Sediment Types with Alternation of Sandy and Rocky Shores Influence the Distribution of Clams in an Area Characterized by High-Energy Hydrodynamic Conditions

Abstract

To identify short-term changes (14–69 days) in the adult abundance of two closely related shallow-burrowing bivalves (Ruditapes spp.), a series of observations and displacement assessments were made during the 2014–2018 period. This study was initiated to estimate the natural displacement of clams in a high-energy hydrodynamic tidal regime along the western coast of Cotentin in Normandy, France (western basin of the English Channel, northeastern Atlantic). The raking of several different surfaces and sediment types at successive periods separated by about one month shows clam displacement in most of the selected quadrats. The mean abundance displacement derived from all the observations carried out in 2014, 2016 and 2018 is estimated at 1.8 ind·m−2 mo−1, a value that should be compared with the mean density of between 2.0 and 12.5 ind·m−2 along the western coast of Cotentin. These displacements are confirmed with experiments using clams marked with an inox metal washer and detected with a Minelab Sovereign GT multi-frequency metal detector. During the course of the experiments, about 20% of the clams show a displacement. Most of the displacements are moderate, being less than 2 m, but some clams could be displaced by more than 20 m.