Cease or prohibit mobile fishing gears that catch bottom (demersal) species and are dragged across the seafloor
Overall effectiveness category Awaiting assessment
Number of studies: 10
Background information and definitions
Mobile bottom-towed fishing gears targeting demersal species (mainly various trawl types and some seine nets) are nets that are hauled or actively towed in contact with the seabed, usually from one or more vessels under power (and hence they are a specific subset of general bottom/demersal fishing gears – see related interventions listed below). Towed mobile bottom-contacting gears typically consist of a conical length of net that gradually tapers in size from a wide opening at the front to a closed end section (codend) that collects the catch. Towed bottom-contacting gears can be deployed over large linear distances for extended periods of time and so can catch large quantities of fish, including unwanted sizes and species. Because they are dragged along the seabed essential fish habitats may be damaged along with non-target fish and organisms that fish may feed on. They can also cause sediment disturbance that reduces water quality (Jones 1992). Banning mobile fishing gears that contact the seabed in an area may reduce overall fishing mortality and the damage and disturbance affecting fish and their habitats.
Evidence for similar interventions relating to the use of towed bottom-contacting fishing gears are summarized under ‘Cease or prohibit all (mobile and static) fishing gears that catch bottom (demersal) species’ and ‘Cease or prohibit shellfish dredging’.
Jones J.B. (1992) Environmental impact of trawling on the seabed: a review. New Zealand Journal of Marine and Freshwater Research, 26, 59–67.
Supporting evidence from individual studies
A before-and-after, site comparison study in 1970–1994 of two areas on the Scotian Shelf, northwest Atlantic Ocean, Canada (Frank et al. 2000) found that closure of a haddock nursery area to mobile bottom (groundfish) fishing activity did not increase the survival or recruitment of young haddock Melanogrammus aeglefinus in the seven years after compared to before, and compared to a fished area. Average rate of haddock survival to age two was lower in the closed area in the period after closure compared to before, but it did not differ in the fished area (data presented as survival index). Conversely, trends in haddock recruitment at age two before and after the closure were similar in both areas. Authors noted possible causes for the effect, including continued fishing in the closure area by fixed gears, and biological and environmental factors. In 1987, a haddock nursery area on the Emerald and Western Banks (4,000 nm2) was permanently closed to mobile groundfish fishing activity, whilst fixed gears were permitted until 1993 when the area was closed to all fishing including fixed gear. The closed area was 13% of the total area of the Northwest Atlantic Fisheries Organization haddock management unit Division 4VW. This unit was compared with a neighbouring fished area without a closure (4X). Haddock numbers and age data for the years 1970–1994 were taken from annual July research vessel surveys in the two areas.Study and other actions tested
A before-and-after, site comparison study in 1981–1998 of a fjord in the North Sea, Denmark (Hoffmann & Dolmer 2000) reported that prohibiting all towed bottom fishing gears in an area had no effect on the abundance and species richness of bottom-dwelling fish in the following 10 years, and compared to open areas. Data were not statistically tested. In trawl surveys, fish abundance (closed: 0–13 kg/30 min, open: 0–31 kg/30 min) and number of species (closed: 4–11, open: 1–9) varied between years but no effect of the closure was detected in either area. In set net and trap samples, catch rates were higher in the fished area (closed: 37–486 g/fishing unit, fished: 132–915 g/fishing unit) but there was no difference in the number of species (closed: 3–8, fished: 4–8). In 1988, a 40 km2 fishing area in the Limfjord (previously fished commercially using poundnets, trawls – types unspecified – but most recently and extensively by mussel dredges) was closed to all towed fishing gears (in practice however the ban was focussed on stopping mussel dredging as little or no other towed gears were being used, static gears allowed). Fish data was collected by two methods: annual trawl surveys from 1981–1998 in August/September at two stations inside and two just outside the closed area; and in 1995, 1996 and 1997, experimental fishing with fixed set nets (48 deployments) and eel traps (38 deployments) at three locations inside and three outside the closed area. Catch rates and number of species were recorded. No fish species groups (other than demersal) or individual species were specified.Study and other actions tested
A site comparison study in 1997 of 14 seamounts in the Indian Ocean, Tasmania (Koslow et al. 2001) reported that the number of fish species varied with historical levels of bottom trawling intensity but was also dependent on seamount depth. Data were not statistically tested. The total number of species of fish recorded/seamount was three for non-trawled seamounts, five for very lightly trawled (1–10 trawls), 12/for lightly trawled (11–100 trawls), seven for heavily trawled (101–1,000 trawls) and zero for very heavily trawled (>1,000 trawls) seamounts. In addition, the non-trawled and very lightly trawled seamounts were generally the deepest and therefore considered less likely to support high species richness. In January and February 1997, fish were sampled with longlines, traps and sleds across 14 seamounts off South Tasmania with peaks between 714–1,580 m depth (deployment numbers not given). The seamounts had been trawled at different fishing intensities and in 1995, a temporary protected area incorporating six seamounts with no or very low trawling was established in which the fishing industry agreed not to trawl for a 3-year period. Trawl samples were taken inside and outside the temporary protected area and across the different trawling intensities. Fishing intensity for the period 1988–1996 was obtained from fisher logbook records.Study and other actions tested
A replicated, site comparison study in 1998 in the Arafura Sea off Australia (Stobutzki et al. 2003) found that in an area closed to bottom (prawn) trawling for five years, abundance, biomass and size of species (fish and invertebrates combined) were typically similar to two open trawled areas. Probability of occurrence was similar in the closed area and a nearby open area for 81–89% of species, and for 74–85% of species between the closed area and a nearby open area and a distant open area. Biomass was similar in the closed area and the nearby open area for 89–94% of species and similar for 85–99% of species in the closed area, the nearby open area and a distant open area. The average size of species was similar between closed and open areas for 39% species. Zero to 9% of species were largest in the closed area and 43–77% in the two open areas. Sampling was done in October 1998 in two regions of a large area (6,648 km2) closed to trawling (types not specified) and in 1983 in an area fished and managed for a commercial prawn species. In each region three areas, one closed to trawling and two open to trawling (near to and distant to the closed area) were sampled by a bottom prawn trawl with 57 mm mesh net and 45 mm mesh codend towed for 0.5 h. Three 6 × 6 nautical mile grids were sampled in each area, with each grid sampled three times in each of four three-day sampling blocks. Full sampling details are provided in the original study.Study and other actions tested
A replicated, randomized, site comparison study in 1992–1993 in four areas of mixed seabed inside the Great Barrier Reef Marine Park in the Coral Sea, Australia (Burridge et al. 2006) found no difference in the biomass of non-commercial unwanted catch (fish and invertebrate discard) or in the number of ‘common’ and ‘rare’ discard species between areas closed to trawling for seven years and adjacent open fished areas. Data were reported as statistical model results. An extensive area (10,000 km2) of the Great Barrier Reef Marine Park was closed to trawling (types unspecified) in 1985. Two surveys were carried out, one in 1992 and one in 1993. During each survey, 25 randomly selected sites were sampled at each of four areas within the marine park, two closed areas, and two fished areas located 10 nm away, using both a benthic dredge and a prawn trawl. A total of 156 dredges (86 in closed areas, 70 in fished areas) and 122 trawls (68 in closed areas, 54 in fished areas) were deployed. For each tow, discard species were collected, identified, counted, and weighed from subsamples (amount not specified). Total weight of discard was estimated from the subsamples. Species were either recorded as ‘common’ (found in at least 11 of the 25 sites) or ‘rare’.Study and other actions tested
A replicated, controlled, before-and-after study in 2000–2002 of 17 sites in a lagoon in the North Atlantic Ocean, Portugal (Curtis et al. 2007) found that densities of long-snouted seahorse Hippocampus guttulatus, but not short-snouted seahorse Hippocampus hippocampus, increased when bottom seine fishing (a mobile gear) was ceased, compared to sites where seining fishing effort remained constant. At sites where experimental seining was ceased after one year, the average density of long-snouted seahorses increased to 0.07 from 0.03/m2 and was higher than fished and unfished sites (0.03/m2) in both years. However, densities of short-snouted seahorses decreased (0.02 to <0.01/m2) and were lower than fished and unfished sites (ceased: <0.01/m2, fished: 0.02/m2). Experimental fishing was done in the Ria Formosa coastal lagoon in southern Portugal using a beach seine in October 2000–October 2002. A total of 12 sites were seined each month during the first year, but no seining was done at these sites during the second year. Three other sites were fished monthly in both years and two sites were unfished (not seined in either year). All sites were surveyed once each year from June–September by scuba divers using standard underwater visual census techniques along three belt transects 30 m long and 2 m wide (180 m2/sampling site). Sea horse species were counted, and trunk lengths recorded. Full survey specifications are detailed in the original paper.Study and other actions tested
A before-and-after study in 1985–2005 of muddy and sandy-mud seabed in the Mediterranean Sea, Sicily (Fiorentino et al. 2008) found that after the closure of an area to all towed bottom fishing gears for 14 years, adult (spawning-stock) red mullet Mullus barbatus had a higher biomass, were larger at two of three depths and recruitment of young mullet increased, compared to before the closure. Biomass of adult red mullet was higher at all depths after the ban (750–4,200 g/haul) compared to before (170–650 g/haul). Average total length of all adult red mullet was higher after the closure at the two depths >50 m, and similar at depths <50 m (data reported as statistical model results). In addition, the number of small fish surviving to reach a larger (fishable) size (i.e. recruitment to the fishery) increased after the closure, and there were several recruitment events recorded throughout the year compared to only one before the closure. In 1990, an area of 200 km2 in the Gulf of Castellammare was closed to trawl nets and all other bottom-towed fishing gear (non-towed bottom gears and pelagic gears permitted). Red mullet data for the periods before (1985–1986) and after (2004–2005) the closure were obtained from 35 experimental trawl survey deployments at three depth ranges (10–50, 51–100 and 101–200 m).Study and other actions tested
Referenced paperFiorentino F., Badalamenti F., D’Anna G., Garofalo G., Gianguzza P., Gristina M., Pipitone C., Rizzo P. & Fortibuoni T. (2008) Changes in spawning-stock structure and recruitment pattern of red mullet, Mullus barbatus, after a trawl ban in the Gulf of Castellammare (central Mediterranean Sea). ICES Journal of Marine Science, 65, 1175-1183.
A randomized, replicated, site comparison study in 2004–2005 on fishing grounds in the Gulf of Mexico, USA (Wells et al. 2008) found that areas not exposed to bottom trawling had different fish assemblages compared to trawled areas, and the effect on overall species diversity and richness (fish and invertebrates) and fish size, varied with the habitat type. Overall, the fish community structure for all three habitat types differed between non-trawled and trawled areas (reported as statistical results). Species diversity and richness (fish and invertebrates) differed between non-trawled and trawled areas on sand and shell habitats, but not reef, and were higher on non-trawled shell habitat but lower on non-trawled sand habitat. Average total length of four of the nine most important fish species (see paper for species individual data) was higher in non-trawled areas over sand habitat (non-trawled: 94–124 cm, trawled: 84–118 cm), and five were larger over shell (non-trawled: 114–254 cm, trawled: 91–239 cm). Data was collected quarterly in 2004 and 2005 by standard otter trawl net for groundfish surveys at three random stations over each habitat type (sand, shell and reef), both exposed and not exposed to bottom shrimp trawling (as determined from annual shrimp-trawling effort data). In non-trawled areas 24 deployments (10-minute tow) were done on sand, 48 on shell and 24 on reef. In trawled areas 21, 33 and 21 deployments were done on sand, shell and reef respectively. All fish (144 species) and invertebrates (70 species) caught were counted, weighed and fish lengths measured.Study and other actions tested
A replicated, site comparison study in 2004–2005 of three gulfs in the Mediterranean Sea, Sicily (Sweeting et al. 2009) found that 15 years after bottom-towed fishing gear (commercial trawling) was banned in an area, total fish biomass but not average fish weight was higher compared to gulfs where trawling was permitted. The total biomass of fish was higher in the non-trawled gulf than in the two trawled gulfs, and differences were greatest in smaller size classes (data reported as normalised biomass spectra analyses). Average fish weight was typically similar in non-trawled (61–89 g) and trawled gulfs (62–70 g), except for significantly greater average weight in spring. However, more than 70% of fish above 500 g were from the non-trawled gulf. A ban on trawl nets and all other bottom-towed fishing gear in a 200 km2 area was implemented in the Gulf of Castellammare, 200 km west of the trawled gulfs, in 1990. Fishing with static gears (trammel and gillnets) by artisanal vessels within the trawl exclusion area continued. All gulfs were subject to the existing country-wide ban on trawling in water <50 m. Fish surveys were carried out over four consecutive seasons in the trawl exclusion gulf from 2004 and in the two trawled gulfs in autumn 2004 and spring 2005. Bottom-dwelling fish were sampled with a benthic otter trawl. At each gulf, random sampling within several 2.25 nm2 areas, at three depths, was done and lengths and weights of all fish recorded.Study and other actions tested
A replicated, before-and-after, site comparison study in 1996–2007 of three coral reef areas in the Indian Ocean, off Kenya (McClanahan 2010) found that landing sites in two management areas (with and without areas closed to all fishing) where beach and all other seine nets had been prohibited for three to six years, had higher average fish lengths of two of five groups, increased overall fish catch rates and varied catch rates of individual fish groups compared to an unrestricted fishing area. Overall, average length was higher in management areas where seine nets were eliminated than in an openly fished site for goatfish Mullidae (managed: 19 cm, open: 13 cm) and parrotfish Scaridae (managed: 18–19 cm, open:14 cm), and no differences were found for the other three groupings of rabbitfish Siganidae, scavengers Lethrinidae, Lutjanidae, Haemulidae and ‘rest of catch’ (managed: 16–19 cm, open: 14–17 cm) (see paper for separate group averages). In the period after beach seines were eliminated (2002–2007), total catch rates increased from 3.0–3.2 to 3.7–3.8 kg/fisher/day in managed areas and averaged 2.0 kg/fisher/day in the open site. In addition, differences in catch composition were found between areas and catch rates differed for four of the five groups with time and management regime (see original paper for data). Fish data was collected between two and 10 days/month at 10 fish landing sites representing three different management regimes: one intensively managed area (small-mesh beach seine nets prohibited in 2001, next to a 6 km2 no-fishing protected area); one moderately managed area (most seine nets prohibited in 2001, and all seine nets in 2004, >30 km from an area closed to fishing); and one with no restrictions on gear (seine nets the dominant gear but also hand lines, spear guns gillnets, traps and fence nets used, 1–10 km from an area closed to fishing). Fish were categorized by the five groups used locally to price and sell the fish. Data for the two managed areas were collected 1996–2007 and for the open area data was collected in 2001–2007.Study and other actions tested