The state of Kerala, lying between S018' and 12" 48' N lat 74'52' and 779' E long. bordered on the East by Western Ghats and on the West by Lakshadweep Sea, is gifted with 41 west-flowing and 3 east-flowing rivers which form about
5% of India's total freshwater potential. Rivers are linear systems which show a gradient of characters along their length associated with their progression through the varied landscapes. Geomorphic processes govern the river form, and based on it several classifications have been suggested for tropical river systems. The primary distinction of the river course as Rhithron and Potamon is more suitable to riverine systems of Kerala even though greater differences exist between the various zones of one river and the homologous zones of different rivers. In a river system from the headwater to the river mouth, the physical variables such as width, depth, velocity, flow rate and temperature present a continuous gradient. This condition is referred to as the " ~ i v e rContinuum Concept" (Vannote et a/., 1980). Existing models of the structure and dynamics of stream communities are largely based on patterns in temperate streams. The tropical river systems, especially the rivers which originate from the Western Ghats, has very scarcely been subjected to studies incorporating the river continuum concept. Of the 41 west-flowing rivers the River Pamba is the third longest (161 kms). About '13
of the total length of this river flows through thick
forest with diverse terrain and substrate. A very high habitat variation exists within a short distance of about 10 km along the river. A typical example of such a topographical variation and difference in microhabitat is exhibited by the River
Pamba in the Perumthenaruvi region. Within a distance of 10 kms, a very intense microhabitat variation exists so as to identify five different sampling stations. The Perurnthenaruvi part of the River Pamba is situated in the lower hilly region of the Western Ghats. The river in this part is a very turbulent fourth order stream with intermittent deep, calm pools and rapids in its course. The topography and gradient in this region is unique. During its curved course of about 10 km it displays habitats which show high physical variations such as pool zone, slowflowing zone, fast-flowing zone and rapid zone, with different grades of substrate such as silt, sand, pebbles, rubble and bed rocks, not met with in any other part of the rivers in Kerala. The vegetation of the riverbank ranges from bushes to thick forest canopy. During dry season 80% of the riverbed is exposed, imposing a lot of restrictions to the surviving fishes to remain in the isolated rocky pools formed in the riverbed. This peculiar phenomenon in the Perurnthenaruvi is quite contrary to the 'Riva Continuum Concept' and hence the physico-chemical parameters and fish communities of this part of the stream were subjected to detailed investigation. This work is an attempt to put together the results of the observations conducted for a period of three years !?om January 2000 to April 2003. The study is presented in five parts.
Part I gives a description of the different species of fishes collected during the present investigation based on morphometric and meristic characters. Extensive collections have been undertaken from five stations selected. Fishes were collected mainly by gill nets, cast nets. fish traps and other manual methods. All the collected fishes were photographed alive so as to present the natural colour
of the body. The specimens newly recorded from Kerala were deposited in the Zoological Survey of India, Regional Centre at Calicut, and indexed. Detailed account of each species has been provided.
Part I1 deals with the physico-chemical characteristics of Perurnthenamvi area. Studies pertaining to the physical and chemical aspects of the
Pemmthenaruvi region of the River Pamba...