reclutas” in the Argentine Sea” [J. Sea Research 60/3 (2008) 193]. Journal of Sea Research 61, 275.}
Study and other actions tested
Referenced paper
Schejter L., Bremec C.S. & Hernández D. (2008) Comparison between disturbed and undisturbed areas of the Patagonian scallop (Zygochlamys patagonica) fishing ground “Reclutas” in the Argentine Sea. Journal of Sea Research, 60, 193-200.
A replicated, site comparison study in 2007 in six areas of rocky seabed in Lyme Bay, English Channel, UK (Hinz et al. 2011) found that closing areas to scallop dredging had mixed effects on the abundance of invertebrates depending on species, after a year. Abundances were higher in the closed areas, compared to areas that remained dredged, for pink sea fans Eunicella verrucosa (closed: 58 vs dredged: 15 individuals/100 m²), bryozoans Pentapora fascialis (27 vs 9 individuals/100 m²), sponges Axinella dissimilis (5.0 vs 1.4 individuals/100 m²), and spider crabs Maja squinado (1.2 vs 0.3 individuals/100 m²). In contrast, there was no difference in abundance between areas for tunicates (ascidian/sea squirt) Phallusia mammillata (6 vs 12 individuals/100 m²), or edible crabs Cancer pagurus (1 vs 1 individuals/100 m²). In March and August 2007, six areas within the bay were sampled: three voluntarily closed to scallop dredging since September 2006 (but where static gear fisheries occurred) and three that remained open. Samples were taken using a video camera (10 recordings/area) towed for approximately 10 minutes in a straight line. Abundances of six species were recorded from the videos.
Study and other actions tested
Referenced paper
Hinz H., Tarrant D., Ridgeway A., Kaiser M. & Hiddink J. (2011) Effects of scallop dredging on temperate reef fauna. Marine Ecology Progress Series, 432, 91-102.
A before-and-after, site comparison study 2009–2011 in two areas of sandy, pebbly and gravelly seabed in Cardigan Bay, Irish Sea, Wales, UK (Sciberras et al. 2013) found that in an area prohibiting commercial scallop dredging year-round, sessile invertebrate community composition, diversity, species richness, and abundance were similar to that of an adjacent seasonally dredged area, after two years. Invertebrate community composition (presented as graphical analyses), diversity (presented as a diversity index), species richness, and abundance, were similar between closed and fished areas both before (richness: closed 7 vs fished 4 species/tow; abundance: 3 vs 3 individuals/m²) and two years after closure (richness: 15 vs 13 species/tow; abundance: 23 vs 7 individuals/m²). Richness, diversity, abundance and assemblage composition changed in a similar manner over time in the closed and fished areas. Two areas of Cardigan Bay were assessed: one permanently closed to scallop dredging in March 2010, another seasonally closed to scallop dredging (May to October). Surveys were conducted before closure (December 2009) and three times after (June 2010 to April 2011). During each survey, a camera was towed behind a boat at 30 m depth for 300 m at six sites/area. More than 40 images/camera tow (covering a 0.13 m² area of seabed) were analysed, and sessile invertebrates were identified and counted.
Study and other actions tested
Referenced paper
Sciberras M., Hinz H., Bennell J., Jenkins S., Hawkins S. & Kaiser M. (2013) Benthic community response to a scallop dredging closure within a dynamic seabed habitat. Marine Ecology Progress Series, 480, 83-98.

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

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