The second important pelagic fishery at Tuticorin is Thryssa spp. The food of T. mystax is mainly zooplankton. Juvenile and adult fish differ in their food habits. The length weight relationship is in accordance with cube law (W=0.00002383 L^3.1063). The point of inflexion in the ponderal index is noticed at 122 mm size which denotes tne size at first maturity of this species. It has a prolonged spawning season right from December to June with a mean fecundity value of 12,703. Based on the ova diameter frequency polygons, it may be inferred that T. mystax spawns during a definite period between December to June.