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Oxytetracycline (OTC), an antibacterial agent, is extensively used in aquaculture practices all over the world. Despite its use, the toxicity of OTC to freshwater fish has been scarcely investigated. In this study, Labeo rohita were exposed to different concentrations (20, 40, 60, 80, 100, and 120 mg L(-1)) of OTC. Based on the survival-to-mortality ratio, an 80 mg L(-1) concentration was selected for sublethal toxicity analysis. Fish were exposed to the above-mentioned concentration for a period of 25 days, during which fish were killed at the end of every 5 days to analyse certain hematological and enzymological parameters. During the exposure period, a mixed trend was observed in hemoglobin (Hb), hematocrit, mean cell volume, mean cellular Hb, and mean cellular Hb concentration, whereas decreased red blood cell count and increased white blood cell was noted. A biphasic trend was observed in the enzymatic levels of aspartate aminotransferase, alanine aminotransferase, and lactate dehydrogenase in the vital organs (gill, liver and muscle) of fish. The alterations of these parameters lead to the conclusion that these parameters may be used as biomarkers in monitoring OTC toxicity in aquaculture and fisheries farms.

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In the present study, the median lethal concentration (LC 50) of neem leaf extract to Cirrhinus mrigala for 24 h was found to be 1.035 g l -l . During the study period, the haematological parameters including Hb, Hct, RBC, MCV, MCH and MCHC levels were significantly decreased in neem leaf extract exposed fish when compared to the control fish whereas WBC count was increased. Similarly, plasma Na + and Cllevels were significantly lower and K + level were significantly higher when compared to the control. In biochemical study, elevated plasma glucose and induced protein levels were noticed. The enzymes, glutamate oxaloacetate transaminase (GOT) and glutamate pyruvate transaminase (GPT) activities were increased significantly in gill, liver and muscle of treated fish compared to that of their control groups. The results of the present investigation suggest that neem leaf extracts affects the hematological, ionoregulatory, biochemical and enzymological parameters of fish and alterations of these parameters can be useful in environmental biomonitoring of neem based products in freshwater environment.