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Phytoplankton Preferences of Young Silver Carp, Hypophthalmichthys molitrix, in Hypereutrophic Mesocosms during a Warm Season

ABSTRACT The feeding behavior of age-1 silver carp, Hypophthalmichthys molitrix (Val.), on phytoplankton species in a shallow hypereutrophic lake was studied in enclosures from 22 May to 18 September, 1997. Over the experimental period, the filter-pore sizes of the fish and the total biomass of the phytoplankton in each enclosure changed little with time. The fish biomass in each enclosure increased with time, while the percentage of weight gain correlated negatively to the stocking density, due perhaps to competition for prey. Analysis of gut contents of silver carp showed a strong similarity between the algae in the foregut and the algal communities in the water and this was significant (p<0.05) for the enclosure with a low fish density. There were weak relationships between the size/shape of phytoplankters and the selectivity of fish on them (r=0.001, p>0.5). Silver carp fed unselectively when cyanobacteria populations were high, even though the selectivity index for diatoms was slightly higher than those for cyanobacteria, green algae, and cryptomonads.