Summaries of reviewed publications references 36 40.

Details are limited to information relevant to the UK marine habitats and species listed in the Habitats Directive and the Birds Directive.

[pr] indicates that the paper is from a peer reviewed journal or report


Natura 2000 Habitats & Species

Fishing Technique




REF: 36

Mudflats and sandflats



Tractor towed cockle harvester

Investigated the use of tractor towed cockle harvester on invertebrate fauna. Smaller interstitial forms were not greatly affected in most cases significant reduction in species numbers occurred immediately after dredging with continued decline for at least two weeks subsequently. After that a few species showed signs of some recovery others did not, although seasonal trends were obviously important for several of the latter type. Effects at Site A (more tube dwelling and sedentary species) were obvious for longer than 3 months and the dredged area was still visible after 6 months. At Site B (more mobile fauna) natural winter weather disturbances resulted in changes of greater magnitude than those caused by dredging. Results suggested the importance of a stable environment, including surface microflora, for maintaining certain diverse community types and also revealed interesting patterns. Some types of benthic intertidal communities would be adversely affected by commercial tractor towed cockle harvesting.

General conclusions from both this study and a 1990 study at Lavan sands are similar in that effects of dredge.

1. Result in a much decreased biomass of the target species, numerical reductions and likely decreased biomass of non-target species.

2. Are much more pronounced in areas with diverse communities and stable environmental conditions have some effects on certain types of sediment and can change sediment parameters at least in the short term.

3. Depend on the time of year the cockle bed is being exploited will be most severe if sufficient recovery time is not allowed.

Results from this study did not agree with the conclusion that recolonisation takes place fully and quickly from nearby areas. Effects were obvious at Site A even at the end of the experiment.

General effects on birds. Reductions in Hydorbia ulvae populations could affect shelduck, knot, dunlin and redshank. Disturbances to bivalve molluscs could affect oyster catcher, shelduck, knot, curlew and eider ducks, the latter however preferring M. edulis. Polychaetes are important in the diet of curlew, dunlin, bar tailed godwit and redshank although the latter prefer Nereis from the upper shore regions. Amphipods figure prominently as food for dunlin, curlew, oystercatcher, knot and shelduck.

Burry Inlet - Loughor Estuary (Llandhidrian sands)

Rostron D. (1993). The effects of tractor towed cockle dredging on the invertebrate fauna of Llandhidrian Sands, Burry Inlet. Subsea Survey. Report to Countryside Council for Wales.

REF: 37

Harbour porpoise

Bottlenose dolphin

Gill nets

Incidental capture of cetaceans in gill nets is geographically widespread and considered a severe problem. Most capture dolphins and porpoises although large cetaceans are also vulnerable to entanglement. Large incidental catches can occur in coastal gill net fisheries which can have a greater impact than oceanic fisheries because coastal cetaceans often have more restricted distributions than oceanic relatives. Several proposals to reduce impact are discussed.


Dawson S.M. (1991). Modifying gill nets to reduce entanglements of cetaceans. Marine Mammal Science 7(3): 274-282.


REF: 38

Harbour porpoise

Bottlenose dolphin

Pelagic trawls

Trammel-gill bottom nets

Both nets and trawls are involved in the incidental capture of dolphins however accurate estimates of by-catch cannot be made because of lack of relevant data. High opening pelagic trawls towed by pairs of boats and combined trammel-gill bottom nets tied together in a row about the continental shelf are perhaps the most likely cause of large dolphin by-catch.

French Atlantic coast

Charreire F. (1993). A report for Greenpeace on recent dolphin strandings along the French Atlantic coast.

REF: 39

Shallow inlets and bays


Pelagic trawls

Trammel-gill bottom nets

Five maerl beds surveyed in the upper parts of the Firth of Clyde. Some information on the impact on maerl habitats obtained from examination of catches during experimental dredge runs. Preliminary findings. Each ground was a focus of high infaunal diversity and biomass consisting primarily of Phymatolithon calcareum.

Immediate effects a bow wave of fine particulates suspended ahead of the gear. Bobbins usually rolled along the surface but ploughed into the sediment by up to 4cm when the two-bar was skewed on impact with large boulder leaving trenches of crushed maerl. Cobbles and boulders up to a 1m3 were dislodged and overturned when hit by the tow bar or dredge mouths. Dredge teeth projected fully into the maerl deposits. Maerl flicked over dredge mouths creating a cloud of suspended sediment in the wake of the bar. Large macroalgae L. sacaharina torn up as dredge dragged through the sediment and large animals Echinus, Echinocardium, Luidia, Mya, Ensis, Ascidella aspersa were either mangled or entrained or flicked into the chain mail bags. Even highly motile elements were caught eg butterfish, plaice, L. depuratur. The dredging has major repercussions for the structure of maerl habitats and associated biota.

Firth of Clyde

Hall-Spencer, J. (1995). The effects of scallop dredging on maerl beds in the Firth of Clyde.

REF: 40

Shallow inlets and bays




(Mudstone reefs, cobble and bulder seabed, sandy areas with boulders and sandy substrates)

Scallop dredging

Single pass of full sized scallop dredge (12 spring-loaded dredges, deployed either side in groups of 6 attached to two beams) along 300m transects. Video recordings before and after and survival studies of specimens in laboratory for 14 days.


Habitat effects - Scallop dredging can alter the substrate composition. Stones and boulders (up to 60cm in length) overturned, small boulders piled against larger boulders, fragments of mudstone reef broken off, sand waves in the dredge path completely obliterated, suspension followed by settlement of fine sediments disturbed by the dredge and displacement of substrate (apart from mudstone, loose rocks brought to the surface and shovelled off the deck once the catch had been sorted). Overall there was a markedly changed appearance the most striking being the covering of all boulders and rocks with a fine coating of sediment. Chipping and movement of cobbles and boulders has implications for the habitat of juvenile crabs, particularly Cancer pagurus, which appears to inhabit the areas of soft mudstone. Of the habitats studied, area of sand waves was probably the least vulnerable to scallop dredging in the long term.

Species and community effects - Changes in species observed before and after dredging due to various factors; revealed by dredge as substrate overturned, dug out of substrate (eg Pomatocerus triquiter, Pecten maximus) or dislodged off the interstices eg Maia squanado; species hidden Porifera, destroyed Pentapora foliacea, injured or killed by action of dredge (adult crustaceans) and attracted by injured specimens in wake of the dredge Pollachus spp crustaceans. Survival of dredged specimens in laboratory tanks showed surprising resilience of juvenile C. pagurus and Pholus dactylus which remained in the honeycomb mudstone, sea squirts died rapidly compared to controls and starfish exhibited comparable survival between experiment and control. No clear cut evidence in the case of P. foliacea and E. verrucosa but these most likely to suffer from being displaced as unlikely to re-establish themselves so mortality of these species seems likely.

Response of the whole system to dredging will depend on resettlement and growth of new stock and whether the substrate is suitable for this. The vulnerability of the system switching to another system would depend on importance of the species affected. If slower growing species with poor recruitment (eg E. verrucosa or slow growing but rapidly recruiting (eg P. foliacea) hold the system in its present form there is a high risk of complete change.

Lyme Bay (Beer Home Ground and Eastern Heads)

Sea Fish Industry Authority (1993) Benthic and ecosystem impacts of dredging for pectinids (reference 92/3506) Consultancy Report No.71

Next Section