Determination of 96-hr LC50 and lethal responses of Wallago attu to water-borne chromium

Water pollution by heavy metals, especially chromium pollution from industrial source can affect aquatic life, all ecosystems and human health directly or through food chain. The present research work was conducted under laboratory conditions to determine 96-hr LC50 and lethal responses of a fish, Wallago attu, to water-borne chromium. The fingerlings of fish were kept in glass aquaria having 35-L water capacity. The toxicity tests were conducted at constant total hardness (250mgL-1), water temperature (28 degree C) and pH (8) with three replications for each test concentration, separately. During 96-hr acute toxicity trials, the observations on fish mortality were made after every 2 hours. Physico-chemical parameters of water viz. total hardness, temperature, pH, carbon dioxide, dissolved oxygen, total ammonia, sodium and potassium were monitored twice a day during each 96-hr trial. The 96 hour LC50 and lethal concentrations of chromium, with 95% confidence interval, were computed by using the Probit analysis method. Regression/correlation analyses were also. performed to find-out relationships among different variables under study. This investigation revealed significant differences between 96-hr LC50 and lethal concentrations of chromium for Wallago attu. The mean 96-hr LC50 and lethal concentrations of chromium for this fish were computed as 59.17 plus minus 1.45 and 90.65 plus minus 1.09mgL-1, respectively. The present investigation also revealed that metallic ion concentrations had significant impacts on the physico-chemical parameters of the test media as total ammonia and carbon dioxide contents showed significant increase with concomitant increase in chromium concentration while dissolved oxygen showed significant decrease. The chromium concentrations in the test media exhibited statistically highly significant correlation with calcium, sodium and potassium while it showed highly significant but inverse relationship with magnesium contents of the test media. Carbon dioxide showed significantly direct relationship with total ammonia while it exhibited highly significant but inverse correlation with dissolved oxygen. The dissolved oxygen revealed negatively significant relationship with total ammonia. Calcium exhibited statistically highly significant but negative correlation with magnesium contents of the test media.