Based on long-term observation data (from April 1993 to December 2002) on megabenthos in Ise Bay, central Japan, seasonal and interannual variations in abundance, biomass and spatio-temporal distributions were examined for the dragonet Repomucenus valenciennei dominating in the bay. Then, to examine when and where their recruits are located within the bay, recruits were collected using small-size trawl nets from May 2000 to December 2002. The abundance and biomass of the dragonet decreased in summer when oxygen-poor water developed in the bay, and increased from autumn to next spring when the oxygen-poor water disappeared. Recruits were mainly located in the innermost part of the bay in spring to winter, particularly in winter. According to the cohort separation based on body length histograms, the decrease of the dragonet abundance in summer was induced by avoiding the oxygen-poor water or by high mortality due to the oxygen-poor water. On the other hand, the increase of the dragonet abundance from autumn to next spring was due to recruitment.
The present study deals with seasonal and interannual variations in abundance, biomass and spatio-temporal distributions of the mantis shrimp Oratosquilla oratoria in Ise Bay, central Japan. The abundance and biomass of the mantis shrimp decreased in summer when oxygen-poor water developed in the central to inner parts of the bay, and then increased due to new recruits from autumn to spring of the following year when the oxygen-poor water disappeared. Planktonic larvae appeared from June to November, whereas two cohorts were detected for new recruits: one in summer (``summer'' cohort) and the other in autumn (``autumn'' cohort), respectively.Usually the``summer'' cohort grew to 50 mm BL by autumn and became a target forˆsheries. This``summer'' cohort largely contributed to establishing and maintaining benthic populations of the mantis shrimp in the bay.
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