A number of effects on the seabed habitats result from scallop dredging. Tracks are created on the seabed, fine sediments are lifted into suspension and large rocks can be overturned^30,40,42,45. A mound of sediment may be carried in front of the toothed bar and deposited around the sides in distinct ridges, most obviously in the case of the spring-loaded dredges³³.

There are reports of the top 100 mm of sediment being disturbed by scallop dredging which has the effect of smoothing out the surface with pits and depressions being filled in and mounds removed³. These physical changes as well as the track marks may still be present months later depending on the conditions at the site. Where there is little current movement the tracks may be visible for a long time and even a relatively minor fishery may have a significant cumulative effect on bottom microtopography³⁰.

Significant decreases in the abundance and number of species have been recorded from dredged areas although, depending on the conditions at the site, seasonal and inter-annual changes such as storm events, may be greater than those caused by dredging^3,16,44,69. It should also be noted that the maximum impact may not be immediate, suggesting that some indirect ecological changes such as exposed organisms becoming more vulnerable to predation, may be taking place³. In one study a 20-30 % decrease in abundance of most species was recorded 3.5 months after dredging, and some differences were still apparent after 8 months. Fragile groups such as nemerteans (unsegmented worms) were directly affected and showed considerable damage³. In another study more than 50 % of the common taxa of macrofauna were affected and significant differences from adjacent reference plots were still apparent after 3 months⁸⁸. The collection and sorting of stones and shells by the dredge can also have an impact by removing encrusting sponges, hydroids, and small anemones and, by reducing habitat complexity may lead to increased predation on juveniles of some harvestable species⁷¹. Burrowing and tube dwelling infauna may be less affected than epifauna⁴². In a study carried out in the Skomer Marine Nature Reserve the numbers of sea anemones, Cerianthus lloydii, Mesacmae mitchellii, and the sand mason worm, Lanice conchilega, within and alongside dredge paths were similar to pre-dredge levels several weeks later. Fragile species such as the bristle worm, Filograna implexa, and the ross coral, Pentapora foliacea, appear to be particularly vulnerable^42,44. Slow growing species will not be able to recover to pre-dredging numbers or sizes even if there is no dredging for several years. In common with other forms of dredging, predatory fish, whelks and hermit crabs are attracted to the track to feed on damaged and exposed animals^30,33.

Effects of scallop dredging across seagrass beds have also been investigated and show significant reduction in seagrass biomass and shoot numbers on both soft and relatively hard seabeds with the potential for both short and long-term effects on settlement of juvenile scallops and other invertebrates⁸⁵.

Investigations into scallop dredging across maerl beds show a major effect on the structure of the habitat and its associated biota¹⁶. The teeth of the dredge may penetrate up to 100 mm and as sediment is suspended by the trawl, fine fractions can be eroded away. Microtopographical effects can be clearly visible 8 months after dredging. Large macroalgae can be torn up and large animals, including highly mobile species can be caught by the dredge. Maerl is crushed and killed through burial which in turn limits opportunities for habitat recovery. The deposition of fine sediment over adjacent areas, smothering of photosynthetic organisms and stress on filter feeders may also occur. Scallop dredging can also have an impact on harder substrates as discussed in the section dealing with reefs.

On gravelly seabeds around the Isle of Man, community composition has been shown to be related to the intensity of commercial dredging effort⁸⁶. Effects may differ from those in areas of soft sediment due to the extreme patchiness of animal distribution, greater abundance of epifauna and the combined effect of the toothed gear and stones caught in the dredges. Impacts may also be apparent in lightly dredged areas, including the loss of a number of species including some potentially fragile tube-dwellers⁸⁵.

There are very few studies which compare the fauna of dredged and undredged areas and therefore give clues about possible long term effects of dredging which may be different from short or medium term effects. One example is a study currently underway off the Isle of Man^57, 86. Provisional results show differences in the epifaunal communities including greater species numbers in the area closed to fishing even under conditions of considerable seasonal variation. A higher density of shallow burrowing and epibenthic species (particularly those noted for their vulnerability to dredging such as the bryzoan, Cellaria fistulosa, and the common sea urchin, Echinus esculentus) have been recorded at the undredged sites. Long-lived and slow recruiting epifauna such as sponges and ascidians are likely to be particularly vulnerable. There is no evidence of longer-lived benthic species being more prevalent at the undredged sites at the moment and it is suggested that this could be due to the relatively short time since effective closure of the area to dredging giving another indication of the time scales required for these species to become re-established in dredged areas.

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