Ecological studies on chironomids in Lake Suwa : I. Population dynamics of two large chironomids, Chironomus plumosus L. and Spaniotoma akamusi Tokunaga.

Population dynamics of two large chironomids, Chironomus plumosus and Spaniotoma akamusi in Lake Suwa were studied through a four-year period from 1967 to 1970. 1. Based on the daily catches of midge by means of light trap, three emergence periods in a year were confirmed in C. plumosus (April-May, June-July and August-October) and only one in S. akamusi (October-November). 2. C. plumosus produces three generations yearly in the littoral region. In the profundal region, however, it usually lacks the second generation and some part of the larvae of the first generation emerges finally in the third emergence period. 3. Larvae of S. akamusi disappeared from the surface layer of mud bottom in May as the result of their burrowing. Pupation and emergence took place after their reppearance at the surface layer in September. 4. In the imagines of C. plumosus trapped through an emergence period, males as a rule predominated over females (males 50-83%). The predominance of males over females was even more striking expressed in S. akamusi (males 83-93%). 5. Population density of larvae of C. plumosus attained its maximum in June (ca. 1200-5400 individuals/m2 in the profundal region) and in October (ca. 1500 to 2100/m2 in the same region) with the appearance of newly hatched larvae in the lake bottom. The maximum of biomass appeared either in correspondence to that of population density or a little later, and reached 15-90 and 10-27 g fresh weight/m2 in the above-mentioned periods respectively. 6. Population density of larvae of S. akamusi reached a maximum (ca. 700 to 4800 individuals/m2 in the profundal region) in the period from December to March. The maximum biomass was found in March (ca. 10-96 g fresh wt./m2) when the growth of the larvae had been almost completed. 7. Survival rate of wintered larvae of C. plumosus was very low (ca. 6-7%) in 1968 and 1969, whereas it amounted to about 70% in 1970, followed by an unusually intensive emergence. 8. The quantity of pupae produced in an emergence period was estimated based on the numbers of larvae before and after emergence. The total amount of pupae produced per year was estimated at ca. 268-354 tons in C. plumosus and at ca. 10-168 tons in S. akamusi for the whole lake. A certain quantitative relationship was found between the total number of trapped imagines and the estimated number of pupae produced in an emergence period in both species.