Chironomid Fauna in the Lower Reaches of the Nagara River, Japan.

The chironomid fauna of the lower reaches of the Nagara River was investigated on the basis of male adults emerging from bottom samples and those collected by light sampling. As a result, a total of 547 males representing 72 species were collected. For bottom samples, one occurrence peak was observed from May to June. The number of species increased remarkably with the distance from the rivermouth.For light samples, two peaks of species richness were observed, one in June and another from September to November. Larval distribution ranges differed considerably with species. Cryptochironomus albofasciatus was recorded from all the sites. Fourteen out of 18 major species were collected only at the upper reaches of more than 20 km from the rivermouth. This suggests the existence of a salinity barrier for freshwater species between 15 km and 20 km. Clear differences in the seasonal occurrence of larvae were observed among species. For example, Polypedilum masudai and P. cultellatum occurred only from spring to summer and summer to autumn, respectively. Paratendipes albimanus was collected only by bottom sampling, suggesting the weak phototaxis of male adults of the species. In contrast, Smittia aterrima was collected only by light sampling, probably due to the terrestrial nature of the species.