Toxicological Alteration of Dimethoate (rogor) Insecticide Against Freshwater Fish Colisa fasciatus and Mystus mystus

Like other organophosphates, dimethoate is an acetylcholinesterase inhibitor which disables cholinesterase, an enzyme essential for central nervous system function. Dimethoate insecticide can be used for both indoor and outdoor purposes to eliminate a number of insect pests under different condition. Dimethoate, one of the most extensively studied pesticides, may induce many significant changes in fishes. The aim of the present study is to determine the toxicological action of dimethoate on freshwater teleost fishes. The Dimethoate has shown strong piscicidal activity in freshwater fish Colisafasciatus and Mystusmystus for all the exposure periods (24 or 96h) in time as well as dose dependent manner. The LC 50 values decreases from 0.084 ppm (24h) to 0.059 pp m (96h) in winter season (water temp.19⁰) Colisafasciatus and LC 50 0.028 ppm (24h) to 0.010 ppm (96h) in summer season (water temp. 23⁰). The LC 50 decreases 0.039 ppm (24h) to 0.022 ppm (96h) in winter season against Mystusmystuus and LC 50 0.040 ppm (24h) to 0.015 ppm (96h) in summer season. The pesticide exposure may also fatal to many non-targeted organisms like fish where it hampers its health through impairment of metabolism, occasionally leading to the death of the fish.


Introduction
Pesticides are employed routinely in the integrated farming practice to protect crops and animals from insects, weeds and diseases. Liberal use of pesticides at different stages of crop production, starting from seed processing to storage of agricultural produce is posing great danger to aquatic environment. These pesticides are carried into aquatic ecosystem by surface runoff from sites of application, where they enter the organisms through food webs and also through contact in water. Therefore, the health of aquatic ecosystem is being adversely affected because they serve as ultimate sink for these pesticides. These pesticides are found to be highly toxic not only to fish but also to other organisms which constitute food of the fishes. ( (WHO,1992). Dimethoate [IUPAC Name-0,0 dimethyl S-(N methyl carbamoylmethyl) phosphoro-dithioate] CAS No. 60-51-5 is an organophosphate available in the market by the trade name of Rogor. It is a systemic insecticide used for control of a wide variety of insect pests of fruits, vegetables and crop plants. Dimethoate is highly selective as insecticide because relative rate of degradation of toxicant by enzymes (esterases and amidases) are very low in insects as compared with those of mammals (Randy et al.,  2004). Like other organophosphates, rogor is also an acetylcholinesterase inhibitor, therefore, works primarily as nerve poison which is reflected in uncoordinated abnormal behaviour of the fish soon after exposure to pesticide. Contamination of aquatic ecosystem by pesticide can cause acute and chronic poisoning of fish and other organism (Heger, 1995).

Fish
Adult freshwater teleost fish Colisa fasciatus of uniform size range (length 6.3±0.86 cm; width 3.6±0.49 cm; weight 2.4±0.24 g) and Mystus mystus(length 7.2±11.3 cm;3.6-10 g; width 7.5±8.5 cm)were collected from different water bodies of Gorakhpur district of Uttar Pradesh, India and kept in glass aquaria containing 50L of de-chlorinated tap water for 7 days to acclimatize them to laboratory conditions. Water quality was measured according to the method of (APHA, 2005). The temperature of the experimental water ranged from 23.4 to 28.6 0 C. The other parameters were within the following range: total alkalinity was 43-62 ppm, pH 6.8-7.7, dissolved oxygen 7.8-10.3 mg/L. Dead fish were removed as soon as possible to avoid water fouling. Fishes were fed daily on commercial fish food manufactured by Tokyo, Japan.

Pesticide
Dimethoate (Rogor) is one of the earliest insecticides widely used against vegetables and fruit sucking aphids, mites, saw flies and boring insects on cereals, cotton, chilli, tobacco and oil seeds. Pesticides are also well known for causing more toxic effects in teleost fish (Scott et al.,1967; Jackson,1968).

Toxicity Experiments
Ten fishes were kept in glass aquaria containing 5L of dechlorinated tap water. Fishes were exposed to four different concentrations of dimethoate (rogor) against fishes Colisa fasciatus and Mystus mystus. Organophosphates pesticides dimethoate was given as the final concentration in the test aquaria. Control fishes were kept in de-chlorinated tap water only. Each set of experiment was replicated six times. Mortality was recorded every 24h during the observation period.

Treatments
Concentration

Results
Like other organophosphates dimethoate (rogar) is also an acetylcholinesterase inhibitor therefore, works primarily as nerve poison which is reflected in uncoordinated abnormal behaviour of the fish soon after exposure to pesticide. The slope values were steep and the results were found to be within the 95% confidence limits of LC values. The't' ratio was greater than 1.96 and the heterogeneity factor was less than 1.0.
The 'g' value was less than 1.00 at all probability levels (tables 1-4). • There was no mortality in control groups.

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Batches of ten fishes were exposed to four different concentrations of rogor.  • There was no mortality in control groups.

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Batches of ten fishes were exposed to four different concentrations of rogor.

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Concentrations given are the final concentration (w/v) in aquarium water.

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Regression coefficient showed that there was significant (P<0.05) negative correlation between exposure time and different LC values. • LCL = Lower confidence limit; UCL = Upper confidence limit. • There was no mortality in control groups.

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Batches of ten fishes were exposed to four different concentrations of rogor.  • There was no mortality in control groups.

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Batches of ten fishes were exposed to four different concentrations of rogor.

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Concentrations given are the final concentration (w/v) in aquarium water.

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Regression coefficient showed that there was significant (P<0.05) negative correlation between exposure time and different LC values. • LCL = Lower confidence limit; UCL = Upper confidence limit.

Discussion
Fish mortality due to pesticide exposure mainly depends upon its sensitivity to the toxicant, its concentration and duration of exposure. Contamination of aquatic ecosystem by pesticide can cause acute and chronic poisoning of fish and other organism (Omitoyin, 2006). The pesticides are found to damage vital organs of fish (Velmurugan et al, 2007; Singh et al., 1996), skeletal system and cause biochemical alterations in the exposed fishes (Singh et al., 2002 (Kumar et al., 2000).
Wide differences in LC50 values of air breathing and carp fishes reflect greater resistance of air breathing fishes to dimethoate which probably occurs due to their ability to adaptively shift towards aerial breathing in contaminated water. Different LC50 value of dimethoate in different fish species occur probably due to differences in susceptibility and tolerance related to differences in rates of accumulation, biotransformation and excretion of toxicant. Loss of balance and drowning reflect the progression towards death as fish succumbs to the continued high exposure of dimethoate. Similar alterations in behaviour of dimethoate exposed fish have been reported earlier in Heteropneustes fossilis  . (Mandloi,1995) in his toxicity experiments, of five organophosphorus pesticides on fingerlings of Cyprinus carpio, reported that 100% mortality was observed at 5 and 2.5 ppm of emulsified phenthoate and quinalphos, respectively. Apart from their well-known cholinesterase inhibitory effect, the organophosphorus pesticides have been found to affect the physiology of fishes (Dalela et al., 1978; Deodhat et al., 1984).
Organophosphate compounds have been reported to produce biochemical changes in tissue of fish. Cythion has been reported to reduce protein level in brain, liver and ovary in fish Channa punctatus (Ram et al., 1985). The slope line is an index of the susceptibility of the target animal to the pesticides used. A steep slope is indicative of rapid absorption and onset of effects. Even though the slope alone is not a reliable indicator of toxicological mechanism, it is a useful parameter for such a study. Since the LC50 of the pesticides of different concentrations lay with in the 95% confidence limits, replicate tests of random samples would indicate the concentration response lines in the same range (Singh et al., 2009).

Conclusion
The considering the hazards of pesticides, it becomes necessary to think about the danger of pesticides and make an effective strategy to keep them far from non-target organisms as far as possible. It is believed that the rogor (dimethoate) is toxic to the freshwater predatory fishes in aquatic medium.