Fishes are important as they indicate the ecological processes and the producer-consumer interactions. Fish can be used for ecological assessments at all levels of biological organization; assessment procedures are available at the levels of ecosystems, populations, individuals, organs and at the cellular and molecular levels (Harris, 1995). Fish can also be used as indicators over a temporal range varying from minutes to decades and spatially from a local scale measured in meters to entire river catchments (Karr, 1991) over thousand kilometers because fish species exhibit diverse morphological, ecological and behavioral adaptations to their natural habitats. Fishes are the integral components of stream and river systems and represent a visible measure of stream ecosystem structure and function. Fish assemblage structure and function are also associated with geographic variation and the understanding the pattern is crucial for effective assessment and monitoring of streams and rivers.
In the world about 22,000 species of fishes have been recorded, of which 3.32% belongs to cold freshwaters, 24.73% belongs to warm freshwater, 6.5% brackish water and 65.45% marine ecosystem (Calestousjuma, 2002). India is endowed with a vast expanse of open inland water in the form of rivers, canals, estuaries, natural and man-made lakes, ponds, backwater and mangrove wetlands. India has a vast potential for development of inland water fisheries. Due to increased demands for reliable supplies of electric power, irrigation and drinking water, the number of new reservoirs increasing dramatically in India.
The Western Ghats are one of the most important biodiversity areas in India. It has a good number of endemic flora and fauna including fish fauna (Jayaram, 1981; Talwar and Jhingran, 1991). In the total 930 species of fish recorded in lentic and lotic freshwaters of India, 168 species found in Western Ghats (Singh et al., 1993). In that, catfish posse’s considerable commercial importance due to the high consumer preference and they are suitable for both mono and polyculture system and has huge market demand (Jayaram, 1981).
During the recent years there is an increasing interest and activity has been shown to enhance aquaculture productivity on scientific basis in fish (Billard and Marcel, 1986; Kai-Wang and Dehadrai, 1992; Sinha and Srivastava, 1991; Hopper, 1995). Special attention has been paid to studies on biochemical, physiological, genetical, biotechnological aspects and also hormonal influence on gonadal development, sex differentiation, sex reversal and production of fertilizable gametes as well as development, growth and brood stock management in relation to egg and larval quality (Guraya, 2000). Knowledge on biology of fish breeding is extremely important for the development of fish culture. Maturity and spawning are important biological aspects to be studied in fishes as it is useful in its management (Anju and Anoop, 2003). Reproductive cycle and gametogenesis are important parameters in understanding the reproduction of native fish species and to the establishment of conservation programmes (Bazzoli and Godinho, 1991 & 2007). The economic value of any fish depends upon relationship between its length and weight. The ratio of length to the weight of fish is known to be a useful index to demonstrate the well being of the fish (Maceina and Murphy, 1988).
Mystus cavasius is widely distributed Siluroid fish found through out India and Burma (Day, 1878; Nath and Dey, 2000) in rivers, canals irrigation channels, beels, ditches, ponds and inundated fields. It belongs to the family Bagridae, order Siluriformes. Mystus cavasius is a commercially important fish having high protein content and good market value. This also forms an important fishery along Bhadra reservoir. The identity of South Asian riverine Bagrid catfishes usually referred to as Mystus cavasius (Hamilton, 1822). This genus comprises some species like Mystus seenghala, Mystus aramatus, Mystus pelusius, Mystus tengra, Mystus montanus, Mystus falcarius and Mystus gulio etc. These species differ from each other in dorsal fin, shape of predorsal profile, coloration and number of rakers on the first gill arch. Barring some works on this freshwater fish viz., on the feeding habits in relation to the size of the gut (Das and Moitra, 1956), the spawning frequency (Qasim and Qayyum, 1961), studies on the biology (Bhatt, 1971), histochemical activity of HSDH steroid activity in ovary (Saidapur and Nadkarni, 1976), reproductive biology from Guntur (Sharma et al., 1996), inter-specific hybridization (Arockiaraj et al., 2005) and identity of Mystus cavasius from Myanmar (Prosanta Chakrabarty and Heok Hee, 2005).
Histology is the science of producing stained sections of preserved tissue on glass slides that can be examined under a microscope. Microbes and parasites, as well as pathological processes and abnormalities can be detected in these preserved tissues. No other studies have been made on physiological, enzyme activity viz., HSDH activity and histology of this fish. Particularly on this Bhadra reservoir area, the culture practice of this fish is not reported due to lack of proper knowledge and guidelines about its enzyme activity and histology. Hence, the hitherto above study of male fish Mystus cavasius was undertaken keeping in view the above background.