Action: Cease or prohibit bottom trawling
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- Four studies examined the effects of ceasing or prohibiting bottom trawling on subtidal benthic invertebrate populations. Two studies were in the Bering Sea (USA), one in the North Sea, and one in the Mediterranean Sea (Italy).
COMMUNITY RESPONSE (3 STUDIES)
- Overall community composition (2 studies): Two site comparison studies (one before-and-after, one replicated) in the North Sea and the Mediterranean Sea found that in areas prohibiting trawling for either 15 or 20 years, overall invertebrate community composition was different to that of trawled areas.
- Overall species richness/diversity (3 studies): Two of three site comparison studies (one paired, one before-and-after, one replicated) in the Bering Sea, the North Sea, and the Mediterranean Sea found that invertebrate diversity was higher in sites closed to trawling compared to trawled sites after either 37 or 15 years, but the other found no differences after 20 years.
POPULATION RESPONSE (3 STUDIES)
- Overall abundance (2 studies): One of two site comparison studies (one paired, one replicated) in the Bering Sea and the Mediterranean Sea found that total invertebrate abundance was higher in sites closed to trawling compared to trawled sites after 37 years, but the other found no differences after 20 years. Both found no differences in total invertebrate biomass.
- Unwanted catch overall abundance (1 study): One replicated, before-and-after, site comparison study in the Bering Sea found that during the three years after closing areas to all bottom trawling, unwanted catch of crabs appeared to have decreased, while no changes appeared to have occurred in nearby trawled areas.
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 bottom trawls are known to be particularly damaging as they are dragged along the seabed. Their use can be stopped or prohibited within an area and only static gears (such as lobster or crab pots/traps) allowed. Ceasing or prohibiting bottom trawling can remove this direct pressure to subtidal benthic invertebrates and potentially allow them to recolonise and recover naturally over time (Hiddink et al. 2017). When this intervention occurs due to the closure of an area to shipping, evidence has been summarised under “Threat: Transportation and service corridors – “Cease or prohibit shipping”. When it is in combination with ceasing or prohibiting dredging, but without separating the effects of the two gear, evidence has been 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 bottom trawling”.
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.
Hiddink J.G., Jennings S., Sciberras M., Szostek C.L., Hughes K.M., Ellis N., Rijnsdorp A.D., McConnaughey R.A., Mazor T., Hilborn R. & Collie J.S. (2017) Global analysis of depletion and recovery of seabed biota after bottom trawling disturbance. Proceedings of the National Academy of Sciences, 114, 8301–8306.
Supporting evidence from individual studies
A paired site comparison study in 1996 of 84 sites of sandy seabed in the eastern Bering Sea, USA (McConnaughey et al. 2000) found that ‘macro’-invertebrate (size unspecified) species diversity and abundance were higher in sites closed to trawling for 37 years, compared to trawled sites, but there was no difference in biomass. Overall across paired sites, species diversity was higher in sites closed to trawling compared to those trawled (reported as a diversity index). Of the 42 invertebrate taxa recorded, 27 appeared more abundant in the closed sites compared to the trawled sites (not statistically tested). In particular, abundances of sponge (Porifera), anemones (Actinaria) and Neptunea snails (gastropods) were significantly higher in the closed sites (data not shown). Invertebrate biomass was similar in sites closed to trawling (1.6 kg/ha) and trawled sites (1.6 kg/ha). Trawling was prohibited in an area in 1959. Macro-invertebrates were surveyed at 84 sampling sites (44–55 m depth) along the boundary of the closed area (42 pairs; one site on either side of the boundary, 1 nm apart) using an otter trawl (3.8 cm liner at the codend). Macro-invertebrates were sorted into groups, counted and weighed.
A site comparison study in 2004 in areas of soft sediment in the southern North Sea, Netherlands (Duineveld et al. 2007) found that an area closed to bottom trawling had different invertebrate community composition, and higher species richness, compared to areas where trawling occurred, after approximately 20 years. Community data were presented as graphical analyses, and richness data were presented as a diversity index. A gas production platform was drilled approximately 20 years prior to the study and a 500 m zone closed to all trawling, established around it. In April 2004, invertebrates were surveyed inside the closed area and in four sites (1 x 1 nm) outside (1.5 nm north, south, east and west of the exclusion zone). Samples were collected using a combination of dredge (6–10 tows/site; invertebrates >7 mm) and sediment cores (seven cores/site; invertebrates >1 mm) at 36–39 m depth. Invertebrates were identified and counted.
A replicated, before-and-after, site comparison study in 1992–1997 in the eastern Bering Sea, USA (Abbott & Haynie 2012) found that during the three years after closing areas to all bottom trawling, unwanted catch of red king crab Paralithodes camtschaticus appeared to have decreased, while no changes appeared to have occurred in nearby trawled areas (results not tested for statistical significance). In the closed areas, average unwanted crab catch tended to be lower after the closure (2–4 crabs/hour) compared to before (6–17). In addition, the proportion of hauls without crabs tended to be higher after the closure (after: 91–95%) compared to before (71–86%). In the continuously trawled areas, unwanted crab catch was similar before (2–8 crabs/hour) and after (2–4 crabs/hour) the closure. Two areas were closed to all bottom trawling in 1995. Unwanted catch data inside the closed areas and in nearby trawled areas (number and location unspecified) between January 1992 and March 1997 were obtained from the North Pacific Groundfish Observer Program (approximately 4,500 observations).
A replicated, site comparison study in 2005 in four gulfs of muddy seabed in the Mediterranean Sea, off the northern coast of Sicily, Italy (Romano et al. 2016) found that, 15 years after prohibiting trawling, overall invertebrate community composition, but not total invertebrate abundance, biomass, or diversity, was different to that of trawled gulfs. Invertebrate communities were different between non-trawled and trawled gulfs (community data presented as graphical analyses), with amphipods reported to dominate non-trawled gulfs, while polychaete worms reported to dominate trawled gulfs. There were no statistical differences between gulfs in total abundance (non-trawled: 683–872; trawled: 448–633 individuals/m2), total biomass (non-trawled: 751–927; trawled: 1,000–1,080 g/m2) and diversity (as a diversity index). Two gulfs (200 and 240 km2) were closed to trawling in 1990 (artisanal fishing with static gears and small purse seines allowed). In May–June 2005, sediment samples were collected in the two closed gulfs and two fished gulfs (18 samples/gulf) using a grab (0.4 m2; 3 grabs/sample) at 40–80 m depth. Invertebrates >0.5 mm were identified to family level and dry-weighed.
- McConnaughey R. (2000) An examination of chronic trawling effects on soft-bottom benthos of the eastern Bering Sea. ICES Journal of Marine Science, 57, 1377-1388
- Duineveld G.C.A., Bergman M.J.N. & Lavaleye M.S.S. (2007) Effects of an area closed to fisheries on the composition of the benthic fauna in the southern North Sea. ICES Journal of Marine Science, 64, 899-908
- Abbott J.K. & Haynie A.C. (2012) What are we protecting? Fisher behavior and the unintended consequences of spatial closures as a fishery management tool. Ecological Applications, 22, 762-777
- Romano C., Fanelli E., D'Anna G., Pipitone C., Vizzini S., Mazzola A. & Badalamenti F. (2016) Spatial variability of soft-bottom macrobenthic communities in northern Sicily (Western Mediterranean): Contrasting trawled vs. untrawled areas. Marine Environmental Research, 122, 113-125