Life-history theory predicts that when fisheries-induced size selection causes the survivability of large individuals to decline, the average growth rate is reduced and the size-at-maturation becomes smaller. These changes may often reduce biomass and also population size, since most species have higher batch fecundities at larger body sizes. Therefore, many papers encouraged fisheries to develop monitoring and management strategies incorporating these changes. However, the fact that also spawning seasons often differ with spawners’ body size has not been well discussed. Here we show a possible example of a population dynamics affected by the shift of the peak of the spawning season, in addition to the decrease of average body length and size-at-maturation. In mantis shrimp, Oratosquilla oratoria, in Results/Conclusions Computer simulations assuming no shifts of size-at-maturity and spawning-season peak showed larger decrease of population size than the actual data. In addition, simulations assuming no seasonal difference in larval survivability and settlement success showed increase. Thus the simulations suggested positive contributions of the life-history change and spawning-season shift to population dynamics. Simulations also showed that reducing fishing mortality on small individuals sustains the population effectively due to the later spawning peak for small individuals.