Abstract
A great number of large-sized chironomid adults, Chironomus plumosus and Tokuna-gayusurika akamusi, have emerged in the south basin of Lake Biwa since the 1970s.
Emergence period, annual changes, distribution, sex ratio and flight distance of chironomid adults, C.plumosus and T.akamusi, were investigated from 1977 to 1992.
The former emerged twice a year: in spring, from the middle of March to April, and in autumn, from the middle of September to October.The latter emerged once a year: in the late autumn, from the beginning of November to December.
Recently the amount of C.plumosus adults has decreased at any parts, compared with the amount in 1981 and 1982.
In 1981 a lot of T.akamusi adults were observed at the southern and middle parts. Lately they have decreased in the southern parts. However, there has been a tendency for them to increase in the middle and northern parts. They have been largely found in the middle parts of the east and west coasts.
The sex ratio (male/female) of C. plumosus which emerged in spring and T.akamusi changed from more than 1.0 to less than 1.0 at the peak of their emergence periods.
Over 500m away from the coast, T.akamusi has decreased drastically, while C.plumosus has gradually decreased around 2100m away from the coast.
