Impacts of shifts in spawning seasonality and size at maturation on the population growth of mantis shrimp in Tokyo Bay

The spawning season of mantis shrimp Oratosquilla oratoria (Stomatopoda) in Tokyo Bay has changed from being unimodal (spring only) to bimodal (spring and summer).This shift in the pattern of spawning arose because reproductive maturation began to occur in smaller females, which presumably spawn in summer, and because the proportion of smaller mature females increased. Cur- rently, recruitment appears to depend entirely on the summer cohort because of significantly lower juvenile survival in the spring cohort. In this study, we propose that the new summer spawning period and spawning by smaller-bodied individuals have supported the population. Our size- and age- structured mathematical model of O. oratoria population dynamics indicated that the number of eggs spawned in spring is lower than that in summer, and 98% of the annual recruitment is currently derived from summer spawning. The model also suggested that smaller-bodied spawners provided 84% of total recruitment and that if the proportion of smaller-bodied mature females had not increased, the population would have shown a larger decline than was observed. In reality, the stock size of O. oratoria has become extremely low, damaging the local fishery. Based on our model predic- tions, an efficient solution for population recovery is to reduce the mortality of small, unsalable indi- viduals, which are currently fished and discarded.