Cockles are generally associated with the intertidal but can be found in the subtidal. Most studies on the effects of dredging for cockles (Cerastoderma edule) have examined the use of mechanical dredges which use a plough-like blade to remove sediment from which the cockles are subsequently sorted.

Tractor harvesting - Studies have shown that tractor-towed harvesters leave vehicle tracks as well as dredging furrows which remain visible for varying amounts of time depending on the conditions at the site⁵. In an area of stable sediment (poorly sorted fine sand) dredge tracks may be visible for long periods (more than 6 months have been recorded) whereas in more mobile sediments there may be no alteration in sediment parameters⁶. On areas of cohesive sediment the tracks appeared to act as lines from which erosion of the surface layer spread out. This appeared to accelerate the erosion phase of a natural cycle of cohesion of the surface sediment by worm tube mats⁶². Dredged areas often had a lot more dead shell scattered on the surface, an effect which can persist for several months. In undisturbed beds, most dead shell is normally under the surface which can create a shell layer limiting the depth to which small drainage channels can normally erode into a cockle flat⁶².

The effect on infauna also depends on the exposure of the site^6,18,36. Research to date suggests that in an area of stable sediments, as well as large reductions in the target species, mechanical dredging can result in a significant decline in numbers of the lower spire shell (Hydrobia ulvae) and decreased numbers of Pygospio elegans, a segmented worm whose tubes may be removed by the dredge^6,18. These effects may still be apparent 6 months later⁶. The sand mason worm (Lanice conchilega), on the other hand, has more robust tubes and can retract below the depth disturbed by the dredge^{18, 62} and although the distribution of white ragworm (Nephtys hombergii) was affected by dredging, populations have been shown to recover within six months⁶.

In Scotland there is a general prohibition on dredging for cockles from or by means of any vehicle, eg. tractor dredging. This was specifically adopted following concerns as to the direct effects of large scale dredging operations on cockle stocks and the indirect effects to the intertidal habitat^15,36,93. In England and Wales some Sea Fisheries Committees have in place byelaws which specify the design or type of equipment that can be used to target cockles. In this way, tractor dredging has been prohibited in many areas.

Suction dredging - Suction dredgers or hydraulic continuous lift dredgers - to be more accurate - are deployed from specially adapted or specially built shallow draft vessels and are used to harvest cockles in the Wash and Thames in particular. Depending on the stability of the sediment surface at the time and the prevailing tide or wind conditions, evidence of the tracks left by the dredge head, can persist for several months⁶². Where dredging was carried out in a sheltered area with eel grass (Zostera) beds, (Auchencairn Bay, Solway Firth), breaking the sward allowed erosion that produced clearly visible grooves down the shore⁶². The immediate effect of hydraulic dredging on the infauna can be significant. Studies have shown up to 30% reductions in the number of species and 50% reduction in number of individuals. Comparison between dredged and undredged areas have shown recovery times varying from 14-56 days ⁹³.

In general the overall decrease in biomass of target species and non-target species is likely to be more pronounced in areas with stable environmental conditions and diverse communities. In sites with moderately mobile sediments it is possible for natural disturbances to have a greater effect than dredging^{6, 77}. Sites with more tube dwelling and sedentary species appear to take longer to recover to pre-fishing levels than areas with more mobile fauna.

The time of year of exploitation will also influence recovery³⁶. Avoiding dredging during periods of larval settlement or spawning for example, can reduce time required for the restoration of infaunal communities. The sediment may change, at least in the short term, but how long this remains the case also depends on the exposure and stability of the site.

Effects on birds are varied. In some cases short-term increases of gulls and waders in the harvesting area, followed by a long term significant reduction in feeding opportunities for these birds= has been noted⁵. In contrast, research linked to the Solway fishery concluded that because natural changes are very large the fishery may not have a significant effect on bird numbers unless a high proportion of the cockles are harvested⁶².

A simulation model tested on the Exe estuary has been developed to explore the consequences of changes in fishing activities and bird numbers on commercial shellfish stocks and on the birds themselves⁶³. Key predictions include that where a number of conditions apply it is possible to exploit shellfish stocks without increasing the winter mortality of shorebirds, that the effects of a given intensity of shellfishing depend crucially on local conditions of the climate and general abundance of food and that as fishing effort increases, shorebird mortality may be hardly affected initially but then may suddenly increase dramatically once a threshold level of fishing effort has been reached⁶³.

Hand gathering - Hand gathering for cockles is the only permitted form of cockle fishery in some areas. No information was found on the effects of large scale hand gathering for cockles. However, disturbance to feeding and roosting birds, which is a concern in relation to bait digging on intertidal flats could also be an issue for cockle gathering from intertidal areas. This issue is addressed in more detail in the related report on collecting bait and other shoreline animals (Fowler, S. 1999).

Next section                     References