Yoshihisa Akamatsu

Habitat suitability of eight threatened gobies inhabiting tidal flats in temperate estuaries: model developments in the estuary of the Kuma River in Kyushu Island, Japan

We developed habitat suitability models for eight threatened goby species inhabiting tidal flats in the estuary of Kuma River in Kyushu Island, Japan. The best models selected several environmental factors, which were supported by area under the curve (AUC) values ranging from 0.793 to 0.976. The habitat suitability curves for each variable demonstrated various slopes, suggesting interspecific differences in habitat suitability for each environmental factor. The results suggested that the range of environmental gradients in tidal flats influences the richness of the threatened goby species.

Sediment and nutrients transport in watershed and their impact on coastal environment.

Sediment and nutrients yields especially from farmlands were studied in a watershed in Ishigaki island, Okinawa, Japan. The transport processes of these materials in rivers, mangrove, lagoon and coastal zones were studied by using various observation methods including stable isotope analysis. They were simulated by using a WEPP model which was modified to be applicable to such small islands by identifying several factors from the observations. The model predicts that a proper combination of civil engineering countermeasure and change of farming method can reduce the sediment yield from the watershed by 74%. Observations of water quality and coral recruitment test in Nagura bay indicate that the water is eutrophicated and the corals cannot grow for a long time. Based on these observations, a quantitative target of the reduction of sediment and nutrients yield in watershed can be decided rationally.

Abiotic and biotic factors influence the habitat use of four species of Gymnogobius (Gobiidae) in riverine estuaries in the Seto Inland Sea

Gobies that are phylogenetically related or coexist in the same marine and estuarine systems often exhibit abiotic and/or biotic habitat segregation. Thus, it is possible that species of Gymnogobius inhabiting the same riverine estuaries also exhibit abiotic and/or biotic habitat segregation. The goal of this study was to determine the differences in abiotic and biotic habitat use between these species by sampling goby and host shrimps, and by examining the physical environments of the rivers where these species are found. The surveys of goby and host shrimps were conducted in the estuaries of the Saba and Ibo rivers, which drain into the Seto Inland Sea, a body of water that separates three of the four main islands of Japan. We used hand nets and shovels to collect goby and host shrimps, and measured median sediment particle size, elevation, and salinity at each site. Generalized linear models (GLMs) were used to assess the preferences in abiotic and biotic habitat use by the goby species. Median particle size, salinity, and elevation were used as the abiotic environmental predictors, whereas the presence/absence of host shrimps were re-organized into four categories consisting of “Upogebia major” only, “Nihonotrypaea japonica” only, “Upogebia major & Nihonotrypaea japonica,” and “Upogebia yokoyai,” which were used as the biotic environmental predictors. The GLMs demonstrated that median particle size had the largest influence of the abiotic variables, with goby species segregating according to differences in sediments; moreover, there was some evidence suggesting that the host and symbiont do not always correlate at the species level. Our results indicated that although there is some overlap in abiotic and biotic habitat use among the four species of Gymnogobius, the differences were broad enough to provide an explanatory mechanism as to how these species can coexist in the same river systems.

Removal of water-surface reflection effects with a temporal minimum filter for UAV-based shallow-water photogrammetry: Removal of Water-Surface Reflection Effects for UAV-Photogrammetry

The recent development of structure-from-motion (SfM) and multi-view stereo (MVS) photogrammetry techniques has enabled semi-automatic high-resolution bathymetry using aerial images taken by consumer-grade digital cameras mounted on unmanned aerial vehicles (UAVs). However, the applicability of these techniques is sometimes limited by sun and sky reflections at the water surface, which render the point-cloud density and accuracy insufficient. In this research, we present a new imaging technique to suppress the effect of these water-surface reflections. In this technique, we order a drone to take a short video instead of a still picture at each waypoint. We then apply a temporal minimum filter to the video. This filter extracts the smallest RGB values in all the video frames for each pixel, and composes an image with greatly reduced reflection effects. To assess the performance of this technique, we applied it at three small shallow-water sites. Specifically, we evaluated the effect of the technique on the point cloud density and the accuracy and precision of the photogrammetry. The results showed that the proposed technique achieved a far denser point cloud than the case in which a randomly chosen frame was used for each waypoint, and also showed better overall accuracy and precision in estimating water-bottom elevation. The effectiveness of this new technique should depend on the surface wave state and sky radiance distribution, and this dependence, as well as the applicability to large areas, should be investigated in future research. Copyright

ENVIRONMENTAL DNA ANALYSIS FOR ESTIMATING THE RESOURCES OF Plecoglossus altivelis IN GONOKAWA RIVER

アユ Plecoglossus altivelis は古くから日本で食料として よく利用されている魚類であり,陸水域における漁業の 対象としてもっとも重要な種の 1 つであるが ,近年西 日本の河川における資源量の減少が報告されているため ,減少要因を把握し,適切な資源管理をおこなう必要 がある. 本研究対象河川である江の川は,中国山地を貫流し日 本海に注ぐ幹川流路延長 194km,流域面積 3,900kmの, 中国地方最大の一級河川である.江の川の中上流域は日 本でも浸食平坦面の最も良く発達する地域であり,流域 内の人口は中上流域にあたる三次盆地と河口部に集中し ている.江の川水系の中でも特に,三次盆地周辺の河川 においては,オオカナダモをはじめとする外来沈水植物 が繁茂しており,アユの漁獲量に影響を与えていると言 われているが ,これまで,広範囲にわたってアユの資 源量を把握する手段がなかったため,詳細は明らかには なっていなかった.しかしながら,近年発展が著しい環 境 DNA分析により ,水中に含まれるアユの DNAが, その箇所に生息するアユの生物量(個体数や現存量)と R が 0.9 前後の非常に強い関係性があることが明らかに なっているため ,環境 DNA 分析をおこなうことによ り,その箇所のアユの生物量の大小を相対的に評価でき る段階にまで至っている. そこで本研究では,オオカナダモがアユの漁獲量に負 の影響を与えているとされる三次盆地の江の川において ,環境 DNA分析を用いてアユの生物量を定量化し,さ らに,アユの生物量に影響を与える可能性が考えられる 環境条件との関係性を明らかにすることを試みた.

REGIONAL ANALYSIS ON RIVER ENVIRONMENTAL MANAGEMENT BY RESIDENTS

It is important that residents do river environmental management, because reducing governmental burden, minimization of the psychological distance of residents and river and strengthening regional social network. Recently, these activities by cooperation of residents and administration are done. The purpose of this study makes clear the regional comparison and changing processes of the residential activities at the river. The year of starting activity party is analyzed in class A river. These activities increased from 1980s, and the most number of established party in 1998. As a result of analyzing the contents of activities, it is shown that formation of a network which supports activity is important. An analysis of a possibility that activity will start was carried out. High potential river of environmental activity is elementary school in the neighborhood, and much a family’s number area.

STUDY ON PRODUCTION AND DETACHMENT OF PERIPHYTON GROWING ON FLAT CHANNEL BED

Laboratory experiments for the kinetics of periphyton growth were conducted to find out the effect of hydraulic characteristics on the production and the detachment of periphyton, in which the amount of periphyton and the rate of detachment were measured. The results of the experiments show that the growth rate of periphyton becomes large in the case turbulence near the channel bed becomes large. It was also found that the detachment of periphyton mainly depends on the physiological action of periphyton.A numerical simulation model of the growth of periphyton was presented to describe the production and detachment in details. The result of the calculation shows that the biomass of the filament type periphyton is mainly determined by the balance between production rate and respiration rate.

FIELD OBSERVATION ON WATER QUALITY AND PRODUCTION OF ATTACHED ALGAE AT RIFFLES AND POOLS

From view points of ecology, it is important for river engineers to mainfest the hydraulic and ecological characteristics of riffles and pools. A field observation was performed at riffles and pools in Tama-river in Tokyo. The hydraulic characteristics, the amount of attached algae and water the qualities were measured using various devices. It was found that the production rate of the attached algae at riffls is larger than that at pools. The primary production of observed riffles and pools are very large, yielding temporal variation of dissolved oxygen during day-time.