In order to examine the distribution of the spawning ground of the Japanese mantis shrimp Oratosquilla oratoria in Tokyo Bay, sampling surveys with small bottom beam trawlers at 10 stations were carried out in 1987 and 1988.
As was already reported, the percentage occurrence of mature females during a spawning season showed two peaks, the first one between April and May, and the second between July and August. In the first half of the spawning season including the first spawning peak, the main spawn-ing ground was almost limited in the area deeper than 30m. Most of mature females were above 2 years old. In the latter half of the spawning season including the second peak, the main spawning ground extended towards the southeastern shallower area of the bay and females above 1 year old matured. At around the second peak of the spawning, it seems that the formation of the main spawning ground is influenced by yearly fluctuation in north-south gradient of bottom DO in summer.
