The present study summarized the conservation and restoration of fourspine sculpin （Cottus kazika） in the Kuzuryu River from the past four research records. Many adults of C. kazika were found at depths of about 3 m（sea water） in the estuary of the Kuzuryu River and coastal shallow area neighboring the river mouth from the late of December to March （main season: from the middle of January to the middle of March）. Larvae and juveniles were collected in sandy beach near the river mouth and shallow area in the estuary from February to April. Young of the year of C. kazika moved upstream with growing along the shallow water sides from April to August. The present remaining habitat of C. kazika was the middle area （23.0 to 29.4 km from the river mouth） in the Kuzuryu River. This indicated that the area of ＂Habitat of Araregako （C. kazika） in the Kuzuryu River （designated as a natural monument）＂ reduced to be one –third from 1990s onward, compared to that of the past habitat. The important habitats of the C. kazika, for both growing up and passing the winter, was riffles with loose bed materials in the river. Larger C. kazika more than 250 mm in total length werenʼt almost caught since 1990s by the traditional fisheries, which suggested that the body size of C. kazika has been shrinked in the Kuzuryu River. This study showed that two important conservation strategies for C. kazika in the Kuzuryu River are; 1） to conserve and create sandy beach in the coastal area neighboring the mouth of river, and riffles with loose bed materials in the present remaining habitat of Kuzuryu river, 2） to re-introduce juveniles of C. kazika with genetic diversity into upstream from Naruka-Oozeki （the past habitat） of the river.

Inter-levee floodplain water bodies (FWBs) have been highlighted as an important habitat for aquatic animals. However, no study has examined the function of FWBs as a frog habitat. We studied the reproductive status of frog communities in FWBs and paddy fields, which are well known as frog habitats, along the Kiso and Ibi Rivers. Four frog species, Hyla japonica, Fejervarya kawamurai, Pelophylax nigromaculatus, and P. porosus brevipodus, were more abundant in the paddy fields than the FWBs. Additionally, the mating calls of the male frogs were mainly recorded in the paddy fields, indicating that they selected paddy fields as reproductive sites. In contrast, the mating calls of Glandirana rugosa were only recorded in the FWBs, and the tadpoles of G. rugosa and Rana japonica were only captured in the FWBs. Furthermore, these two species were recorded in multiple FWBs of our two study rivers in previous studies. These results suggest that FWBs can serve as critical habitats for G. rugosa and R. japonica, which require water bodies as winter-spawning and/or overwintering habitats during a non-irrigation period in paddy fields.

Flood disturbance is amongst the most universal event in rivers and streams, and known to exert an overriding effect on lotic ecosystems. Ongoing climate change is causing the frequent occurrence of massive floods worldwide. However, there is limited information on the impact of such extreme flood disturbances on lotic organisms owing to its rarity of occurrence and thus the difficulty in observation. In September 2017, an unprecedented flood occurred in the Shigenobu River on Sikoku Island, southwestern Japan. This paper reports the changes in invertebrate and fish assemblages caused by the extreme flood based on a rare dataset obtained 10-11 days before and 60-61 days after the peak flow. We also compared the response of the assemblages among four representative channel unit types in a targeted river segment. We observed significant decrease in invertebrate density (88.7％) and taxon richness (41.0％) after the flood. Invertebrate communities after the flood were characterized by the dominance of mobile taxa, that were supposed to recolonize faster than others. In contrast, fish differently responded to the disturbance event compared with invertebrates. While fish species richness did not show clear change after the flood, the abundance decreased in riffles and rather increased in backwater habitat. Analyses of the individual species showed the aggregation of Opsariichthys platypus at backwaters after the flood, implying that such semi-lentic habitats serve as a flow refugia for water-column fishes during high flow. Consequently, we showed strong impacts of an extreme flood event on invertebrate and fish assemblages, and that the effects differed between these taxa and between channel unit types. Our results give support to the conservation and the restoration of lentic habitat in low water channel from the viewpoint of disturbance ecology.

Development of submerged angiosperm vegetation is believed to induce higher transparency and inhibit bloom of cyanobacteria in Japan. However, the researcher who proposed an opinion that transparency becomes higher as submerged angiosperm grows thick denied his own opinion in 2007. Instead, development of Charophyte is suggested to raise transparency in developed countries. Charophyte grows thick to cover the bottom sediment, and would not cover the water surface to decrease the vertical mixing and induce hypoxia. In addition, it is regarded as a cause to raise transparency that phosphorus deposits in calcium contained in Charophyte. In the south basin of Lake Biwa and Lake Shinji, revived submerged angiosperms excessively grow, and bloom of cyanobacteria occurred simultaneously. The excess growth of submerged angiosperms takes out most of dissolved carbon dioxide and increases the pH of the water. Because most of the inorganic carbon exists as hydrogen carbonate in alkaline solution, diatoms and greens cannot uptake inorganic carbon, and only cyanobacteria can grow by using hydrogen carbonate. By the time that the submerged angiosperms disappeared in the lakes in plains due to the use of herbicides in rice paddy, people were intensely harvesting submerged angiosperms to utilize them as fertilizer. Such an intense harvesting prevents the excessive growth of the submerged plant, and thus prevented hypoxia in Japan. In Europe and America, not only submerged angiosperms but also benthic macro green algae overgrew in several shorelines, inducing several problems. At Great Lakes, anaerobic environment is formed at the pile of benthic macro green algae, which induces the botulism and death of waterfowls. The excess growth of benthic macro green algae is induced by the increase of phosphorus supply. Thus, adequate removal of submerged angiosperms is required to prevent the phosphorus elution form the sediment through the decrease of vertical mixing due to the excess growth of submerged angiosperms.

The Japanese eel, Anguilla japonica, is one of the most important fishery species in Japan, however, population of this species has been decreased, with the species listed as endangered (EN) on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. To investigate habitat use of Japanese eel inhabiting brackish water lake, behavior of 10 individuals were monitored using acoustic biotelemetry. Eels with ultrasonic transmitter were released into study area that included rock revetment area, reed area, concrete revetment area and offshore. Acoustical signals were received by 14 receivers and position of each individual was estimated when possible. Signals were recorded more frequently in the night time than in the day time, indicating nocturnal behavior of this species. Some eels spent longer time near shore but others spent most of time offshore, and we could not find any clear tendency shared by the majority of the eels tested in this study. For about eels that used shore area, rock revetment area appears to be preferred than reed or concrete revetment areas. The results obtained in this study indicate that Japanese eels inhabiting brackish water lake utilize both shore and offshore areas.

The Japanese eel, Anguilla japonica, is one of the most important fishery species in Japan, however, population of this species has been decreased, with the species listed as endangered (EN) on the International Union for Conservation of Nature (UCN) Red List of Threatened Species. To understand use of stone piles as refuge by Japanese eel, behavior of 8 individuals in an experimental pond was monitored using acoustic biotelemetry. The sediment of the pond was sand and mud, and 6 stone piles with about 1 m diameter and 0.7 m height were made as refuge. According to the results of two-dimensions Kernel density estimate, some eels stayed at stone piles and others stayed places without stone piles （sand-mud sediment）. When stone piles were filled with sand, eels did not stay at the filled stone piles. Burrowing behaviors were sometimes observed in this experiment. Although Japanese eels might prefer stone piles as refuge, they can also use sand-mud sediment as refuge by burrowing themselves.

In the Lake Biwa basin, the number of laying eggs in Ayu, Plecogossus altivelis altivelis, has decreased recently, especially the number in 2017 was very small. Therefore, improving the spawning environment of Ayu is an urgent issue. As Ayu prefers soft riverbed for spawning, in order to restore the favorable spawning environment of Ayu, it is necessary to quantify the spawning riverbed hardness. In previous studies, quantification of the spawning riverbed hardness of Ayu was carried such as Hasegawa type soil intrusion meter, but there was a problem in accuracy and reproducibility. In this study, we conducted quantitatively evaluate the hardness of spawning riverbed of Ayu by using Yamanaka type soil hardness tester which has not been used as it is. The field survey period was conducted in September 2014 and the field survey area were in the four Lake Biwa inflow rivers: Echi, Ado, Shiotsuoh and Ane Rivers. We surveyed both the spawning riverbed at head of rapid riffle area and the non-spawning riverbed at flat riffle area on dried parts near edge of water of sandbars in these rivers. As a result of the survey, median value of the spawning riverbeds hardness was 0.22 kgf/cm² and the non-spawning riverbeds hardness was 1.40 kgf/cm². Statistical analysis of riverbeds hardness and occurrence of egg laying by mixed effect logistic regression model formula showed the tendency that the egg production probability became 40% or more when the riverbed hardness was 1.0 kgf/cm² or less from the model formula. Finally, we discussed the usefulness and the issues of quantification of monitoring about surface hardness of spawning riverbed of Ayu by usingYamanaka type soil hardness tester.

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Conservation of S. histophorus is based on the "S. histophorus conservation plan for Nakagusuku Bay" during port development in Okinawa Prefecture. Avicennia marina, an introduced mangrove, is not native to Okinawa Island, but now covers many muddy tidelands, compromising the habitat of the rare fish S. histophorus in Nakagusuku Bay. We preliminarily explored how to exterminate A. marina, and then put our findings into practice.