Changes in reproductive patterns in relation to decline in stock abundance of the Japanese mantis shrimp Oratosquilla oratoria in Tokyo Bay

We investigated changes in the reproductive patterns of mantis shrimp Oratosquilla oratoria concurrent with stock-abundance decline in Tokyo Bay, Japan. Stock abundance was high in the mid to late 1980s but decreased abruptly in the early 1990s. The yearly change in annual mean larval abundance was similar to that of stock abundance. Mantis shrimp in the bay have two spawning seasons, an early season (May–June) for ≥1-year-old individuals and a late season (July–September) for 0–1-year-old individuals. This general reproductive pattern does not differ among different stock-abundance levels. However, the monthly pattern in larval abundance has changed with stock-abundance decline; larval abundance from the early spawning season was highest in the high-stock-abundance period, and it decreased significantly in the low-stock-abundance period, probably as a result of decreased spawning-stock abundance of large female mantis shrimp ≥1-year-old. Correlation analysis on the egg production index and larval abundance suggested that during this low-stock-abundance period the population is supported mostly by late-hatched larvae spawned by small, 0–1-year-old female mantis shrimp. Considering the reproductive pattern and the present status of the fishery, the stock of small female mantis shrimp should be conserved to enhance reproduction of the population for stock recovery.