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

Abstract

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. Currently, 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 agestructured 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 predictions, an efficient solution for population recovery is to reduce the mortality of small, unsalable individuals, which are currently fished and discarded.

Population dynamics of mantis shrimp Harpiosquilla harpax and Oratosquillina sp. in the waters south of Madura Island, Indonesia

“…According to Ahyong et al (2008), mantis shrimp reach the maturity stage at a length of 10-30 cm. On the other hand, Nakajima et al (2010) reported that for Oratosquila oratoria, maturity started from 10 cm. The sex ratio of the two species of mantis shrimp differed, which may be related to differences in the behavioral pattern of spawning.…”

“…This result was related to recruitment periods in the preceding months. Nakajima et al (2010) reported that in the period of May-July the lengths of Oratosquilla mantis shrimps were < 10 cm in length, while in August the Oratosquilla were bigger (i.e. >10 cm in length).…”