Natural food
Grass carp are herbivorous and feed primarily on aquatic plants but also take insects and other invertebrates (Ni and Wang, 1999), and readily accept formulated pellets under culture conditions. Grass carp are often used for aquatic weed control (George, 1982).
The mouth gape of larvae is small and pharyngeal teeth are not developed. Larvae first feed on protozoa and small zooplankton including rotifers and then switch to larger zooplankton including cladocerans (Dabrowski and Poczyczynski, 1988) and copepods. At 26 mm TL pharyngeal teeth develop and the intestine increases in length and begins to convolute. At this stage the fish switch to a macrophytic diet. As the fish grow the mouth gape increases and pharyngeal teeth become falciform and their cutting ability is improved (Ni and Wang, 1999).
Feeding behaviour
Feeding behaviour is temperature dependent (Cai and Curtis, 1990) (see Table 2). Optimum temperature for feeding ranges from 22-30°C (Ni and Wang, 1999). Feeding intensity and feed intake is also affected by diet type. Cui et al. (1993) reported that grass carp do not show a distinct feeding periodicity. Grass carp fed on duckweed or Elodea feed continuously throughout the day, while fish fed on animal diets (e.g. tubificids) only fed for about a quarter of the diel cycle. On the other hand Ni and Wang (1999) found that when grass carp feed on macrophytes they do so continuously for 2-3 h with a break of less than one hour between feeding sessions (Table 3).
Hybrid grass carp (female Ctenopharyngodon idellus x male Hypophthalmichthys nobilis) show a distinct preference for Lemna gibba, Chara sp., Najas guadalupensis and Potamogeton peciinatus, while macrophytes such as Ceratophyllum demersum and Myriophyllum brasiliense are least preferred (Cassani and Caton, 1983). Hybrid grass carp have a greater range of optimal feeding temperature (12–28°C) than pure grass carp.
Maximum feed intake of grass carp as a function of temperature and body weight is described by Cmax=4.9W-0.24 [(37.5-T)/(37.5-34)]0.68 e [0.68(T-34)/(37.5-34)], where Cmax = maximum feed intake in g feed/g fish/day), T = temperature in °C and W = body weight in g (Wen et al., 1995), while maximum ration as a function of temperature is described by RLmax = 0.04T2.32, where RLmax, % body weight /d and T= temperature in °C (Cui et al. 1995).
Feeding behavior is affected by dissolved oxygen (DO) and grass carp stop feeding when DO falls below 3 mg/l and the optimum range for feeding is 5-8 mg/L (Ni and Wang, 1999). Feeding rate as well as growth also decreases with increased salinity (Kilambi, 1980; Gou, 1998).
