Abundance of four exotic fish species cyprinus carpio, ctenopharyngodon idella, hypophthalmichthys molitrix and aristichthys nobilis in the urpod beel of Goalpara District of Assam.

The present study was carried out to find out the abundance of four exotic fish namely, common carp, grass carp, silver carp and bighead carp in the Urpod beel of Goalpara district of Assam. The Urpod beel has a rich ichthyofaunal diversity and available almost all native fish species of Assam. During last few years the beel become habitat of some exotic fish species and their number has been increasing day by day, causes disturbances to the native fish species. The rate of exotic fish infestation in the beel was determined through field investigation and catch record from January 2011 to December 2011 in the fish landing centres of the beel. Abundance Index was calculated for the four exotic species from each catch data. A marked seasonal variation of the fish catch of native and exotic fishes was noticed. Among the four exotic species common carp recorded highest catch (4.99% of total catch) and bighead recorded lowest (2.84% of total catch).

Functional ureogenesis and changes of amino acid metabolism in amphihaline shad hilsa (Tenualosa ilisha, Hamilton-Buchanan) while inhabiting in estuary and freshwater habitats.

The possible occurrence of a functional ornithine-urea cycle (OUC) and changes of activity of key amino acid metabolism-related enzymes were studied in the amphihaline shad hilsa (Tenualosa ilisha) that were collected from estuarine water of Kakdwip and from freshwater river basin of Bhrahmaputra during the breeding season. Very high concentration of urea was detected in different tissues and plasma of shad hilsa collected from estuarine water compared to the one collected from freshwater river basin. This observation clearly suggests that the shad hilsa has the potential of synthesizing and retaining urea inside the body for the purpose of osmoregulation while living in hypertonic saline environment of estuary. This was accompanied by the presence of high activity of all the five OUC enzymes in hepatic and in certain non-hepatic tissues such as the kidney and muscle of shad hilsa in support of its potential ureogenic capacity while inhabiting in estuarine water. The activities of different key amino acid metabolism-related enzymes such as glutamine synthetase, glutamate dehydrogenase, alanine aminotransaminase and aspartate aminotransaminase were also found to be significantly higher in shad hilsa of estuarine water compared to the one collected from freshwater habitat. Thus the adjustment to amino acid metabolism in shad hilsa in different environmental salinities appears to play significant roles for osmotic balance and also for proper energy supply in addition to the presence of a functional OUC while migrating between marine and freshwater habitats throughout their life cycle.