1. Introduction
Grass carp (Ctenopharyngodon idella) are now widely cultivated in
China as well as in many other countries as edible fish or as biological
control agents for aquatic weeds. It is hotly debated as to whether the
grass carp is truly herbivorous. Fischer and Lyakhnovich (1973) stated
that 40–100 g grass carp displayed very low growth rates when fed
with plant food. They argued that the optimal diet for grass carp should
include 75% animal food, and that the high growth rates of grass carp in
natural waters attested to its omnivorous nature. Stanley (1974)
reported that grass carp fed elodea (Egeria densa) exhibited a negative
nitrogen balance, but Liu et al. (1963) showed that 10 g grass carp
could achieve very high growth rates when fed with duckweed. High
growth rates were also reported for grass carp as small as 3 g fed a
range of macrophytes (Cai and Curtis, 1989; Shireman et al., 1978).
Cui et al. (1993) found that grass carp fed with plant diet spent longer
time on feeding, had higher feeding intensities, and consumed less
dry matter per bite than those fed with animal diet. Grass carp fed
with plant diet feed almost continuously for most of the diet cycle.