The growth and dynamics of Ensis directus in the near-shore Dutch coastal zone of the North Sea

Abstract

Here we present data on the wax and wane of a subtidal Ensis directus population which settled in 2009 off the coast of Egmond (North Holland Coast in The Netherlands). Initial densities decreased from a maximum of 700m−2 in early 2010 to about 50m−2 in June 2013. In this period the average length increased from ~4cm to ~12cm. In 2011–2012 the population was sampled at 3 to 6week intervals and near bottom environmental conditions were monitored continuously. Samples of animals that were collected were used to follow the change in gonadal mass, tissue glycogen content, tissue weight and shell length. On the basis of these data well defined seasonal cycles were observed. The data indicate that the maturation of gonadal tissue already starts early in the year, initially at the expense of somatic tissue. Main spawning takes place in May. After spawning net somatic tissue growth starts after compensation of losses due to spawning. Somatic growth precedes shell growth which starts at water temperatures exceeding 12–14°C. Mortality, growth and production are comparable to those found for populations in close-by intertidal areas. As such there is no indication that this offshore population significantly suffers from nearby beach nourishments along the Dutch Coast.