Program

Correct identification and aquatic habitat mapping of the seagrass field is very important since the seagrass plays a crucial role in fisheries as a nursery ground of some economically and ecologically important fishes. In addition, seagrass has a great capacity for carbon storage, and therefore is significant component of blue carbon. Physical or divers observation of seagrass is labor and cost intensive and poor insight of its habitat spatial distribution. Other techniques, such as optical-based observation are limited by the poor clarity of the waters that is commonly found in aquatic habitat. Acoustic method is one of the promising tools to identify and map the seagrass in relatively short time. However, seagrass field is difficult and challenging to identify and map the species due to its patchiness and located in relatively shallow waters. A recent field measurement on seagrass was conducted in the Seribu Island using single beam acoustic transducer by varying mechanically acoustic detection angle (900, 450, and 300) on the four tropical species of seagrass, namely Cymodocea sp., Thalassia sp., Syringodium sp., and Enhalus sp. (more detail detection angle for Enhalus sp.) in the Bintan Island, Indonesia, where the species is abundant in the area. The operating frequency of the acoustic transducer is 200 kHz. The results show that the acoustic backscattering strength coefficients of the aforementioned seagrass vary from species to species and depend on its acoustic incident angles.

Eutrophication of shallow coastal area can lead to the high growth rate of macroalga. This research was conducted to clarify marine environmental condition after eutrophication surrounding seaweed cultivation area. The study was performed in two periods. The first period was June – October 2009 while temporal eutrophication occurred with the high concentration of nutrients and bloom of microalgae and epiphytes macroalga. The second period was June – October 2013 while concentration of nutrients return to normal and there are no blooming microalgae. During our research, in the end of September 2009, inorganic nitrogen (nitrate + nitrite) and phosphate concentrations were 2.4–4.2 µM and 2.59–3.45 µM, respectively. High concentrations of ammonia, nitrate + nitrite, and phosphate were found at location of disposal of fertilizers. Since July – September 2013, ammonia, nitrate + nitrite, and phosphate concentrations were in range between 0.3–0.6 µM, 0.4–0.7 µM and 0.16–0.58 µM, respectively. There were significant differences in ammonia, nitrate + nitrite, and phosphate in 2009 compared to 2013. During May 2009, the initial growth of Kappaphycus alvarezii was represented by the increase from 50 g in first weight (Wo) to 235 g after 50 days. However, in June 2013, the weight incereased from 50 g in first weight to 262 g after being cultivated during 50 days. There was no signifcant growth of K. alvarezii while fertilizer was used by farmers in 2009.

Since the 1960s, many parts of Japan have experienced drastic losses of submarine vegetation in the coastal ecosystems. Indeed, the loss of large kelps, Sargassum, and seagrass ecosystems have been accelerating world-wide, as a consequence of climate change and human activity. To better understand what these changes in the ecosystem may imply, we have been studying the dynamics of oxygen, temperature, and light in sargassum beds and seagrass meadows in Omura Bay, Nagasaki, Japan. From July 2013 to March 2017, we have been continuously monitoring the state of these ecosystems, for periods of three to ten days at two or ten minute intervals. This data is then processed using a Bayesian state-space model, to estimate gross ecosystem production in each ecosystem. Our results suggest that on the scale of tens of m2, maximum gross ecosystem production for both ecosystems occurs in July, however minimal rates occur during December for the seagrass meadow and during February for the sargassum bed. Furthermore, annual gross ecosystem production in seagrass meadows were estimated to be 3290 mg O₂ m^-2 d^-1, whereas it was 1969 mg O₂ m^-2 d^-1 in the sargassum bed. We believe that these measurements provide an alternative solution to measuring productivity of these ecosystems without destructive and intrusive sampling methods, which is an important consideration when monitoring sensitive ecosystems.

A “dead zone” represents an uninhabitable aquatic area where dissolved oxygen (DO) availability is below the limit of tolerance in aerobic organisms. As spreading of this life-threating zone has become evident in many coastal areas around the globe, it is urgently needed to assess responses of aquatic biota to hypoxia in order to help predict the ecosystem consequences. Omura Bay in Nagasaki is a typical enclosed bay that develops strong seasonal hypoxia (dead zone) every summer, yet the bay still holds rich marine resources and active fisheries. Aiming at (1) clarifying microbiological mechanisms by which dead zones are formed, and (2) obtaining quantitative understanding on how diverse types of inhabitants respond and adapt to the low oxygen condition, we began analyses of community structure and functions of micro-organisms, and monitoring distributional and behavioral patterns of macro-organisms in relation to hypoxia at the center of the bay. Major findings include (1) a uni-modal relationship between DO and sediment bacterial richness occurred with a peak at suboxic condition, which may reflect a transition where both strict anaerobic and aerobic bacteria can co-exist; (2) hypoxic condition favored a group of meiofauna (nematode) that mostly fed on bacteria, suggesting greater importance of organic matter transfer from bacteria through nematode in hypoxia than in normoxia; (3) a school of small pelagic fishes kept migrating into near-bottom water soon after DO declined to less than 4 mg/L; and (4) finless porpoises in Omura Bay also seemed to come often in and/or around the hypoxic water mass in the center of the bay. These results suggest that the hypoxic water mass in Omura Bay is not simply a “dead zone”, but it serves as an ephemeral “dining zone” for diverse life forms.

Mangrove ecosystem is one of the most biologically and fishery important ecosystem in the tropical region; however, the importance of it in Japan, northern limit of mangrove distribution in the Indo-Pacific, has not yet been well evaluated. To clarify how the presence or absence of mangrove habitats affects the assemblage structure of fishes in the Ryukyu Islands, southern Japan, daytime fish sampling was conducted in mangrove-rich and mangrove-free rivers in Ishigaki Island (24ºN) and Okinawa Island (26ºN) in 2015 and 2016 using seine net (n = 20 in each river in each year). The mean species richness and abundance of fishes per tow in mangrove-rich rivers were higher than those in mangrove-free rivers in both the islands and years. Benthic invertebrate feeder was the most dominant feeding group in both mangrove-rich and free rivers, and species richness and abundance of the latter feeding fish were two to three times higher in the former than in the latter rivers. Cluster and ordination analyses based on the abundance of each species demonstrated that the fish assemblage structures were markedly different between mangrove rich and free rivers in each year and each island. Some species (e.g., Apogon amboinensis and Sphyraena barracuda) occurred exclusively in the mangrove-rich rivers while the others (e.g., Caranx papuensis, and Zenarchopterus dunckeri) occurred in both mangrove-rich and free rivers. Commercially important fishes, such as Lutjanus fulvus, Lutjanus argentimaculatus, and Siganus guttatus, showed higher abundance of juveniles in mangrove-rich rivers than in mangrove-free rivers. The results of this study indicated that rivers with the presence of mangroves have greater diversity of fish species and could function as important nursery habitats for commercially important fishes than rivers without mangroves.

The artificial change of river environments, where most Japanese eels live, is one of the factors that is decreasing Japanese eel stocks. In particular, recent studies have revealed that across-river structures and concrete revetment of river shorelines affect the migration and food supply for the eels. However, since there is little knowledge about preferable environments for Japanese eel habitat in rivers, it is necessary to evaluate the habitat usage of eels in rivers to discuss habitat reductions by river modifications. The purpose of this study was to determine the suitable environments for Japanese eel habitat at the river-reach scale. Eel samples were collected by an electric-shocker from four rivers (totally 78 points in 39 areas) in Kagoshima Prefecture, Japan. The most important environmental factors (the distance between each point and the river mouth, depth, velocity, substrate material and percentage of revetment) were then determined using statistical model selection. Analysis was carried out by distinguishing between large (≥261 mm) and small (≤260 mm) eels, and the tidal and non-tidal zones. The number of eels was higher in the tidal zone than in the non-tidal areas. In particular, small eels were concentrated in the tidal zone. Analysis of the relationship between total length of large eels and environmental factors indicated that total length was smallest near the tidal limit and tended to increase going upstream and downstream. Analysis of the relationship between population density and environmental factors indicated that the environmental factors affecting population density and their effects were different between tidal and non-tidal zones, as well as between large and small eels.

Hiroshima Bay is one of the largest fishing ground for black sea bream Acanthopagrus schlegelii in Japan. Recently, the landing of this species in the bay has been gradually decreasing. Therefore, frameworks for reproductive biology of this species are essential. To get the basic knowledge of their spawning biology, we assessed horizontal and vertical distribution of pelagic eggs of the black sea bream in Hiroshima Bay.
We collected pelagic eggs using submersible pomp at 14 sampling stations (7 station each at inner and outer bay) from 5 and 10m in depth from late-April to early-July in the bay. Species identification for black sea bream eggs was performed using monoclonal antibody method. Simultaneously with field survey, we collected matured adults for measurement of gonadosomatic index (GSI).
Black sea bream eggs collected in late-April and then settles down in mid-Jun. The peaks in egg density and highest GSI of adults were marked in early-May. Water temperature to commence and settle down at spawning of black sea bream in Hiroshima Bay are 17 and 22 ℃, respectively. Based on the horizontal egg distribution, egg density in outer bay was much higher than that of inner bay. Particularly, high egg density was recorded near the oyster farming area at 10m in depth. Black sea bream usually utilize at abundant oyster rafts as main spawn ground. On the contrary, significantly lower egg density at inmost bay area suggested that adults avoid the low salinity area for spawning ground. Our findings will be advantage of future management for black sea bream.

Tango Bay is a typical region of freshwater influence (ROFI) opening to the Sea of Japan. A strong current (i.e., Tsushima Warm Current) passes offshore of the bay, and a large river (i.e., Yura River) flows into the bay. This ROFI is an important nursery ground for temperate seabass, Lateolabrax japonicus, which spawn in the offshore areas during winter. The coastal circulation plays an important role in the landward transport of the eggs and larvae. In this study, therefore, we developed a three-dimensional Lagrangian particle tracking model to clarify the physical mechanisms of the transport of seabass eggs and larvae. The model can reproduce the anticyclonic circulation in the mouth and the estuarine circulation inside the bay. This circulation directly affects the transport of eggs and larvae. The eggs drift clockwise in the surface layer of the bay mouth due to the anticyclonic circulation. After hatching, larvae migrate vertically from the surface to the middle layer and successfully reach the nearshore nursery areas. These results correspond to the observations that showed eggs were restricted in the surface layer near the bay mouth and larvae were widely dispersed in the middle layer inside the bay. Thus, we conclude that seabass larvae are transported landward effectively by the estuarine circulation, as they control their vertical position in the water column.

Discarded plastics are now globally ubiquitous across freshwater and marine habitats. Much of this material is smaller than 5 mm in size, and is referred to as microplastics. As a consequence of widespread contamination, microplastics have been reported in fisheries products, including in fish, shellfish and sea salt. The microplastics we find in aquatic habitats and animals are made up of a diversity of polymers and are associated with a cocktail of chemicals, including those that are added during manufacturing (i.e. BPA, PBDEs, phthalates, lead) and that sorb to the material from ambient seawater (e.g., DDT, PCBs, PAHs, copper). It is now understood that microplastics and their associated chemicals can have adverse impacts at several levels of biological organization – from suborganismal to population and assemblage. As such, there is concern regarding whether and how microplastics pose a threat to fish and shellfish stocks. Using recent insights from my own work and the work of others, this presentation will review the contamination of microplastics in seafood products globally, the impacts of plastic debris and associated chemicals on fish and shellfish and potential impacts to fish and shellfish stocks.

The ocean faces a multitude of interconnected threat that is unprecedented in modern history. Some less-visible threats, such as hypoxia (a dissolved oxygen concentration of < 2.8 mg/L) and microplastics (MPs, < 5 mm) in the costal water are only just beginning to be understood and remain beyond the awareness of public. Hypoxia and MPs contamination have been occurred the world's coastal waters, with severe consequences for marine life, including death and catastrophic changes. As a filter-feeder organism, bivalves are sensitive to hypoxia and can ingest MPs while feeding. Therefore, bivalves are likely to be impacted by binary of hypoxia and MPs exposure. To assess the impact of hypoxia and MP on the physiology of the bivalves, juvenile pen shell (Atrina pectinata) were experimentally exposed to hypoxia, polystyrene fluorescent MP (2 μm in diameter; 0.025 mg/L) and their mixture, respectively. By next generation sequencing (NGS), we obtained the de novo transcriptome of hepatopancreas in pen shell, with 96382 assembled transcripts. Among them, 141 differential expression of genes (FDR < 0.05) were observed in heatopancreas of pen shell after exposure to hypoxia, no significantly changes (FDR < 0.05) in pen shell after exposure to micro-PS. Interstingly, 2084 genes were significantly regulated (FDR < 0.05) in pen shell after exposure to the mixture compared with control group. With gene ontology and pathway analyses, most genes are involved in macromolecular metabolism, immune and anti-oxidant defense, such as heat shock protein, lectin and cathepsin c. These results suggest the physiological effects of hypoxia, MPs and their mixture on filter feeder organisms.