Based on new material collected in June 2008, twenty-five taxa of aquatic oligochaetes belonging to three families, Naididae, Tubificidae and Enchytraeidae, were recorded from Lake Izunuma and Kabukurinuma Marsh in Miyagi Prefecture, northern Japan. The records from L. Izunuma included the northernmost record of the tubificid Rhyacodrilus hiemalis. The offshore benthic oligochaete assemblage in L. Izunuma was dominated by Limnodrilus hoffmeisteri and devoid of Tubifex tubifex, and the composition resembled those of several other shallow and eutrophic lakes in Japan. A diverse naidid assemblage was found in the vegetated littoral L. Izunuma, and an oligochaete community of diverse ecological niches was found in Kabukurinuma Marsh, both of which could be associated with habitat characteristics. It is suggested that past records of two oligochaete species were erroneously identified.

We conducted to collect earthworms in order to clarify the earthworm fauna around Lake Izunuma-Uchinuma. We could collect six speices of Megascolecidae including one undescribed species, and two species of Lumbricidae. To our knowledge, it is first time to record Apporectodea trapezioides (Dùges, 1828) in Tohoku district. Twenty nine previous described species of earthworms exist in Miyagi prefecture including our result.

Acheilognathus tabira tabira is a species of bitterling (Cyprinidae, Acheilognathinae), and is listed as endangered species by ministry of the environment. In recent years, there is no report of A. t. tabira collection in Lake Biwa. In the present study, five A. t. tabira were collected by eri fish traps in north–part of Lake Biwa in 2008. The population size of A. t. tabira in Lake Biwa may increase at one hand, while invasive fish populations declining by eradicating activities at the other. The need for greater attention to the conservation of A. t. tabira in relation to the population dynamics of invasive fishes was suggested.

The bloom of “Hikarimo” was recorded from mud samples collected from Lake Izunuma. Morphological features observed under light microscope showed the Chrysophycean algae having golden colored floating and motile cells. The motile cells had long and short flagella at the tip of the cell, and a single cup-shaped chloroplast with a semi-embedded pyrenoid. These morphological features were consistent with those of Chromophyton vischeri (= Ochromonas vischeri). We determined the nucleotide sequence of the 18SrRNA gene from isolates, and compared it with known sequences. The obtained sequence showed 99.9% similarity to C. cf. rosanofii CCMP2751. Although we have not observed a bloom of “Hikarimo” in the field yet, Lake Izunuma might be a northern limit of distribution of C. vischeri in Japan.

We investigated the spawning grounds of Micropterus salmoides using Side scan sonar in Lake Izunuma in 2006. In the 2006 survey, we were able to determine that the spawning grounds of Micropterus salmoides were concentrated in areas of sandy bottoms and no ripple marks. In a 2007 survey, fine-grained bottom sediments moved due to shear induced by wind over speeds of 5 meter-per-second to make ripple marks. On the basis of these results, we searched for spawning areas of Micropterus salmoides by the bottom sediment distribution map made by Side scan sonar in 2008. Our results from this field work confirmed that Micropterus salmoides laid eggs in these spawning areas. In addition, the bottom sediment environment for breeding was similar to other studies of Micropterus salmoides. These results show that we were able to validate locating potential spawning habitat of Micropterus salmoides using Side scan sonar.

Freshwater sponges were collected from five sites in the water near the edge of the Lake Izunuma-Uchinuma and were identified based on the morphology of spicules. Although this investigation was carried out to make a search for Ephydatia fluviatilis and not a comprehensive survey of sponge species, we found three species of freshwater sponges: Eunapius fragilis, Ephydatia muelleri, and Spongilla lacustris. This report made the first record of sponges from Lake Izunuma and Uchinuma since about 75 years ago and Eunapius fragilis was newly recorded from these lakes.

A survey of benthic fauna was carried out from April 2006 to September 2008 at the central point in Lake Izunuma situated in north part of Miyagi prefecture, Japan. A total of 8 taxa were captured; 6 taxonomic groups of Chironomidae (Diptera), Tubificidae spp. (Oligocheata), and Kamaka morinoi Ariyama (Amphipoda). Density of total zoobenthos (mean ± SD) was 359 ± 329 inds.m-2. Dominant taxa were two chironomid species, Chironomus plumosus (Tokunaga) (152 ± 114 inds.m-2) and Tanypus sp.(155 ± 239 inds.m-2), and Tubificidae spp.(192 ± 114 inds.m-2). Chironomus plumosus is a biological indicator of eutrophic and hypertrophic lakes. Propsilocerus akamusi (Linnaeus) (Chironomidae), a biological indicator of eutrophic and hypertrophic lakes, was also captured in a low densities (6 ± 10 inds.m-2). The chlorophyll a concentrations in the lake water were mostly within the range of eutrophic lakes. Carlson's Trophic State Indices (TSI) were calculated from chlorophyll a concentrations and secchi depth, respectively. The TSI values in Lake Izunuma were within the range of eutrophic lakes. Therfore, the present trophic state of Lake Izunuma is determined to be eutrophic from the benthic fauna and water quality. Comparing the present benthic fauna in this study with those from 1986 to 1987, no common taxa of chironomid larvae was found. Because no indicator chironomids of eutrophic or hypertrophic lakes were found in the survey in1986 and 1987, Lake Izunuma is suggested to be eutrophicated for last 20 years.

In the Ohbetsu River, the tributary of the Bekanbeushi River, eastern Hokkaido, Japan, 20 species of freshwater fishes were collected from May to July in 2000-2003. Freshwater type of Pungitius pungitius and Gymnogobius castaneus were collected throughout the study period, suggesting that sampling areas constituted major habitat for these species during May-July period. In contrast, anadromous fishes such as Pacific Ocean form of Gasterosteus aculeatus, Osmerus eperlanus mordax and Salvelinus leucomaenis, were thought to migrate upstream for breeding and get through the Ohbetu River during spawning migration. These differences in occurrence pattern of fish species in the river appeared to be related to various life history patterns of fishes. It is also conceivable that the vast estuary of the river provides diverse habitat environment, enabling many fishes with various life history patterns to inhabit this river.

We report the survey results documenting the habitat distribution of spined-loach in a fallow field of a hill bottom valley. Spined-loach populations were concentrated at the water entrances and exit in the field. Mostly juvenile fish were caught, with highest populations concentrated in fast flowing deep water, and where the bottom substrate consisted of sandy or silt sediment. The species entered the fallow field through connections to an upper paddy field with a network leading to a ditch in the hill-bottom valley.

Largemouth bass invaded a small pond in Northern Miyagi Prefecture. We captured all the invaded 82 individuals of largemouth bass immediately after discovery. The record of the environmental factor showed that we found the largemouth bass 13 days after the invasion. Gastric content analysis showed that each largemouth bass ate an average of 3.0 individuals of freshwater fauna such as Acheilognathus melanogaster and Paratya compressa improvisa. The total population of freshwater fauna which inhabited in the pond was estimated at about 9,000 individuals before the largemouth bass invaded. The estimate based on these results showed that the largemouth bass ate 3,172 individuals (35.2%) of prey during the 13 days. The initial influence on fauna by the invasion of the largemouth bass has been reported. This report provides important evidence about the influence of predation pressure of largemouth bass causing a decrease of freshwater fauna.