2008‒2010年の西日本沿岸域におけるマイワシの性成熟，産卵期およびバッチ産卵数

抄録

We examined the sexual maturation, spawning period and batch fecundity of Japanese sardine Sardinops melanostictus collected from coastal waters off Kochi Prefecture (KP), northwestern Kyushu (NWK), and the San-in district (SD) during the 2008-2010 period of low stock abundance to understand the reproductive characteristics of the current stock biomass. All male specimens were mature in all age classes examined in the three locations. Similarly, most females in these locations were mature although inter-annual variation in the percentage of mature 1-year-old females was found in SD in association with differences in the body length and relative condition factor of individuals. Monthly changes in ovarian condition showed differences in the peak spawning period among sampling locations. The relative condition factor of females in the three locations decreased and reached a minimum at the peak spawning period, thereafter it increased from the late spawning period. Spatial variation in the relative condition factor of mature females was evident as females in SD had a better condition than those in KP and NWK. There were no significant differences in the relative batch fecundity between specimens at age 1 and 2+ years and among the three locations. These findings indicated that the majority of 1-year-old specimens off western Japan matured and spawned in contrast to the evidence found during the past time of higher and lower stock abundance.