Recruitment and growth patterns of the portunid crab Charybdis bimaculata in Kagoshima Bay, southern Japan

We studied the recruitment and growth patterns of the portunid crab Charybdis bimaculata in deep water for the first time. The study area, Kagoshima Bay, is a semi-enclosed, deep-water bay characterized by steep slopes, with unique caldera walls where C. bimaculata is one of the dominant decapods and acquiring commercial importance. The monthly length-frequency method was used from a total of 8265 individuals collected over a three study periods—1990s, 2010s, and 2020s—at the central basin (174–229 m depth) and one study period 1990s at the channel area (122–136 m depth). Carapace width ranged from 9.5 to 42.7 mm in males, and 9.5 to 35.0 mm in females. The overall sex ratio significantly deviated from 1:1 favoring females. Recruitment was estimated to occur between September to November in this bay. Recruitment seemed to be synchronized with the peak spawning season with a time lag of 2–4 months. Growth was appositely delineated by the von Bertalanffy growth function for both sexes. The growth equations revealed that males had lower K values and higher asymptotic sizes than females. Spatial and temporal comparison in growth parameters between the same sexes found no statistical differences except in one case.