Basically, a dredge is a bottom-towed gear and consists of a metal frame with a blade or teeth, to dig into the sediment and extract the shelled molluscs, and a mesh bag to collect the catch. Whereas hand dredging is observed on some intertidal beds (Donax trunculus in France), the use of dredge generally needs a fishing boat. There is a great diversity of dredge types, with specific designs used to target different species in particular beds: Manual dredges for Donax (Donax trunculus), Warty venus (Venus verrucosa) and clams (Venerupis pulastra, Ruditapes sp) Dredges for King scallops (Pecten maximus) Dredges for Warty venus (Venus verrucosa) Dredge for Queen scallops (black and white, Chlamys varia and Chlamys opercularia) Dredges for small bivalves : Thick trough shell (Spisula ovalis), palourid (Venerupis rhomboïdes), bittersweet (Glycymeris glycymeris) Dredge for Mussels (Mytilus edulis) Except in the case of scalloping where the dredges designs are widely used, fishermen European used a variety of local designed dredges adapted at the target species. The inter-specific selectivity of these gears is generally very high and the by-catch rate is low : however, in the norman-breton gulf it is possible to catch bittersweet or whelks with a warty venus dredge and to catch a mixed of bittersweet palourdi and spisula with the small clam dredges.