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We investigated effects of severe hypoxia (dissolved oxygen <1 ml l-1) on recruitment of mantis shrimp Oratosquilla oratoria in Tokyo Bay. Ten-year field surveys were conducted to examine quantitative relationships in annual mean densities of larvae and juveniles, and spatial distribution of juveniles and severe hypoxia. There was no significant correlation between annual mean densities of larvae and juveniles, suggesting that mortality during larval or juvenile stages varies among years, which might have regulated abundance of young-of-the-year juveniles. Juvenile density was low in the severely hypoxic area, implying that hypoxia could affect survivals and spatial distribution of juveniles. Meanwhile, there are yearly fluctuations in juvenile density in normoxic areas of both northern and southern part of the bay. This evidence suggests that abundance of post-settled juveniles might have been determined by not only effects of hypoxia, but also other factors influencing mortality during the early life stages. [ABSTRACT FROM AUTHOR]

We investigated the reproductive biology of the stomatopod Oratosquilla oratoria for both males and females of different size classes. Samples were collected from Tokyo Bay, Japan, by monthly bottom trawl surveys from May 2004 to October 2005. Histological examinations of testis, vas deferens, and penis for males and ovary and seminal receptacle for females were conducted. Males attained sexual maturity at a smaller body size (between 4 and 5 cm body length [BL]) than did females (BL > 7 cm). Although there were significant changes in the developmental stage of the testis between months, medium- to large-sized males (BL ≥ 7 cm) stored sperm in the vas deferens and penis throughout the year, indicating year-round functional maturity of the males. On the other hand, females showed clear seasonal patterns in ovarian maturation. Mature females were found between March and September. The early (May-June) and late (July-September) spawning peaks involved large (BL ≥ 10 cm) and medium (7 ≤ BL < 10 cm) size classes of females, respectively, confirming a similar trend shown by previous studies for size-related seasonal patterns in ovarian maturation. In addition, sperm was found in the seminal receptacle concurrent with ovarian maturation, suggesting that the timing of the onset of copulation is regulated by the sexual receptivity of females. This study is the first to reveal this size-related reproductive pattern in both sexes in the Stomatopoda. [ABSTRACT FROM AUTHOR]

We examined the age and growth of the marbled sole Pseudopleuronectes yokohamae collected from Tokyo Bay, Japan, during two study periods between 2002 and 2008, when the stock size was consistently low. The von Bertalanffy growth equations were found to differ significantly between sexes, and females attained a larger standard length than males of the same age. These results were then compared with those reported for 1974–1983 and 1986–1988, two study periods when the stock size was high. The growth of both sexes of marbled sole in Tokyo Bay during the 2000s was found to be significantly higher than that in the 1970s and 1980s. To explore possible factors causing these changes in the growth, we examined bottom water temperature and population density. We found that the population density decreased and the bottom water temperature increased between the late 1970s and the late 2000s. However, despite evidence of changes in population densities and water temperatures, further study is needed to determine the main factors causing the observed changes in growth. [ABSTRACT FROM AUTHOR]

Abstract: Three-dimensional distributions of fluorescent whitening agents (FWAs: more specifically, DSBP and DAS1), which are sewage-derived water-soluble markers, were observed in Tokyo Bay water through multi-layer sampling of water at 20 locations. In summer, FWAs predominated in the surface layers, with trace but significant concentration of FWAs in bottom water due to stratification of seawater. In winter, on the other hand, FWAs were extensively mixed into the bottom layers because of the vertical mixing of seawater. In the surface layer, FWA concentrations and the DSBP/DAS1 ratio (the concentration ratio of DSBP to DAS1) were lower in summer than in winter, suggesting more efficient photodegradation of FWAs in euphotic zones during the summer due to stronger solar radiation. Horizontally, FWAs were widely distributed over the surface layer of Tokyo Bay. Surface water with DSBP concentrations above 50ng/L, corresponding to <200 times dilution of sewage effluent, was found to have spread up to 10km from the coastline. In addition, an offshore decline in FWA concentrations was observed, showing a half-distance of 10–20km. The decrease was caused by dilution by seawater of fresh water containing FWAs. The eastern part of the bay was different with respect to surface layers, with higher concentrations seen in northeastern parts. Furthermore, dispersion of combined sewer overflow (CSO)-derived water mass was observed in Tokyo Bay after heavy rain. [Copyright &y& Elsevier]

Abstract: We examined changes in the growth and reproductive traits of dragonet Callionymus valenciennei concurrent with decrease in stock size in Tokyo Bay. Stock size in mid 2000s decreased to 14% of that recorded in early 1990s. The minimum standard length at which dragonet attain gonadal maturation was smaller in 2000s (4.8 cm) compared to that in 1990s (6.0 cm). In addition, timing of the onset of the first spawning became earlier in 2000s (starting from spring) than that in 1990s (from summer). We also found significant changes in growth for both sexes from 1990s to 2000s; growth of dragonet after they attained sexual maturation showed a significant decrease in 2000s against that in 1990s. Changes in the life history traits may reflect trade-off for allocating available energy resource to reproduction rather than to somatic growth under limited prey abundance for enhancing a chance for stock recovery. However, these changes in the life history traits might have not led to the resilience of the population due to negative effects of hypoxia on settlement of larvae from the spring spawning season. [Copyright &y& Elsevier]

We investigated factors that might have disturbed the stock recovery of marbled flounder in Tokyo Bay by focusing on the early life stages. Field surveys in Tokyo Bay from 2006 to 2011 revealed that mature adult biomass increased from 2006 to 2008 and decreased thereafter. Meanwhile, larval and juvenile densities were high in 2006 and 2008 but low in other years. Discrepancies in the yearly trends of these parameters suggest that mortality during life stages between spawning and early larval phases might have affected the abundance of the subsequent life stages. Monthly mean water temperature between January and February, in which hatching and pelagic larvae occur in the bay, was lower in 2006 (8.6 °C) and 2008 (9.6 °C) than was observed in other years (10.4–11.4 °C). Significant negative correlation between water temperature and larval density implies that mortality during pre- and post-larval stages would be higher in warmer winter years (>10 °C). To test this hypothesis, we examined the effects of water temperature on mortality and development in egg and larval stages under controlled laboratory conditions. Hatching rate was high in a water temperature range of 9.2–12.7 °C (66.6–82.5%), whereas it decreased in cooler (3.7% at 5.9 °C) or warmer (33.9% at 14.8 °C) conditions. Meanwhile, days from fertilization to hatching, size of larvae at hatching and survival rate of larvae after 18 d from hatching were monotonically and significantly decreased as water temperature was elevated. Combined evidence of the field and laboratory studies suggests that a warmer reproductive season (>10 °C) might induce mortalities of marbled flounder larvae in Tokyo Bay. [ABSTRACT FROM AUTHOR]

Abstract: This study demonstrated the spatiotemporal patterns of the environmental conditions and benthic macrofauna in Tokyo Bay, Japan, and investigated the factors causing disturbances in the assemblage structure. In the north-central areas, the density and species diversity of the macrobenthos was low. Although hypoxia appeared in July, defaunation occurred in August. The delayed defaunation and recolonization soon after the abatement of hypoxia were attributed to several polychaete and bivalve species that were tolerant to the hypoxic environment. In the southeastern areas, however, the density and species diversity of the macrobenthos was high throughout the year, and no defaunation was recorded. Multivariate analyses showed that the disturbance in the macrofauna correlated with organic enrichment in the sediment and bottom-water hypoxia. There is a concern about further impairment of the macrofauna in the bay due to the expansion of sediment with high levels of organic matter towards the southern regions that could cause hypoxia and subsequent defaunation. [Copyright &y& Elsevier]