Effects of water flow rate and stocking density on the early pre-fattening of Manila clams (Ruditapes philippinarum) farmed in a North Adriatic “Valle da Pesca” (Italy)

Abstract

The Manila clam (Ruditapes philippinarum) is one of the main products of Italian aquaculture and is primarily farmed in the north-Adriatic lagoons. To support this important farming sector, there is a need for increased production of clam seed pre-fattened to a size suitable for sowing in the lagoon, i.e., approximately 8–10 mm. The early pre-fattening up to 4–5 mm in shell length is critical and preferably requires controlled upwelling systems similar to those adopted by hatcheries. These could be implemented in the Italian “valli da pesca,” which are semi-natural confined environments, historically exploited for extensive fish-farming and located in the innermost areas of the north-Adriatic lagoons. The implementation of clam pre-fattening activity in these environments of high naturalistic value, which are currently no longer profitable, presents an excellent opportunity to enhance their valorization and preservation. In this study, two pre-fattening trials were conducted, in autumn and in spring respectively, in an experimental “valle da pesca” equipped with an upwelling system. The aim was to define the optimal conditions of stocking density (approximatively 100–300 clams/cm²) and water flow rate (approximatively 10–20 mL/cm²) for clams with an initial shell length of 1.8–2.6 mm. The clam specific growth rate (SGR) varied between 2.3 and 5%/day, decreasing as stocking density increased and water flow rate decreased. The results suggest that a water flow rate per unit of biomass > 15 mL/min/g fresh weight (FW) (preferably > 20 mL/min/g FW) could support satisfactory clam growth rates. Chlorophyll-a maintained an average concentration of 1.3 mg/m³ in spring and 3.8 mg/m³ in autumn, respectively, but without a clear impact on the clam SGR.