Size selectivity of hand dredges targeting black clam (Villorita cyprinoides)

Abstract

Black clam (Villorita cyprinoides) fisheries in India support thousands of small-scale fishers and account for more than 90% of estuarine clam landings in Kerala. Harvesting is conducted using hand-held dredges, yet the selectivity of these gears has never been quantified. Recent adoption of small mesh sizes in the fishery has raised concerns regarding retention of juveniles below the minimum legal width (MLS = 20 mm). This study experimentally quantifies the size selectivity of hand dredges equipped with two mesh sizes in Vembanad Lake (25 and 30 mm nominal). Furthermore, clam morphometrics were measured to develop a morphology-based predictive framework for dredge size selectivity by linking clam circumference to mesh circumference. Among five hypothesized shell–mesh contact modes, one mode reproduced the experimental selectivity curves for both dredges with high accuracy (R² > 0.999), indicating that retention is primarily governed by geometric constraints. Model predictions demonstrate that the recent adoption of smaller meshes (18 mm – 20 mm) shifts the 50% retention rate below the MLS, whereas meshes of approximately 28–29 mm are a better match. This study provides the first quantitative assessment of hand dredge selectivity in a tropical estuarine clam fishery and establishes a mechanistic basis for evidence-based mesh regulation, offering a framework that can guide mesh regulations for clam fisheries worldwide.