Studies on the fish assemblages in the achenkovil river system with special reference to their niche segregation habitat usage

Abstract

River Achenkovil (l0ng:76~-77"El, a t: 9°-100N) is one of the least polluted rivers in Kerala, which harbours a large variety of fish species. The present investigation was carried out over a period of three years from June 1990 to May 1993 in six selected study stations in the river along the course length of 128 kilometers at intervals of altitude variation. The study describes the niche segregation in the fish communities of Achenkovil river in the light of resource partitioning. Species, habitat and feeding methods were determined from field and laboratory observations. 360 seining collections made systematically at six stations over one year resulted in the capture of about 2160 specimens representing 64 species; the numerically dominant groups being cyprinids (13 species) percids (4 species) and silurids (3 species). Niche segregation among the assemblage members was primarily by diet, vertical position in the water column and horizontal distribution related to water velocity. Habitat structure and physico-chemical tolerance are the most important factors in regulating the distribution of fishes in the stream. Habitat diversity is depended on water column depth, flow rate and substrate type, whose temporal variability was noted greater in upstream than in downstream. Interactive segregation frequently implicated as the primary mechanism regulating the structure of stream fish assemblage, although unpredictable environmental disturbances also may substantially affect stream fish populations. Most species showed a high degree of morphological specialization, especially in structures related to feeding, which were interpreted ecologically based on character correlation, gut content analyses and field observations. The study reveals that the size of the prey is the most important factor and the vertical position of the prey being the next in the separation of diet among the different species. It is observed that the assemblage members reside in distinct microhabitat in either the upper water column (Surface feeder) (Barilius bakeri, Danio aequipinnatus); mid-water column (Column feeder) (Rasbora daniconius, Puntius fllarnentosus), lower water column (Bottom feeder) (Garra mullya). Thus it can be concluded that the pattern of segregation has resulted by a combination of predatory avoidance and differential evolutionary adaptation rather than by interspecific competition for space.