In this study, the relationship between the behavior of fish school, Jack mackerel Trachurus japonicus and oceanic conditions was investigated in Tateyama Bay, Chiba Prefecture, Japan. We showed that the fish school entered from offshore to the inside of Tateyama Bay with the flood tide inflow of water masses having lower temperature and higher salinity at the bottom layer. The fish school appeared in the inside of Tateyama bay along with mixed water masses of Tokyo Bay, Sagami Bay, and the offshore Kuroshio water. The Kuroshio water mass was distributed over the depth from several tenth of meter to around 100 m depth off Tateyama Bay. These results indicated that the fish school behaves with a diurnal tidal cycle in accordance with the movement of the Kuroshio water, and as a result, Jack mackerel was fished near the bottom layer where the mixed water was flowed into the bay.
Sargassum forests form important habitats in coastal waters worldwide. Sargassum polycystum C. Agardh is a dominant species consisting of Sargassum forests and distributed widely in the Gulf of Thailand. We studied seasonal variations of S. polycystum on the intertidal reef flats in Samaesarn Island off the northeast coast of the Gulf of Thailand by monthly quadrat sampling, observation of S. polycystum and measurements of environmental variables from January 2014 to December 2015. Percent cover, thallus length, standing crop, percentages of the numbers of immature and mature plants of S. polycystum per 0.25 m² were the maximum during the dry and cold season from November to February. They showed a significant negative correlation with water temperature（p<0.05）and significant positive correlation with DO and phosphate (p<0.05). Percentage of the number of holdfasts, main without stipes of S. polycystum per 0.25 m² , which was the highest in among the numbers of juvenile, immature and mature plants, and main holdfasts without sipes of S. polycystum per 0.25 m² in March 2014 and December 2015, showed a significant and negative correlation with current speed（p<0.05). Plant density, percentage of the number of juvenile plants of S. polycystum were the maximum during the rainy season from May to September. These results indicate that the monsoon drives environmental variables controlling the seasonal pattern of the growth and reproduction of S. polycystum. Its maturation and reproduction occur under a calm sea condition and low water temperature with sufficient solar radiation in January-February at the end of dry season.
Species of Sargassum are widely distributed along the coasts of Thailand. Sargassum oligocystum Montagne is a dominant species consisting of Sargassum beds, playing an important ecological role in a marine ecosystem along the east coast of the Gulf of Thailand. However, there is little information available on the early life stages of S. oligocystum . To fill the gap in this ecological knowledge, fertilized eggs obtained from the receptacles of wild matured individuals were cultured and morphogenesis in the early life stages of S. oligocystum due to their development was observed through laboratory culture. A fertilized egg divided transversely into one large cell and one small cell. The latter gradually induced rhizoidal cells after several divisions and many rhizoidal cells came out at the basal part of germling in 3 day culture. Finally, they became the holdfast of germling. In the large cell, cell divisions occurred and apical part came out in 1 day culture. It developed into the first cauline leaf in 7 day culture and the fourth cauline leaf was appeared in 30 day culture, which were lanceolate. Cauline leaves were lanceolate to spatulate in 60 day culture and broad spatulate in 90 day culture. Three-month-old juveniles of S. oligocystum were cultured in two 500 L fiberglass tanks set outdoor under a roof with translucent windows, and one was filled with seawater and another was filled with seawater with urea dissolved at a concentration of 4 g t-1. When juveniles cultured in two different conditions for five weeks, the growth rate of the germlings of S. oligocystum cultured in seawater was always higher than that of culture in seawater with urea dissolved. The results suggest that S. oligocystum has a potential to adapt to grow under lower nutrient environment.
We analysed datasets of fish assemblages for two timescales（decadal and annual, composed of four periods: 1993-1994; 2004-2005; 2005-2006; 2006-2007）to estimate the temporal dynamics of assemblage structures and changes in estuarine use functional groups（EUFGs） along a modified shoreline. The datasets were obtained from monthly sampling using a light trap at a mooring berth site reinforced with a vertical hard structure in an estuarine canal in Tokyo, Japan. A comparison of the fish community parameters during the sampling periods showed significant decadal variation in some factors, but no significant variation across sequential annual timescales. Total species richness increased due to increased richness in fish in the marine EUFG category. The percent abundance of marine fish also significantly increased, whereas the abundance of estuarine fish（primarily Acanthogobius flavimanus predominated）decreased. Habitat degradation around the mooring berth site accompanying city planning might be responsible for the decline in estuarine fish species over several decades, whereas the increased species richness reflected a recent recovery in water quality in the Tokyo Bay. These results suggest that nowadays the study site functions as an ecological corridor between natural shore
sites rather than as a true habitat for estuarine fish species.