We examined use of small perennial agricultural canals by fish according to seasons and life stages and habitat characteristics in 4 regions, southern Gifu, central Japan, in irrigation (June and August) and non-irrigation (September and February) periods. Seasonal changes in the number and size composition of six dominant fish species showed that the canals function as habitat for growing and overwintering. Total abundance of all fish species was higher in earth canals, which had natural bed materials, than in concrete canals in all study periods other than August. Fish diversity was higher in earth canals than in concrete canals only in winter (February) . Abundance of each of most dominant fish species was positively related to the bed materials, such as silt, sand, or gravel, in warmer seasons (from June to September) . In winter, that was primarily explained by cover ratio or water depth. In addition, we found close positive relationship between bitterling and mussel abundance in three study periods including spawning and other seasons. These habitat characteristics according to the study periods were satisfied in earth canals. Our findings suggest that a set of natural bed materials and covers, such as bank vegetation and undercut bank, is important when canals are considered as fish habitat throughout irrigation and nonirrigation periods including winter. Therefore, the bank-lined canal with natural bed should be adopted for canal construction and modification, although unlined earth canal is ideal. If the canal bed is necessarily lined, the hydrological conditions that allow sediment deposition should be ensured.

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Influence of the dams in the Kamo and Nakayama River systems in the Ehime Prefecture on the common freshwater gobies Rhinogobius flumineus (non-diadromous species), R. sp. CB, R. fluviatilis and R. kurodai (amphidromous species) were surveyed using three microsatellite loci. Genetic variability was high in R. sp. CB (0. 900-0. 919) and low in R. flumineus (0. 191-0. 278). Considerable genetic differences were observed among the species. The effects of a dam with a reservoir were examined for R. flumineus and R. sp. CB. In R. flumineus (non-diadromous species), genetic differences were not observed between populations above and below the Nakayama-gawa Dam (Nakayama R.). In R. sp. CB (amphidromous species), migratory history was also examined using otolith Sr/Ca ratio. The otolith Sr/Ca ratio analysis confirmed that the population above the Kurose Dam (Kamo R.) was landlocked by the dam. However, genetic heterogeneity was not observed between populations above and below the Kurose Dam.

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In Japan, some of introduced exotic fish such as largemouth bass (Micropteraus salmoides) and bluegill (Lepomis macrochirus) are highly competitive over indigenous fish species, causing loss of biodiversity and degradation of aquatic ecosystem in a site. As this issue became apparent, many strategies have been implemented to control the exotic species. In a breading season of largemouth bass and bluegill, from May to June, they flock to shallow areas for spawning. In a front reservoir of Miharu dam, encircling nets were placed parallel to shores in order to lock the exotic fish in the target sites. As the reservoir level was lowered by a drawdown operation, fish in the areas were inescapably driven into the nets. This drawdown capturing method has been carried out from 2007 to 2011. Among the fish captured, the invasive exotic species, largemouth bass and bluegill, were eliminated and the others were released. Large sized individuals of largemouth bass were collected more in a joint area (about 5m width) between the front and main reservoir. Bluegill does not migrate around the reservoir, but they gather in shallow areas in spawning season. As a result of the consecutive practice, abundance of the largemouth bass and bluegill (2nd year and older) have been decreased. Whereas, the number of indigenous juveniles seen, such as Carassius auratus langsdorfii, has increased and increase of the indigenous populations has been affirmed.

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To evaluate the effects of weirs on the distribution of diadromous fish species, we surveyed fish fauna below and above the weir located the lowest reaches of two tidal rivers (Takada and Oshi River) draining into the Tone River estuary in summer(July-August) and autumn(October) in 2010. Of 15 fish species captured, 10(66.7 %) were diadromous fish species. One river with lower weir height(50 cm at max) had diadromous fish both below and above the weir. The other river with higher weir height(55 cm + 130 cm) , on the other hand, had no diadromous fish above the weir, and similarity index between below and above the weir was quite low. Physical environmental condition of the tributary was almost similar between below and above the weir, suggesting weir-induced substantial differences in fish fauna of the tributary. To solve the problem, mitigation of the height of weirs (small-scale fishways and creating a ramp of cobbles and gravels downstream of weirs) might allow diadromous fish passing such instream obstructions during high water level is proposed as a valid management action in tidal rivers.

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[in Japanese]

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Evaluating the effectiveness of control measures provides helpful information for invasive species management. Here, we evaluated the impacts of removal rates of eggs, juvenile fish, and adult fish on the equilibrium abundance of bluegill sunfish (Lepomis macrochiru), by using an age-structured population model developed in the Hyco Reservoir (North Carolina, USA). When controlling eggs, juveniles, and adults independently, removal rates of 0. 84-0. 92 were necessary to eradicate the bluegill population (i.e., the equilibrium abundance < 1 individual). Given the constant removal of eggs, additional efforts to control juvenile or adult abundance could lead to the population extinction with smaller removal rates, compared with controlling them independently. Furthermore, inverse U-shape responses of the equilibrium abundance to independent removals of eggs and adults were observed; the bluegill abundance increased until the removal rates increased to approximately 0. 8 and 0. 6, respectively. This is because the population model used here employs the Ricker-type density dependence to model the relationship between the number of eggs and age-0 fish abundance. This result suggests the importance to estimate and consider the densitydependence effects in the actual managements. Although the application of our results to practical managements of bluegills in Japan requires further consideration, as illustrated here, the use of population model can estimate and compare the impacts of control measures, and help to select better strategies.

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Spread of the largemouth bass (Micropterus salmoides) and bluegill (Lepomis macrochirus) has serious impacts on freshwater ecosystems in Japan. Irrigation ponds are known to serve as major habitats for largemouth bass and bluegill, which these alien fish are expected to disperse downstream via irrigation and drainage ditches. In this study, emigration of largemouth bass and bluegill from an irrigation pond was investigated during the irrigation period (from April to July in 2008) using traps set in irrigation and drainage ditches. In total, 12 and 37 individuals (body length > 3 cm) of largemouth bass and bluegill, respectively, were caught by the traps when water was discharged into the ditches. These fish that emigrated from the pond contributed 4. 0% (largemouth bass) and 7. 1% (bluegill) of the total number of each species (body length > 3 cm) that were captured in the pond by pond draining. These results indicate that dispersal of largemouth bass and bluegill from the irrigation pond is not uncommon. Eradication of such exotic fishes from irrigation ponds is important for conservation of aquatic communities around the ponds.

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[in Japanese]

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Japanese kissing loach Parabotia curta (Cypriniformes, Botiidae) is one of the most endangered freshwater fishes in Japan. This species inhabits in a narrow region of western Honshu Island. The loach inhabits rivers and irrigation channels with gravel substrates hiding in crevices or holes, and spawns for a few days in the early rainy season at temporarily submerged, flooded grounds, which were originally very common lowland environments in monsoon Asia. However, recent artificial environmental changes, especially river improvements and farm land consolidation, have destroyed such environments and resulted in many local population extinction. Volunteers and Japanese/local governments are performing restoration and maintenance of artificial floodplains for the spawning as well as surveillance of poaching, but this loach is still critically endangered with some serious problems. In the agricultural area which has many restrictions, conservation techniques cannot be fully put to practical use. Although the technique of the ecology and civil engineering is effective for the restoration of floodplain environment and improvement of habitat, the sociological approach is crucial to utilize the technique in the local community.

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The microhabitat characteristics of the Tokyo bitterling, Tanakia tanago, designated a natural monument of Japan, were investigated in its natural habitat. To elucidate the habitat requirements of juvenile bitterling, we analyzed the relationships between the occurrence (presence/absence) of juveniles and microhabitat variables using a generalized linear mixed model (GLMM). The model showed that the occurrence of juveniles was positively correlated with the presence of underwater cover and the number of bitterling eggs deposited in adjacent mussels, and negatively correlated with current velocity. When the habitat was evaluated using this model, it predicted that the occurrence probability of juveniles was low in the upper reach of the canal. Therefore, the upper reach habitat was restored (i.e., the bottom was excavated, stakes were installed, and mussels were introduced) after considering the habitat requirements of juveniles. To assess the effect of these restorations, we compared the occurrence of juveniles between restored and control (non-restored) stations. Over the duration of the observations, 99 and 41 juveniles were found at restored and control stations, respectively; 71 % of the total number of juveniles was observed in the restored sections. These results suggest that the restoration scheme could increase suitable habitat for juvenile bitterling. Assuming that the juveniles observed in the upper reach of the canal survive and breed, the size of this population should increase. The habitat-requirement model constructed in this study should be applicable for maintenance and reconstruction of habitats of this species.

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[in Japanese]

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This study provides basic information to increase understanding of study on fish ecology in a paddy field (including agricultural canal) and proposes future issues of the study field based on a short review. First, we identify fish species that spawn in paddy fields in the Kiso River system and emphasize the importance of paddy fields for fish communities and populations. Second, we describe features of irrigation and river networks, structures of paddy field and canal, and irrigation methods. Finally, we review and classify previous studies on fish ecology in paddy fields and propose future issues.

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