RESUMO
The present investigation was carried out on the Karalla River, a tributary of the river Teesta from February, 2009 to July, 2010 at three stations to study the seasonal change of physico-chemical factors and ichthyofaunal diversity. The constituents monitored included air and water temperature, turbidity, pH, total dissolved solid, total suspended solid, total solid, depth, conductivity, dissolved oxygen, biological oxygen demand, free carbon dioxide, chloride, salinity, total alkalinity, hardness, nitrate, phosphate and silicate. Through out the study period the water was hard and a significant variation of the rest parameters was observed. In respect of Ichthyofauna diversity, a total of fifty five species belonging to eight orders and twenty families were identified. Among the 55 species, thirty one species were found under the order Cypriniformes, ten species were found under the order Perciformes, seven species were found under the order Siluriformes, three species were found under the order Synbranchiformes and a single species was found under the order Osteoglossiformes, Anguilliformes, Beloniformes and Tetraodontiformes each. This observation indicates that Cypriniformes order is the most dominating of all other seven important orders. There are thirty three species which were common to all the three sampling stations and hence can be considered as migratory ichthyofauna. In this lotic system the diversity and distribution of fishes were uneven. The un-even distribution and diversity may be due to physico-chemical factors of water, substratum soil quality, and unscientific fish capture technology. The increase in species richness at down streams (SII and SIII) also may be due to the occurrence of submerged weeds.