Feeding habits of silver carp, Hypophtalmichthys molitrix (Val.), and the problem of clean water

Increase in biomass of benthos due to the effect of silver carp must presumably also be attributed to changes in the quality of phytoplankton. In order better to understand and foresee the role played by silver carp in ecosystems it is essential to obtain a larger amount of accurate data, particularly as to food preferences, amounts of food consumed, amount of faeces and products of metabolism, their effect on decomposition processes and significance in circulation of matter, development of plankton and benthos and functioning of different ecosystems with different stocks of silver carp. Such studies should be carried out within a wide range of conditions — temperature, trophic state, circulation and others, with different abundance and qualitative composition of groups of phyto- and zooplankton and ichthyofauna.

Streszczenie w języku angielskim

24 cm

Strony 258-268 ; 24 cm

Bibliographical references (pages 265-267)

Pages 258-268 ; 24 cm

Abstract in English

The chief components of the food of the silver carp, Hypophtalmichthys molitrix (Val.), are phytoplankton, detritus and zooplankton which occur in greatly differing proportions under different conditions. Growth of these fish is good even when detritus predominates in their food, which offers promising prospects of introducing silver carp into bodies of water in which there is periodical or even constant low plankton biomass. It would appear that zooplanton is a preferred food when biomass of phytoplankton is very high; this seems to apply also phytoplankton under conditions in which it is particularly poor. Species preference of phytoplankton is an undoubted fact, but it is difficult to grasp any regularities in this connection. The question of digestion of different groups of algae is also not clear. Preferences for species or groups of zooplankton is a matter which is both confused and controversial. In the cases examined silver carp usually decidedly reduced the biomass of planktonic crustaceans. The biomass of phytoplankton was in some situations distinctly reduced, in others distinctly increased. This increase could occur even when density of silver carp was extremely great — up to 157 g/m3. Decrease took place in phytoplankton biomass under conditions in which substances were removed from water due to sedimentation of the faeces of silver carp. In shallow habitats, particularly with intensive mixing of bottom sediments by benthos consuming fish, the reverse effect was observed — the faeces of silver carp stimulated destruction and increased the amount of substances present in circulation. Thus, improvement in the purity of water and reduction of phytoplankton biomass by silver carp is more likely to occur in stratified bodies of water, where long-lasting fall-cut of substances from circulation in the epilimnion may take place. Even under such conditions, if the trophic state of water is very high and there are luxuriant water blooms, it would probably be necessary to ensure very 'high stocks of silver carp — at least several hundred kg/ha.The indirect effect exerted by silver carp through the consumption of filter - -feeding zooplankton seems to be even more important than the direct effect of these fish on phytoplankton by. its elimination; decreased abundance of filter- -feeding zooplankton results in reduced in elimination of nannoplankton. On the other hand elimination of macrophytoplankton changes competition relations within the phytoplankton all these modify also physical and chemical factors, etc.Both with a decrease and increase in phytoplankton biomass caused by silver carp (in the few cases when a detailed study was made) here was a decrease in the biomass of blue-green algae and green algae and increase in the biomass of diatoms and nannophytoplankton. These favourable changes in quality of phytoplankton must presumably be attributed to indirect effect. The mechanisms of these changes are not always clear.

Bibliografia na stronach 265-267