REF: 1

Sandbanks

Shallow inlets and bays

Pre and post experimental investigation, within 30m depth contour, with 7 tonne, 12m beam trawl including 5x22mm and 3x18mm tickler chains, 3x20mm and 8x14mm net tickler chains, mesh size of 9cm in the cod-end. Area trawled three times over 2 days and samples taken up to 2 weeks after trawling.

Habitat effects - Tickler chains penetrate at least 6cm into the sediment surface indicated by catches of Echinocardium cordatum and Arctica islandica. Tracks made by the beam trawl shoes still apparent on sidescan sonar after 16hrs.

Species and community effects - Some benthic species show a 10-65% reduction in density after trawling the area three times. There was a significant lowering of densities (40-60%) of echinoderms Asterias rubens and small E. cordatum, and of polychaete worms Lanice conchilega and Spiophanes bombyx. Vertical distribution in sediment appears to be an important factor in catchability. Decrease in density (10-20%), although not significant for small crustaceans and larger Tellina fabula and E. cordatum. Except for the starfish A. rubens most of these animals live in the sediment at a depth up to 15cm. The effect of beam trawling on densities of small individuals tends to be much greater than on densities of large individuals (larger animals tend to live deeper or have better escape possibilities). The polychaete worm Magelona papillicornis showed a considerable increase in numbers, this may be attributable to a change in the vertical distribution of the species in the sediment. The numbers of small Ophiura living in the top centimetre of sediment did not change after trawling the area three times, suggesting the species escape unharmed through the net mesh. Also no direct effect on densities of molluscs (except T. fabula) and worms (except Magelona papillicornis, L. conchilege and S. bombyx). Less abundant worm species (including Spio filicornis Scolelepsis bonnieri, Scoloplos armiger and Owenia fusiformis) and less abundant molluscs (including Thracia sp. Venus striatula, Montecuta ferruginosa and Mysella bidentata) showed no change in total density after trawling. About 90% of A. islandica caught by the 22m trawl were severely damaged.

Bergman M.J.N. & Hup M. (1992) Direct effects of beam trawling on macro-fauna in a sandy sediment in the southern North Sea ICES Journal of Marine Science. 49:5-11

REF: 2

Sandbanks

Shallow inlets and bays

Experimental beam trawl over a 4x2km area, at a depth between 26 and 34m. commercial beam trawl, weighing 3.5 tonne fitted with a chain matrix and 8cm diamond mesh cod-end used. Waylines were fished either 10 or 20 times to adequately disturb trawl area.

Habitat effects - Physical characteristics of the surface sediment were altered by the passage of the beam trawl but effects varied in different parts of the experimental area. Surface roughness of the relatively uniform, stable, flat areas were not altered by trawling but lowered in fished sites in the SE sector which was characterised by sand waves and some ripples. In the latter case the surface ripples were flattened but the megaripples were unaffected. Passage of the chain matrix may have caused sediment to become unconsolidated as shell and gravel currents. Conclusions were that particle size distribution was not affected and observed changes may only be in the superficial layers of the sediments. Newly exposed shell and gravel material would provide surfaces for recolonisation and settlement, epizoites on surfaces which were overturned would be smothered.

Kaiser M.J. & Spenser B.E. (1996) The effects of beam trawl disturbance on infaunal communities in different habitats. Journal of Animal Ecology. 65:348-358.

REF: 3

Sandbanks

Large scale investigations on soft sediment communities depth between 12-15m, 2km offshore. Six vessels towing 3m wide commercial 'Peninsula’ dredge with scraper/cutter bars not extending below the dredge skids. Site dredged for 3hrs day^-1 over 3 days covering the dredge area at least twice. Dredging intensity typical of local commercial fishing intensity.

Habitat effects - Typically top 2cm of surface sediment disturbed but up to 6cm. Observations 8 days after dredging revealed seabed formations such as pits and depressions filled in and mounds formed by burrowing shrimps removed. Parallel tracks from dredge skids apparent after dredging. Physical changes in the seabed still apparent one month post-dredging. Six months post dredging most physical features reformed (abundance and size of callianassid mounds similar to those present before dredging) however some flattened areas still apparent. No physical differences between dredged and control sites after 11 months.

Species and community effects - Number of species in dredged areas decreased significantly. Maximum impact did not occur immediately after dredging suggesting some indirect ecological changes such as uncovered organisms becoming more vulnerable to predation by invertebrates and demersal fish. Most species decreased in abundance by approximately 20-30% in the 3.5 months after dredging. The duration of the decrease in abundance species varied with effects still apparent in some species after 8 months and in two species up to 14 months although this was possibly due to undersampling in the pre-impact period. 11 animals not found in the sample area after dredging, mostly sedentary and therefore unable to re-establish except by larval recruitment.

Susceptibility to dredging not correlated to feeding type or rarity. Fragile groups such as nemerteans were greatly damaged by dredging, polychaetes probably cut and killed by passing dredge. Other species may have been affected by high rates of dredging induced sedimentation, which may be 2-3 orders of magnitude greater than storm produced sedimentation, or buried when depressions filled in. Two species showed significant increase in abundance following dredging (Diamorphostylis cottoni and Oedicerotid sp.) whereas the isopod Natalolona carppulenta decreased sharply and then increased to be consistently higher on the dredged plot for 8 months possibly due to greater availability of prey.

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.

REF: 4

Shallow inlets and bays

Sandbanks

Witbaard R. & Klein R. (1994). Long-term trends on the effects of the southern North Sea beamtrawl fishery on the bivalve mollusc Artica islandica L. (mollusca, bivalva). ICES Journal of Marine Science. 51: 99-105.

REF: 5

Estuary

Mudflats and sandflats

Experimental dredging using tractor towed cockle harvester.

Habitat effects - Vehicle tracks and dredging furrows created.

Species and community effects - Dredging attracted black-headed and common gulls which fed on very small prey items lying on the surface of harvested furrows including Crangon, Corophium, broken cockles, intact small cockles which pass through the drum, and polychaetes. The number of birds attracted and the places they fed depended on the abundance of prey items revealed by harvesting and presence of people. Peak count at Llanrhidian was 200 black-headed gulls and 55 common gulls, mostly adults which fed preferentially in the most recently harvested furrows. Other species present were curlew, dunlin and oyster catchers. The increased feeding activity of birds was short lived, 14 days for oystercatchers and 7 days for gulls and small waders. Significant reduction in bird feeding activity apparent thereafter and still detectable after four months. Oystercatchers responded more quickly to changes suggesting harvesting may have been less disruptive or recovery quicker.