Floating seaweeds play important ecological roles in offshore waters. Recently, large amounts of rafting seaweed have been observed in the East China Sea. In early spring, juveniles of commercially important fish such as yellowtail accompany these seaweed rafts. Because the spatial distributions of seaweed rafts in the spring are poorly understood, research cruises were undertaken to investigate them in 2010, 2011, and 2012. Floating seaweed samples collected from the East China Sea during the three surveys contained only Sargassum horneri. In 2010 and 2011, seaweed rafts were distributed only in the continental shelf and the Kuroshio Front because they had become trapped in the convergence zone of the Kuroshio Front. However, in 2012, seaweed was also distributed in the Kuroshio Current and its outer waters, and massive strandings of seaweed rafts were observed on the northern coast of Taiwan and on Tarama Island in the Ryukyu Archipelago. Environmental data (wind, currents, and sea surface height) were compared among the surveys of 2010, 2011, and 2012. Two factors are speculated to have caused the unusual distribution in 2012. First, a continuous strong north wind produced an Ekman drift current that transported seaweed southwestward to the continental shelf and eventually stranded seaweed rafts on the coast of Taiwan. Second, an anticyclonic eddy covering northeast Taiwan and the Kuroshio Current west of Taiwan generated a geostrophic current that crossed the Kuroshio Current and transported the rafts to the Kuroshio Current and its outer waters. Such unusual seaweed distributions may influence the distribution of fauna accompanying the rafts.

The invertebrate fauna associated with floating Sargassum horneri (Turner) C. Agardh, 1820 in the East China Sea was investigated in terms of ecology and biogeography. Floating algal rafts consisting of only S. horneri were collected at 16 stations in the East China Sea during a cruise of research vessel (R/V) Tansei-Maru in February, 2011. A total of 53 rafts were obtained for faunal investigations at 14 of the 16 stations. In addition to fish eggs, 10 invertebrate taxa were found on the floating algae. Of the collected phytal animals, harpacticoid copepods were most abundant in terms of number (80%), followed by cirripedes (15%), amphipods (4%), and others (1%). The faunal diversity on the algae was correlated to the algal weight. The faunal diversity differed significantly between stations and was highest in the northernmost part of the East China Sea. Density and abundance of animals were highest in areas close to the Kuroshio Current. Considering the direction of flow of the Kuroshio Current and the density of cirripedes immigrating from the surrounding water onto the floating algae, most of the floating S. horneri possibly originated in the southern part of the East China Sea. On the other hand, the Sargassum rafts collected in the northernmost part of the East China Sea could have originated from the coast of eastern China. Most of the animals found on the floating Sargassum were pelagic taxa that complete their life cycles on the rafts in situ. Typical coastal animals, including gammaridean and caprellidean amphipods, tanaidaceans, nematodes, gastropods, polychaetes, halacarid mites, bryozoans, and hydrozoans, were also obtained from the floating Sargassum, but only occasionally and their densities were quite low. These facts suggest that dispersal events among benthic habitats via Sargassum rafts are relatively rare.

The body tilt angle of a fish has a large effect on the acoustic target strength. For an accurate estimation of fish abundance using acoustic methods, it is necessary to measure body tilt angles in free-ranging fish. We measured diurnal body tilt angle distributions of threeline grunt (Parapristipoma trilineatum) while swimming in schools in a fish cage. Micro-acceleration data loggers were used to record (for 3 days) swaying and surging accelerations (at 16 Hz intervals) of 10 individuals among 20 forming a school in a fish cage. Time series analysis of 1-h mean body tilt angles revealed significant differences in body tilt angles between day (−7.9 ± 3.28°) and night (0.8 ± 5.89°), which must be taken into account when conducting acoustic surveys. These results will be useful for calculating