Results of a study of the abundance, biomass and production of larvae of the lakefly Chaoborus edulis in Lake Malawi are presented. Temporal variations in larval abundance were evident, but no distinct seasonal pattern was discernable. The maximum biomass of larvae occurred in September-October, following increased turbulence of the water column in June-July: this turbulence brings nutrients from the hypolimnion to the epilimnion and promotes enhanced primary and zooplankton production. There was no clear pattern in the macrodistribution of C. edulis larvae, other than a general, but inconsistent, tendency for numbers to be lower in the extreme south of the lake.
Swarms of adult flies occurred throughout the year and throughout the lake. In the extreme south, swarms were seen mainly between November and March. In the relatively shallow southern part of the lake, mortality of late instar larvae and pupae from fish predation is likely to be sufficiently severe to prevent numbers of emergent adults from reaching the high densities required for swarming. There was no marked lunar-related periodicity in swarming behaviour, other than an indication that swarms were scarce during about 4-10 days prior to the new moon.
Annual production of C. edulis in Lake Malawi was estimated as 2.2 g dry
wt m-2 in 1992, and 6.4 g dry wt m-2 in 1993. Mortalities of
second and fourth instar larvae exceeded 75%, probably mainly as a result of food
limitation and fish predation, respectively.