Distribution of Japanese Eel Anguilla japonica Revealed by Environmental DNA

絶滅危惧種ニホンウナギの分布域を環境DNA解析で推定. 京都大学プレスリリース. 2021-03-03.The abundance of Japanese eel Anguilla japonica has rapidly decreased in recent decades. Following a re-evaluation of the possibility of extinction, the Japanese Ministry of the Environment and the International Union for Conservation of Nature listed the Japanese eel as an endangered species in 2013 and 2014, respectively. However, their abundance and precise distribution have never been clarified owing to their nocturnality and difficulty in their capture. In this study, the distribution of Japanese eels was investigated by monitoring for environmental DNA (eDNA), a non-invasive and efficient detection method. A total of 365 water samples were collected from 265 rivers located throughout Japan. High concentrations of eDNA of Japanese eels were detected in rivers on the Pacific side, but were low in the Sea of Japan side. In particular, very little eDNA amplification was confirmed from Hokkaido and the north of the Sea of Japan. The eDNA distribution in Japanese rivers coincides with the transport of the larvae in the ocean, as estimated by numerical simulations. Generalized linear mixed models were developed to explain the distribution of eDNA concentrations. The total nitrogen concentration emerged as an important factor in the best model. These results indicate that the distribution of Japanese eel is mostly determined by the maritime larval transport, and their survival and growth depend on the abundance of food in the river. The findings of the present study are useful for the management of populations and in the conservation of Japanese eels

Key biogeochemical processes evaluated by the stable nitrogen isotopes of dissolved inorganic nitrogen in the Yodo River estuary, Japan: significance of estuarine nutrient recycling as a possible source for coastal production

Isotope analysis of dissolved inorganic nitrogen (DIN) and a mixing model along the salinity gradient were used to clarify the dominant biogeochemical processes controlling nutrient dynamics within a shallow eutrophic estuary in Japan. Although delivery of riverine DIN into the estuary is largely dominated by mixing with seawater during most months, internal biogeochemical processes of DIN assimilation by phytoplankton and DIN production by remineralization and subsequent nitrification within the estuary offer the most reasonable explanation for observed deviations in the isotopic compositions of NO3- from mixing behavior. However, the balance of each process changed over time. During phytoplankton blooming in summer, co-occurrence of NO3- assimilation and NO3- regeneration inhibited the accumulation of NO3- within the estuary. Moreover, assimilation of NH4+ as well as NO3- by phytoplankton complicates the nutrient dynamics within the estuary. However, mostly conservative or productive behavior of DIN as well as PO43- showed that recycled nutrients are significant net source within the estuary. These results suggest recycled nutrients within the estuary could have a non-negligible impact on eutrophication in Osaka Bay

Relative contributions of external forcing factors to circulation and hydrographic properties in a micro-tidal bay

The dominant external forcing factors influencing estuarine circulation differ among coastal environments. A three-dimensional regional circulation model was developed to estimate external influence indices and relative contributions of external forcing factors such as external oceanic forcing, surface heat flux, wind stress, and river discharge to circulation and hydrographic properties in Tango Bay, Japan. Model results show that in Tango Bay, where the Tsushima Warm Current passes offshore of the bay, under conditions of strong seasonal winds and river discharge, the water temperature and salinity are strongly influenced by surface heat flux and river discharge in the surface layer, respectively, while in the middle and bottom layers both are mainly controlled by open boundary conditions. The estuarine circulation is comparably influenced by all external forcing factors, the strong current, surface heat flux, wind stress, and river discharge. However, the influence degree of each forcing factor varies with temporal variations in external forcing factors as: the influence of open boundary conditions is higher in spring and early summer when the stronger current passes offshore of the bay, that of surface heat flux reflects the absolute value of surface heat flux, that of wind stress is higher in late fall and winter due to strong seasonal winds, and that of river discharge is higher in early spring due to snow-melting and summer and early fall due to flood events. (C) 2017 Elsevier Ltd. All rights reserved

Upstream migration mechanisms of juvenile temperate sea bass Lateolabrax japonicus in the stratified Yura River estuary

Hydrographic conditions and distributions of juvenile temperate sea bass Lateolabrax japonicus were observed in early spring from 2009 to 2012 in the Yura River estuary, which is highly stratified due to its small tides and consequent seawater intrusion into the bottom layer of the river as a salt wedge. In all four studied years, the upstream expansion of their distribution coincided with the timing of the salt wedge intrusion from the lower to upper estuary, indicating that juvenile fish used salt wedge intrusions to ascend the estuary in early spring. However, juveniles sometimes remained in the nearshore area even when the salt wedge intrusion had already occurred, indicating that other triggers are also likely to be necessary. We therefore evaluated the effects of temperature on upstream migration behaviors. The relationship between the mean temperature they experienced from hatch until starting the ascent and mean age of each cohort at the upstream migration fitted with the law of effective cumulative temperature. Most cohorts ascended the river at an effective cumulative temperature of approximately 500 °C–days. This suggests that higher temperatures would lead to a shorter period prior to the upstream migration

Isotopic trophic-step fractionation of the freshwater clam Corbicula sandai

Diet-switch experiments with three different species of microalgae were conducted to estimate diet-tissue isotopic fractionation of carbon (C) and nitrogen (N) isotopes in the freshwater clam Corbicula sandai. The delta C-13 and delta N-15 values in the soft tissues of C. sandai changed in each experiment, with little inter-individual deviations, thereby reflecting the switch in diet. Isotope values of the clam reached the asymptotic value at around 40 days after the switch. Equilibrium isotopic signatures, as well as turnovers of carbon and nitrogen in the whole soft tissues, were estimated by exponential decay models. Fractionations for C. sandai varied from 0.1 to 0.7 aEuro degrees for carbon, and from 2.1 to 3.6 aEuro degrees for nitrogen; these values fell within or were close to the range of previously accepted fractionation values (0-1 aEuro degrees for carbon and 3-4 aEuro degrees for nitrogen). The half-life values for carbon in the bivalves (12-22 days) were about twofold longer than those for nitrogen (7-9 days). The specific fractionation values estimated in this study contribute towards improving our understanding of inter-specific trophic relationships and aquatic food webs

Partial migration of juvenile temperate seabass Lateolabrax japonicus: a versatile survival strategy

Partial migration describes intrapopulation variation in the migratory behavior, i.e. some individuals from a population migrate to low-salinity river areas, while others remain in coastal areas. This paper reviews the partial migration pattern of juvenile temperate seabass Lateolabrax japonicus, which is a migration pattern not commonly seen in Japan. Seabass spawn offshore, and eggs and larvae are transported to coastal areas. Some of these juveniles then ascend rivers, while others remain in coastal areas. Juveniles efficiently use physical structures in their habitat; they use tidal currents to ascend rivers in macrotidal estuaries, while they use the salt wedge in microtidal estuaries. Once juveniles ascend the river, they can feed on the abundant prey and attain more rapid growth than those remaining in coastal areas. As estuaries are highly productive areas, they play significant roles as nurseries for juveniles of various fishes. However, compared with coastal areas, the relative area of estuaries is considerably smaller and its environmental conditions are more variable. For example, nearly 40% of adult seabass in Tango Bay were estimated to use estuarine areas as a nursery, while the other 60% use coastal areas during their juvenile stage. Using both estuaries and coastal areas through partial migration during the juvenile stage is concluded to contribute to the stabilization and yield of seabass populations

Salt-wedge intrusion of seawater and its implication for phytoplankton dynamics in the Yura Estuary, Japan

We monitored hydrographic and biological conditions in the Yura Estuary and its coastal area from April 2006 to March 2008. Hydrographic conditions changed seasonally, and were mainly determined by river discharge and sea level, with tides playing a minor role.In summer, when river discharge was low and sea level was high, a salt-wedge regime was established until ~18 km upstream from the river mouth. Seawater intrusion into the river was however restricted in winter, when the river discharge was high and sea level was low. Water stratification caused by the seawater intrusion played an important role in phytoplankton dynamics. Phytoplankton developed in the middle layer of the estuary utilizing riverine nutrients, corresponding to the seawater intrusion into the bottom layer in summer. In winter, however, the phytoplankton production remained low in both estuary and sea

Dynamics of iron in the Chikugo River Basin : Comparison of iron with nitrogen and phosphate input to the estuary

Iron (Fe) dynamics in the Chikugo River were investigated in order to understand the relationship between the characteristics of Fe concentrations in the basin and the river estuarine ecosystem and that of the inner area of the Ariake Sea. Analysis of Fe concentration from monthly samples at 29 sites in the Chikugo River from July 2011 to May 2012 was conducted together with analyses of nitrogen (N) and phosphate (P) concentrations. Fe concentration changed slightly from the upstream to the middle basin, whereas in the estuary it increased sharply at 15 km from the river mouth and decreased again near the river mouth. This trend is different from the trends for N and P concentrations, and the Fe distribution was distinctive throughout the year. Further investigation was conducted focusing on the Fe dynamics in the estuary and the inner Ariake Sea. It was found that the trend of Fe concentration corresponds to the trend of turbidity in the estuary. The reasons of the characteristic Fe dynamics are suggested that supplied Fe is accumulated by the scavenging effect of particulate organic matter in the estuarine turbidity maximum to form floccules. As these particle materials are important for forming the ecosystem in the inner Ariake Sea, it is suggested that Fe dynamics have an influence in forming the ecosystem in the Chikugo River and the Ariake Sea