To understand the process of decrease in biodiversity of Naikos, seasonal and annual changes in fish fauna were surveyed in Katada Naiko, which is located at the western part of Lake Biwa. Specimens were collected by a casting net sampling for 20 min each night from August 2001 to July 2006. In the first and second years, 3 cyprinid species—Carassius sp., Pseudorasbora parva, and Rhodeus ocellatus ocellatus—were the most abundant. Over the five year study period, the frequency of the three most abundant cyprinid fishes combined dramatically decreased from 83.3%, 84.3%, 68.9%, 18.6%, to 15.5% whereas that of two invasive species—bluegill Lepomis macrochirus and largemouth bass Micropterus salmoides—increased from 3.9%, 4.5%, 16.2%, 68.9%, to 78.1%. The amount of economic loss, calculated by the production method, also increased during the period. The decrease in dominant species was simultaneous with the increase in bluegill and largemouth bass, and induced the potential economic loss. We discuss the requirement of removing the two invasive species, and methods for the reconstruction of the biodiversity.

In Lake Izunuma-Uchinuma basin, freshwater fishes comprised 12 families with 36 species. Most belong to non-diadromous freshwater fishes including endemic species in eastern Japan such as Acheilognathus typus, A. melanogaster, Pseudorasbora pumila, and Pseudobagrus tokiensis. The basin consists of open water such as Lake Izunuma-Uchinuma and influent rivers, and closed waters such as irrigation ponds. In Lake Izunuma-Uchinuma and the influent rivers, the fish communities showed lower density of native fishes due to influence of non-native largemouth bass. In the irrigation ponds, the existence or nonexistence of largemouth bass affected to the fish communities. The ponds with largemouth bass had a degraded native fish community, whereas the ponds without largemouth bass had many native species. These native species included several rare species that had become extinct in Lake Izunuma-Uchinuma. These results indicated that irrigation ponds served a role as refugia for rare fishes when the influence of invasive species became widespread.

Three pond snails, Cipangopaludina japonica, C. chinensis malleata, and Sinotaia quadrata histrica, were collected from different habitats in the Izunuma area. We analyzed their fatty acids composition for inferring food assimilation. Although each pond snail lives in a different habitat, fatty acids composition for each snail showed similar patterns, and high contributions of green algae, cyanobacteria, and bacteria fatty acids were detected. This indicated each pond snail has similar food assimilation tendencies. Though each pond snail has a potential to live in either pond, rice field, or river habitat, the coexistence of three pond snails rarely occurs. This might be a result of exclusive competition for food caused by same food preferences.

The habitat use of amphibians was investigated in paddy fields and adjacent habitats in areas around Lake Izunuma-Uchinuma, Miyagi Pref., Japan. Field censuses were carried out in May, June and September 2006. Five species of anuran amphibians were found: Hyla japonica Günther, Rana japonica Günther, Rana porosa porosa Cope, Rhacophorus schlegelii Günther and Rana catesbeiana Shaw, an invasive alien species. Frogs other than Rana catesbeiana bred in paddy fields. After the breeding season, H. japonica inhabited paddy fields, marshy woods and open grassy areas on embankments of the lake, Rana japonica were found in paddy fields and marshy woods. Rhacophorus schlegelii were found in marshy woods. Rana porosa porosa was the most abundant frog found in the study area, and it always inhabited paddy fields. Rana catesbeiana was always found in ponds and ditches.

A fish survey was conducted in the influent rivers in Lake Izunuma-Uchinuma basin. Based on their distribution, fish species were categorized into three groups. Group-A was distributed in upper area of the rivers, and included six species such as Lethenteron reissneri and Pseudobagrus tokiensis. Group-B was distributed in the lower area of the rivers, and included largemouth bass Micropterus salmoides and Tridentiger brevispinis. Group-C was distributed in the wide length of the rivers, and included two species such as Rhinogobius sp. OR and Misgurnus anguillicaudatus. The number of fish species in the upper area of the river was higher than in the lower. Comparison of the results with a previous study conducted fifteen years earlier indicated populations of several fish species in the lower area of the rivers have decreased due to environmental degradation and largemouth bass.

In Lake Izunuma, a site survey by Side scan sonar in 2006 suggested spawning area of Micropterus salmoides corresponded to the location occupied by sandy sediments without ripple marks. In a 2007 survey, ripple marks were not confirmed, but soil sampling of bottom sediments and their grain size analyses showed fine sediments like silts and mud at the ripple mark location. It was also observed that reproduction season in 2006 had much stronger winds than 2007. This evidence indicates a possible mechanism for making ripple marks. They occur in areas with water depth of the lake between 1 meter or 1.5 meters; and very slow water current throughout the year. Then fine-grained bottom sediments move due to shear induced by wind over speeds of 5 meter-per-second to make the ripple mark. Wind direction is almost consistently from in the reproduction season, so the ripple marks form at nearly the same location according average wind strength of the season. This model helps in the aim of ecologic control of Micropterus salmoides. It would be useful to arrange a map of lake-bottom sediment type and of distribution of marks, whose are based on the Side scan sonar survey.

Investigations of environmental factors and plant community structures were conducted to find the relationship between periodical cuttings of Phragmites communis and species composition of plant communities. Quadrats at of P. communis communities with periodical cuttings have more plant species than those without cuttings. Carex species were more abundant in terms of species and individual numbers in Quadrats with periodical cuttings. Cover degree of P. communis tended to be lower in Quadrats with periodical cuttings than in those without cuttings, especially in spring. Height of the plant community in Quadrats with periodical cuttings also tended to be lower in spring, but was almost similar height with those without cuttings in autumn. Those facts suggested that periodical cutting management affected coverage and height of P. communis in spring and created more suitable habitat for plant species in open wetland such as Carex spp.