MEPS 324:197-206 (2006)  -  doi:10.3354/meps324197

ABSTRACT: Bottom hypoxia is considered to be one of the factors affecting the recent stock decline of the mantis shrimp Oratosquilla oratoria in Tokyo Bay, Japan. We used field surveys of the bay to investigate the spatiotemporal pattern of bottom hypoxia and the early life history of O. oratoria, and we examined the effects of bottom hypoxia on early life history by nonparametric analyses. Bottom hypoxia in Tokyo Bay began to appear in April, occupied more than half (55 to 67%) of the whole bay area in July and August, and disappeared from November onward. Newly settled juveniles appeared in September–October in the northeastern shallow coastal area, where the hypoxic bottom water had disappeared. After the hypoxia had abated, the distribution of juveniles expanded to the south-central deep area. Classification and regression tree analysis showed that the threshold level of bottom dissolved oxygen concentration for the existence of juveniles was 2.78 ml l^–1, implying that hypoxia restricted the spatial distribution of juveniles. A generalized additive model showed that sampling date, bottom dissolved oxygen concentration, depth, latitude, and longitude had significant effects on the occurrence of juveniles, suggesting an adverse effect of hypoxia on the time and location of settlement of O. oratoria. Our results suggest that hypoxia is directly and/or indirectly associated with mass-mortality events during early life history.

KEY WORDS: Hypoxia · Settlement · Recruitment · Juvenile · Oratosquilla oratoria · Tokyo Bay