Evaluation of Ruditapes spp. Clam Stock on the Western Coast of Cotentin (English Channel)

Abstract : The Manila clam Ruditapes philippinarum (Adam and Reeve 1850) was introduced into France in the 1970s and now dominates the European clam Ruditapes decusattus (Linnaeus 1758) in the intertidal zone of the western Cotentin (English Channel). While it has successfully colonized a large variety of sediment habitats, from muddy sand to gravel, the densities remain< 20 ind. m-2. In this extensive intertidal zone, it is difficult to estimate the density as well as the stock of such dispersed species of clam. Our study has three main objectives: i) to determine a better sampling strategy to estimate the density of Ruditapes spp. clam populations; ii) to assess the spatial distribution of clams at the scale of the western coast of Cotentin (30 km of coastline from North to South), and iii) to evaluate the stock and annual quantities of clams fished by recreational and professional fishing in this area. For the estimation of densities and stock of Ruditapes spp., it is better to continue using a quadrat area of 1 m² for a large number of random sampling points (≥ 400) to estimate with the greatest possible spatial precision the local and regional abundance of such low-density populations. Sampling techniques with smaller quadrats and more sampling points lead to higher standard deviation of the mean densities. The species are distributed in four main patches covering between 0.54 to 3.14 km² out of a total of 10.47 km² (~ 10% of the intertidal zone). The 2015 stock (Ruditapes> 40 mm, legal catch size) in these four patches is estimated as 382 t (fresh weight with shell) for a total of harvest of 122 t (32% of the adult stock), which corresponds to a catch of 8 t by professional fishers and 113 t by recreational fishers.