Commercial shell and fin-fisheries potentially can have a large effect on the integrity of these biotope complexes, their physical structure and their biological components (Jennings & Kaiser 1998; Elliott, 1998). The effects of commercial fishing depend on the type of gear used, substratum type and nature of the resident fauna. Megafaunal benthic species (i.e. organisms >10mm) are in general more vulnerable to fishing effects than macrofaunal species because they are slow growing and thus slowly recover from disturbance and harvesting. If recovery is not permitted, the changes in community structure, such as a decrease in diversity, may be permanent (Jones, 1992).

The potential effects of fisheries on or adjacent to subtidal mobile sandbanks and intertidal areas are summarised as:

• removal of non-commercial sized fish (especially juveniles) and thus affecting the nursery function of the biotope complexes (e.g. juvenile plaice on mobile sandbanks);
• removal, scattering or damage to individuals of non-commercial benthic and demersal species, especially the larger sessile benthic fauna such as the urchin populations (Kaiser, 1996);
• the reduction of community diversity and species richness, for example by commercial digging for worms and other macrofauna in intertidal mud and sand flats (Brown & Wilson, 1997);
• the effects of discards, e.g. an increase of scavengers and a deterioration of water and sediment quality through the organic input;
• the removal of target species leading to community and population changes at the ecological and genetical levels, and the effects on competing predators, e.g. the removal of bait organisms such as Arenicola from intertidal flats and the effects on shorebird predation, and the removal of sandeels, Ammodytes, from subtidal mobile sandbanks and the effects on seabird populations;
• delayed effects on the sea bed, including post-fishing mortality of organisms and long-term change to the benthic community structure (Jones, 1992);
• the change to the physical integrity of the sediment system (through scraping, digging or ploughing of the seabed and intertidal sediments, destruction or disturbance of bedforms, and damage to the benthos) (de Groot, 1984);
• the change to the physical integrity of the water column (by increased resuspension during trawling);
• contamination of the area (through discharges of soluble pollutants and large and small particulate materials, including litter and gear loss); an
• the creation of infrastructure e.g. harbours, jetties (leading to habitat loss, especially of highly productive intertidal areas, and to a disturbance of hydrographic patterns).

The direct and indirect effects of commercial fishing activities on subtidal mobile sandbanks and intertidal sedimentary habitats are poorly quantified; there are few precise case-studies of the effects of multiple fishing methods on these biotope complexes. Similarly, there is no quantitative information of the effects of fishing over other habitats which then has a knock-on effect to the biotope complexes considered here. For example, the margins of subtidal mobile sandbanks adjoining rocky areas are likely to support edible crab and lobster populations and thus will be the site of commercial potting and creel fishing which could disrupt the communities. Similarly, adjacent areas are likely to support beam trawling which will disturb the substratum and the benthic infaunal and epifaunal populations, such as shrimps, and remove juvenile flatfish using the areas as a nursery.

By their nature, the mobile sediments of subtidal sandbanks and their infauna may be more resistant to the sediment disturbance caused by commercial fishing, especially trawling. However, the recovery from the removal of biota, both the target species, often the predators (fishes and macro-crustaceans), and other species, is less well known both for subtidal sandbanks and intertidal flats.

An example of an environmentally-sensitive and conservation important area with multiple fishing activities and sedimentary habitats is Strangford Lough in Northern Ireland (DOENI, 1994). Its commercial trawl fishery for the queen scallop, Aequipecten opercularis, creates conflict with conservation groups through the perceived damage caused to the Lough bed by trawling, principally of the diverse communities associated with the horse mussel, Modiolus modiolus populations (Brown, 1989). The direct effect of trawling may be to reduce benthic diversity by disturbing the Modiolus reefs which provide further niches for colonisation (Erwin et al, 1986). However, the effect of the fishery on adjacent subtidal sandbanks and intertidal flats is not known.

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