The filter feeding capabilities of the Indian major carp Labeo rohita (Ham.) and Catla
catla (Ham.) and the Chinese carp Hypophthabmichthys molitrix (Val.) were studied under laboratory conditions. An ultrastuctural and histological study of the pharyngeal apparatus associated with filter feeding of laboratory reared fish between 30 and 90 mm standard length (SL) showed that a large number of mucus cells, both small goblet and large clavate cells, were present in the bucco-pharyngeal region of all species studied and on the wall of the suprabranchial organ of H. molitrix. The density of taste buds was observed to be much greater on the roof of the pharynx and comparatively low on the roof of the mouth in all species. Among the carps studied, H. molitrix had the greatest concentration of taste buds on both the roof and floor of the bucco- pharyngeal region.

An important part of the study was the development of a sound methodology for counting algal cells and colonies. An automated electronic particle counter, the Coulter Multisizer, was used to increase the speed of counting. A study was also carried out to determine the effects of the preservatives Lugol’s iodine and buffered formalin at different concentrations on both long-term and short-term storage of Pediastrum boryanum. Microcystis aeruginosa and Chlorella vulgaris. Both Lugol’s iodine and buffeted formalin of different concentrations caused significant decreases in the numbers of cells and colonies during preservation. Lugol’s iodine of different concentrations caused cell shrinkage in C. vulgaris and M. aeruginosa after 24 h of preservation.
The abilities of carp fry and fingerlings to ingest a range of algal species were investigated. H. molitrix was found to be the most efficient among the carps studied in ingesting P. boryanum and M. aeruginosa, although C. catla was found not to be able to ingest M. aeruginosa. Ingestion rate was found to be positively correlated with algal density. It was also observed that algal size had a positive effect on ingestion rates. In a separate trial it was found that silver carp did not ingest a toxic strain of M. aeruginosa.

An attempt was also made to evaluate whether these filter feeding carp could detect and ingest unattached bacteria by exposing them to the bacterium Chromobacterium violaceum in suspension. Examination of the gut contents of the exposed fish showed that the numbers of H. molitrix and L. rohita ingesting viable bacteria increased with the concentration of the bacterial suspension and with exposure time. However C. catla ingested far fewer bacteria per unit time than the other two species.