A system dynamics model for the management of the Manila clam, Ruditapes philippinarum (Adams and Reeve, 1850) in the Bay of Arcachon (France)

Abstract

The Manila clam Ruditapes philippinarum (Adams and Reeve, 1850) is one of the mollusc species that, driven mainly by the shellfish market industry, has extended throughout the world, far beyond the limits of its original habitat. The Manila clam was introduced into France for aquaculture purposes, between 1972 and 1975. In France, this venerid culture became increasingly widespread and, since 1988, this species has colonised most of the embayments along the French Atlantic coast. In 2004, this development resulted in a fishery of ca. 520 t in Arcachon Bay. Within this context, evaluation of the clam stock in Arcachon, as undertaken by the French Institute for the Marine Research (IFREMER) in 2000, 2003 and 2006, underlined: (a) the presence of disequilibrium in the population structure and (b) the need for an improved knowledge of the population dynamics. In order to meet these requirements, a research project was initiated. The main objective of this study was to provide an improved knowledge of the biotic and abiotic parameters which control the dynamics of the clam population, within Arcachon Bay (growth, mortality, reproduction, trophic sources, etc.). With these data, the aim of the present study is to develop a system dynamics model, capable of predicting clam population evolution within the embayment, in response to different management measures. Once the model was constructed, the effect of diverse management measures and environmental scenarios on the stock response, over time, was simulated: (i) sensitivity to exceptional environmental changes (sea water temperature); (ii) changes in the minimum legal size for capture; (iii) changes in season capture duration; (iv) an increase of the surface area of no-take zones. Considering all the assumptions made here, the best management decisions are those which are able to maximise, at the same time, the captures and the number of shellfishers; likewise, minimise stock losses, as a consequence of this exploitation. Taking into account this approach, the best management decisions are, in order of efficiency: (i) the minimum legal size for capture maintenance; (ii) a reduction in the capture season; (iii) an increase of no-fishing zones.