Improving Pre-Fattening Protocols for Manila Clam (Ruditapes philippinarum): A Technical Comparison of Upwelling and Flat-Bottom Rearing Systems

Pre-fattening (also referred to as nursery culture) of Manila clam is a priority for this sector of aquaculture, as it allows hatchery-produced seed (1–3 mg) to reach sowable juvenile sizes of 30–100 mg and reduces reliance on natural juvenile recruitment. This study evaluated the efficiency of two early pre-fattening systems, both in economic terms and in product quality: conventional upwelling units (a high-density system) and flat-bottom tanks (a mid-density system), the latter tested with and without a sand layer. The 51-day trial was conducted under autumn environmental conditions (temperature 13–25.8 °C; salinity 25–28 ppt; chlorophyll-a 3–24 µg/L), starting with 1.34 mg seed maintained under a water flow rate ≥ 15–20 mL/min/g. In upwelling units, the initial density was ~216 ind./cm2. Four grading events produced four size classes, with total mean weights ranging from 6.4 mg in the smallest (tails) to 46.3 mg in the largest (heads). The overall population mean size was 19.0 mg, with a specific growth rate (SGR) of 5.2%/day and mortality of 17.6%. Flat-bottom tanks, stocked at ~30 ind./cm2, achieved higher growth (overall weighted mean: 28.0 mg; SGR ~6%/day), but exhibited higher mortality (26.0% on average), with no significant effect from the presence of bottom sand. Overall, flat-bottom systems showed promising growth performance with reduced labor requirements, suggesting that this system could represent a viable alternative to upwelling. However, the associated rearing protocol could still be improved by optimizing stocking density and water exchange rates.