In situ fast marking study of manila clams (Ruditapes philippinarum)

Arcachon Bay, in the southern Bay of Biscay, is the first French production area of Manila clam (Ruditapes philippinarum) with an annual commercial fisheries production around 400 tons in recent years. This bivalve mollusc was introduced into the bay in the 1980s for aquaculture purpose and quickly succeeded in natural settlement so that its high market value led to the progressive establishment of a perennial fishery from the mid-1990s. Local management plans applied to this species are based on a licensing system for the commercial, protected fishery areas and days of prohibited fishing for both commercial and recreational fishermen. A minimum catch size is also prescribed by European regulation, and so growth rates are an important matter in this context. This work focuses on studying shell growth from the recapture of shells, previously marked using a chemical marker (calcein) and then grown in natural conditions, and was performed in order to determine shell growth patterns. Manila clams were marked by immersion in situ in seawater containing calcein. In order to study the limitations of calcein on the mark readability in situ and manila clam mortality, various exposure times, from 30 minutes to 1 hour, and concentrations, from 50 mg.L-1 to 200 mg.L-1 were tested. After a period of 35 days, 69 growth micro-increments were observed, and, it was found that increment deposition in manila clams occurs with a tidal periodicity. Moreover, this study showed that a method for fast-marking of manila clams, and potentially other species, is feasible in situ.