Age, Growth and Reproduction of Two Species of Scad, Decapterus macrosoma and D. macarellus in the Waters off Southern Kyushu

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Growth and reproductive characteristics of shortfin scad Decapterus macrosoma and mackerel scad D. macarellus collected in the waters off southern Kyushu were determined based on otolith readings and gonad histology, respectively. Translucent and opaque zones on sectioned otoliths were identified, and the number of opaque rings counted. Von Bertalanffy growth parameters of both species did not significantly differ between males and females, and the combined growth curves of D. macrosoma and D. macarellus were as follows: FLt = 346 [1 - exp {- 0.456 (t + 0.771)}] and FLt = 428 [1 - exp {- 0.310 (t + 0.821)}], respectively, where FLt is fork length (mm) at age t. The spawning period was evaluated to be from May to August for D. macrosoma and from April to July for D. macarellus based on the monthly changes in the gonadosomatic index and histological observations. The fork length at 50% maturity of females was estimated as 238 mm for D. macrosoma and 258 mm for D. macarellus, and maturity age was estimated for both species as 2 years old.