Fisheries Experimental Station, Wakayama Research Center of Agriculture, Forestry and Fisheries, Kushimoto, Wakayama 649-3503, Japan
The age at maturity, spawning season and spawning frequency of threeline grunt Parapristipoma trilineatum occurring along the south-western coast of the Kii Peninsula, Japan, were investigated. Histological observations of gonads demonstrated that no females at age 1 year and all females over 2 years were mature, compared with all males over 1 year. Seasonal changes in ovarian maturation, together with seasonal occurrence of postovulatory follicles, suggested that the spawning season in the region was from May to August, peak spawning occurring in June. Seasonal changes in gonadosomatic index (GSI) differed slightly from the former in females over 3 years, peaking in May and dropping in June, thereby indicating a rapid deposition of yolk from April to May, followed by highest spawning activity in June. Decreased GSI in June seemed attributable to the increased number of individuals in which gonad weight had temporarily decreased due to high spawning activity. The estimated spawning frequency in each month between May and August was 0.17, 0.51, 0.26 and 0.19, respectively.