Abstract
Seasonal changes in crop contents and the food preference of 2nd, 3rd, and 4th instar larvae of Chaoborus Flavicans were studied in a small, eutrophic pond during April-November in 1987. Major food items in the crops were a dinoflagellate Ceratium hirundinella, rotifers, a cladoceran Daphnia rosea, younger Chaoborus flavicans larvae, and chironomid larvae.
Microscopic observations revealed that Ceratium hirundinella was the major diet ingredient for all instars of Chaoborus Flavicans during May-July. The 4th instar larvae fed on Ceratium hirundinella even though the available zooplankton was abundant. In April and November, Daphnia rosea was the major food of the 4th instar, but was rarely found in the crops of 3rd instar larvae. About 98% of D. rosea predated by 4th instar larvae was <1.25 mm.
Seasonal variation in forage ratio E of Chaoborus flavicans differed widely among prey species. E values for D. rosea and rotifers (Keratella cochlearis and Trichocerca sp.) varied more frequently and widely than for Ceratium hirundinella. In May July, the percentage contribution of C. hirundinella to the diet of Chaoborus flavicans was high (52.8-99.8%), even though the E values were only 0.79-1.31. On the contrary, even though the E values for rotifers were sometimes as high as 55, their contribution to the diet of C. flavicans larvae of each instar was less than 5%. The crops of 4th instar larvae in sediment samples were empty more often than in water samples; this tendency was especially marked in November.
