Comparative studies on the trophic niche of two planktivorous cyprinids, silver carp and bighead, by examination of chlorophyll A amount in their gut contents and feces

To study trophic niche of two planktivorous cyprinids-silver carp and bighead, comparative studies on chlorophyll a (Chl-a) amount in their gut contents and feces were made. The fish used for gut contents examination were cultured in a net cage in Lake Donghu, and the contents from the anterior end of the intestine to the middle of its first loop were collected as fore-gut contents, and the contents in the intestine within 5 cm from the anus were as hind-gut contents. Chlorophyll a content per unit dry material was in the order of fore-gut < hind-gut < feces for both fishes cultured in the net cage. The ratios of phaeophytin (Phaeo) to Chl-a in the feces of the carps were considerably lower than in their gut contents near anus. When the fishes were cultured in small aquaria with the lake water, both bighead and silver carp showed much similar ratios of Phaeo/Chl-a and Chl-a to particulate solids (SS) in their feces. However, when cultured in the net cage in the lake, Phaeo/Chl-a ratio in the feces of bighead was markedly higher than in those of silver carp while Chl-a/SS showed a reverse relation, suggesting a divergence of the trophic niche between the two carps, i.e., food resources containing higher concentration of phaeophytin such as zooplankton, suspended detritus or sediment night be more important for bighead than for silver carp