The nutrition of stomachless phytoplanktivorous fish in comparison with Tilapia

The contents of alimentary tracts of stomachless phytoplanktivorous fish, including silvercarp (Hypophthalmichthys molitrix), Esomus danrica thermoicos, and Amblypharyngodon meletinus were investigated microscopically and compared with those of Tilapia (Oreochromis mossambicus, a fish with stomach).The predominance of detritus in the food and the low efficiency with which phytoplankton is utilized prove, that stomachless fish are not primarily herbivorous. The possibility of omnivorous feeding as an alternative is discussed. These results may contribute to our understanding of the role of filterfeeders in aquaculture and ecosystems.     

Size-hierarchy in a year-class and "condition" of common minnow Esomus danricus (Ham.) in relation to various ecological factors operating in the pond environment

Investigations were made on the existence of size-hierarchy in specimens of Esomus danricus (Ham.) belonging to the same year-class, and the variations in condition factor of the different size-groups. Divergence in the growth rates of individual fish resulted in the development of size-hierarchy in the population. Marked changes seemed to occur in condition factor of the different size-groups. A multitude of factors operating simultaneously in the pond environment appeared to govern the condition factor of fish.     

Length-weight relationship and condition factor of common minnow, Esomus danricus (Ham.) from different freshwater environments.

Investigations were made on the length-weight relationship and condition factor of Esomu danricus (Ham) from ponds and flowing water channels. These relationships could be represented by the regression equations, log W = -2.6252 + 3.2451 log L (for channel fishes) and log W = -1.5390 + 2.6318 log L (for pond fishes). The correlations were significantly high (P < 0.001). The mean values of condition factors were 0.529 and 0.713 for pond and channel fishes, respectively. The additive action of various factors in stagnant ponds, such as oxygen deficiency, stratification of indispensahle nutrients at the bottom, accumulation of body wastes of fishes, and unoxidized chemical compounds, competition for many necessities of life as space, food, shelter, seemed to adversely affect the growth rate and condition factor of pond dwelling fishes. Contrary to this the circulation of water induced by currents in the channels appeared to cause mixing of water at all depths to even out differences in dissolved oxygen concentration, temperature, nutrients, besides minimizing the effect of repressive factor. Such conditions invariably offered more opportunities to fishes to conserve almost all the available space and other resources in the habitat. This ensured rapid growth as evidenced by higher condition factor of the flowing-water fishes as compared to their counterparts occurring in ponds.