Silver carp - Natural food and feeding habits

Feeding behaviour
Silver carp are phytoplanktivorous filter feeders (Ni and Jiang, 1950), although zooplankton is also ingested. During the larval stage (<15 mm TL) silver carp feed on zooplankton and the digestive tract is less than the body length. At 21–23 mm TL the digestive track increases to 6–7 times of body length, where after the fish start to feed mainly on phytoplankton (Liu and He, 1992). Filtration rate is affected by particle size and the fish show a higher feeding rate for particles larger than 70μm (Smith,1989). Filtration rate increases with increasing temperature and fish size and this is described by the equation FR= 1.54 W0.713, where FR is the maximum filtration rate in L/hr and W is the weight of fish in grammes. The relationship between suction volume (ml/mouth) (Bh) of age 1+ silver carp and standard length (L, cm) is described by  Bh=0.561L–8.94, while for fingerlings (5.6–11.0 cm) the relationship is described by Bh= O.l70L–0.837 (Dong, et al., 1992)

Natural food
Under natural conditions silver carp actively select for phytoplankton, although zooplankton is also ingested. The relative proportion of phytoplankton and zooplankton in the intestine varies seasonally and fluctuates with natural plankton availability and composition (Spataru and Gophen, 1985; Chen, 1990; Xie, 2003) (Table 2). Between 118 and 1 550 g, silver carp show a higher degree of selection for phytoplankton, organic detritus and bacteria than for zooplankton (Chen, 1990). Silver carp are capable of selecting phytoplankton of 8–20 μm and are also able to strain particles as small as 4.5–10 μm. Using Ivlev’s selectivity index Xie (1999) showed that silver carp actively select specific phytoplankton species. For example he showed that the index was significantly higher for Cyclotella than for Cryptomona. Turker et al. (2003) also reported that silver carp actively select cyanobacteria. Feeding intensity, as measured by the intestine fullness index, is dependent on dissolved oxygen and temperature (Figure 2) (Chen, 1990) and this is illustrated by the significant relationship between intestine evacuation rate (Y, hours) and temperature (X, °C) of silver carp between 100 and 1 150 g. This relationship is described by Y=270.63X-0.6408 (n=11, r=0.9924). Similarly daily food consumption (Y, percent body weight) is related to water temperature (X,°C) and is described by Y=0.2683e0.1503X (n=11, r=0.9929). The composite relationships between daily food consumption, water temperature and body size is illustrated in Figure 3 (calculated from Chen 1990).