Abstract
Reproductive cycle, maturation mode, and body size and age at maturity of the hexagrammid fish, Hexagrammos agrammus, at Aburatsubo in central Japan were examined by analyzing the seasonaltrends in gonadosomatic index (GSI) and gonadal maturity as revealed by histological observations.Gonads were assigned to the following 5 phases of maturity: (0) immature phase, (I) maturing phase, (II) early spawning phase, (III) late spawning phase, and (IV) recovery phase, based on conditions ofthe most developed oocytes and the presence of empty follicles or atretic oocytes for females, and onthe presence and the amount of spermatocytes or sperm for males. Females appeared to be multiplespawners, i.e., spawning more than oncein a spawning season, judging from the coexistence of oocytesat the pre-mature stage and empty follicles in mature ovaries. Spawning, corresponding to maturity phases II and III, occurred from late November to late January. All females aged 1+and older matured.Smaller 0+females less than 105 mm in standard length (SL)did not spawn within the current spawningseason, but larger fish (≥105mm SL) spawned in the season. Testes of all males were ripe at age 0+ (≥88mm SL), but the firstegg-guarding age was 1+ (121mm SL).
