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Providing evidence to improve practice

Action: Cease or prohibit all towed (mobile) fishing gear Subtidal Benthic Invertebrate Conservation

Key messages

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  • Eight studies examined the effects of ceasing or prohibiting all towed fishing gear on subtidal benthic invertebrate populations. One study was in the Limfjord (Denmark), two in the English Channel (UK), three in Georges Bank in the North Atlantic Ocean (USA and Canada), one in the Ria Formosa lagoon (Portugal), and one in the Irish Sea (Isle of Man).

 

COMMUNITY RESPONSE (4 STUDIES)

  • Overall community composition (3 studies): Two of three replicated, site comparison studies in the Limfjord and the English Channel, found that areas excluding towed fishing gear for either an unspecified amount of time or two to 23 years had different overall invertebrate community composition compared to areas where towed-fishing occurred and one found that ceasing towed-gear fishing for nine years had mixed effects.
  • Overall species richness/diversity (3 studies): Two replicated, site comparison studies in the English Channel reported that areas excluding towed fishing gear for either an unspecified amount of time or two to 23 years had different or greater invertebrate species richness and diversity to areas where towed-fishing occurred. One site comparison study in Georges Bank found no difference in invertebrate species richness between an area closed to mobile fishing gear for 10 to 14 years and a fished area.

POPULATION RESPONSE (3 STUDIES)

  • Overall abundance (3 studies): Two site comparison studies (one replicated) in the English Channel and Georges Bank found that sites excluding towed gear for either two to 23 years or 10 to 14 years had greater overall invertebrate biomass compared to sites where towed-gear fishing occurred, but one also found that abundance was similar in both areas. One replicated, controlled, before-and-after study in the Ria Formosa lagoon found that ceasing towed gear for 10 months led to increases in the cover of mobile but not sessile
  • Mollusc abundance (2 studies): Two site comparison studies (one replicated) in the Irish Sea and the English Channel found that areas closed to towed fishing gear for either two to 23 years or 14 years had more scallops compared to adjacent fished areas.
  • Mollusc condition (1 study): One site comparison study the Irish Sea found that an area closed to towed fishing gear for 14 years had higher proportions of older and larger scallops compared to an adjacent fished area.
  • Starfish abundance (1 study): One replicated, site comparison study in Georges Bank found more starfish in areas closed to towed fishing gear for five to nine years compared to adjacent fished areas.
  • Starfish condition (1 study): One replicated, site comparison study in Georges Bank found that starfish arm length was similar in areas closed to towed fishing gear for five to nine years and adjacent fished areas.

OTHER (1 STUDY)

  • Overall community biological production (1 study): One before-and-after, site comparison study in Georges Bank found an increase in the biological production from invertebrate in sites closed to towed fishing gear for approximately five years compared to adjacent fished sites.

Supporting evidence from individual studies

1 

A replicated, site comparison study in 1997 of 17 sites in the Limfjord, northern Denmark (Hoffmann & Dolmer 2000) found that ceasing towed gear fishing in an area for nine years had mixed effects on invertebrate community composition. Sites in the northern part of the area closed to towed gear had different invertebrate composition to adjacent northern fished sites, but sites in the southern part of the closed area had similar assemblages to adjacent southern fished sites (community data were presented as graphical analyses and statistical model results). Within the closed area, northern sites also had different composition to southern sites. Authors suggest towed gears might not have been the cause of the observed changes in invertebrate and fish compositions prior to the closure. A 40 km2 area was closed to towed gears (static gears allowed) in 1988 following changes in invertebrate and fish assemblages. In September 1997, divers identified and counted sessile invertebrates at 17 sites (ten 0.24 m2 quadrats/site) across four areas: northern fished area (four sites), northern closed area (five sites), southern closed area (four sites), and southern fished area (four sites).

2 

A replicated, site comparison study (year not stated) in eight areas of mixed sediment off the south Devon coast, English Channel, UK (Kaiser et al. 2000) found that areas excluding towed fishing gear (for an unspecified amount of time) had different species richness, diversity and overall invertebrate community composition compared to areas where towed-fishing occurred either seasonally or year-round. Species richness and diversity data were not presented. Community composition in areas closed to towed gears was reported to be dominated by higher biomass and organisms that increased habitat complexity (community data were presented as graphical analyses). In areas where towed-fishing occurred, the community was reported to be dominated by smaller bodied fauna and scavenging taxa. In 1978 a zoned fishery management system was established in a 500 km2 area, which included static-gear-only areas. Eight areas were surveyed (year of study unspecified) at 15–70 m depth: three non-towed (static only), two seasonally-towed (six months/year), and three towed year-round. Invertebrates were sampled at nine stations/area. Invertebrates were sampled with a beam trawl and a dredge, identified, counted and weighed.

3 

A before-and-after, site comparison study in 1994–2000 of four sites of sandy and gravelly seabed on Georges Bank, North Atlantic Ocean, USA and Canada (Hermsen et al. 2003) found an increase in invertebrate biological production in shallow and deep sites closed to towed fishing gear compared to adjacent fished sites approximately five years after closure. Biological production (a measure of biomass regeneration over time) from invertebrates at shallow (45–62 m) sites closed to fishing increased following closure (before: 17; after: 215 kcal/m2/year), and was higher than at shallow fished sites where production did not vary over time (before: 32; after: 57 kcal/m2/year). Production at deep (80–90 m) sites closed to fishing also increased following closure (before: 174; after: 256 kcal/m2/year), and was higher than at deep fished sites where production did not vary over time (before: 52; after: 30 kcal/m2/year). In January 1995, a combined area of approximately 10,000 km2 of Georges Bank was closed to all bottom towed fishing gear. Invertebrates (>5 mm) were sampled with a dredge (6.4 mm mesh) at four sites across the two depth ranges (‘shallow’ and ‘deep’). Shallower sites are subject to more intense and regular fishing. At each depth, one closed and one fished site were sampled. Animals were identified, counted and weighed. Individuals from the 20 most abundant species were measured. Biological production was estimated from a combination of biomass and length-frequency distribution data.

4 

A replicated, site comparison study in 2002 of seven sites of mixed coarse seabed off the south Devon coast, English Channel, UK (Blyth et al. 2004) found that sites excluding towed gear, for either two or 24 years, had greater invertebrate species richness and biomass, different community composition, and more great scallops Pecten maximus compared to sites where towed-fishing occurred. More species were recorded at long-term untowed sites (untowed for 24 years; 16–21 species/tow) and short-term untowed sites (untowed for less than two years; 23–25 species/tow) than at towed sites (8–10 species/tow). Biomass was higher at long-term untowed sites (9.2–9.7 kg/tow) than short-term untowed sites (4.0–8.1 kg/tow); and both were higher than towed sites (0.8–1.5 kg/tow). Community composition at long- and short-term untowed sites (combined) were only 11% similar to that of towed sites. In addition, abundance of great scallops was higher at long-term untowed sites (4–53/tow) and short-term untowed sites (3–15/tow) than at towed sites (0–2/tow). In 1978 a zoned fishery management system was established in a 500 km2 area, which included a static-gear-only area. In 2002, seven sites were surveyed: two long-term untowed (static-only), two short-term untowed, and three towed sites. Dredges were towed for 10 mins three times at each site (two standard dredges to collect great scallops >100 mm in length; one scientific dredge for other invertebrates). Species were identified and wet-weighed (individuals combined per species).

5 

A site comparison study from 1989–2003 in two sites of soft seabed off the southwest coast of the Isle of Man, Irish Sea (Beukers-Stewart et al. 2005) found that an area closed to towed fishing gear for 14 years had more and larger great scallops Pecten maximus compared to an adjacent fished area. Fourteen years after closure, abundance of scallops was higher in the closed area (14/100 m2) compared to the fished area (3/100 m2). In addition, the proportions of older and larger scallops were higher in the closed area (41% over 5-year old; 52% over 130 mm in length) compared to the fished area (5% over 5-year old; 12% over 130 mm). A 2 km2 exclusion zone was closed to towed fishing gear in 1989 following a bylaw (static gears allowed). Abundance, size, and age of scallops inside and outside the exclusion zone were obtained from a combination of dive surveys and annual dredge surveys carried out during multiple studies between 1989 and 2003 (see paper for details). In the fished area, all surveys were carried out during the closed scallop season June–October). Only data for 2002–2003 were statistically tested.

6 

A replicated, controlled, before-and-after study in 2000–2002 of 14 sites within seagrass beds in the Ria Formosa lagoon, southern Portugal (Curtis et al. 2007) found that ceasing towed gear fishing led to increases in the cover of mobile invertebrates, but not non-moving (sessile) invertebrates, after 10 months. Cover of mobile invertebrates increased after towed-gear fishing stopped (3.9%) compared to before it stopped (1.1%), but not cover of sessile invertebrates (before: 2.8%; after: 2.7%). No changes were reported at sites where experimental fishing continued and at sites never fished (data not provided; no statistical comparisons were made with sites where fishing stopped). The use of towed demersal gears for commercial and recreational purposes is prohibited in the Ria Formosa. Experimental towed gear fishing started in October 2000 at 12 sites (monthly 10 m tow of a beach seine, 9 mm mesh) and stopped at nine of them in September 2001. Cover of sessile and mobile invertebrates (>2.5 cm) was surveyed at all sites and two nearby sites that were never fished during underwater visual surveys (180 m2/site) before fishing stopped (August–September 2001) and 10 months after (June–July 2002).

7 

A replicated, site comparison study in 2000–2003 in sites of soft seabed on Georges Bank, North Atlantic Ocean, east of Massachusetts, USA (Marino II et al. 2007) found that there were more starfish Asterias spp. in areas closed to towed fishing for five to nine years compared to adjacent fished areas, but there was no difference in starfish arm length. Across all years, starfish abundance was higher in the closed areas (0.1–0.6 starfish/m2), compared to the fished areas (0.0–0.3 starfish/m2). However, the average arm length of starfish was similar in the closed (20–73 mm) and the fished areas (20–42) and varied between years. In 1994, three areas (17,000 km2 in total) of Georges Bank (located 13–150 m depth) were closed to towed fishing gear. Portions of the closed areas were re-opened from 1999–2001 for a short-term limited fishery. Between 1999 and 2003, video surveys were undertaken within each closed area and in three areas of Georges Bank opened to fishing. A total of 3,209 stations were video-surveyed, and four 2.8 m2 video-quadrats/station assessed. All Asteria spp. starfish (>2 cm diameter) were counted and their arm lengths measured.

8 

A site comparison study in 2004–2008 in two areas of gravelly and sandy seabed on Georges Bank, northwest Atlantic Ocean, USA (Smith et al. 2013) found that, 10–14 years after closure, an area closed to commercial towed fishing gear had a higher biomass of invertebrates attached to the seabed (epifauna), but not a higher total abundance or species richness, compared to a fished area. Epifauna biomass was significantly higher in the closed area (33–109 g/L) compared to the fished area (26–57 g/L). Total epifauna abundance was similar in closed (6–15 individuals/L) and fished areas (6–10 individuals/L). The effect of closing commercial fishing on species richness varied with years, but overall across year species richness was similar in both areas (closed: 26–39 species; fished: 32–41 species). An area on Georges Bank was closed to all commercial fishing gear capable of retaining ground fish (trawls, scallop dredges, gill nets and hook gear) in December 1994. Annually between 2004 and 2008, one site in the closed are and one site in an adjacent fished area were surveyed at 45–55 m depth. Epifauna were collected using a dredge (2–3 samples/site/year; 6.4 mm mesh liner), identified, counted, and wet-weighed.

Referenced papers

Please cite as:

Lemasson, A.J., Pettit, L.R., Smith, R.K., and Sutherland, W.J. (2019) Subtidal Benthic Invertebrate Conservation: Global Evidence for the Effects of Interventions. Synopses of Conservation Evidence Series. University of Cambridge, Cambridge, UK.