carp, the freshwater fish species, are going blind in Punjab. The reason, a study reveals, is a pesticide used in rice fields nearby. Monocrotophos, the pesticide, causes a cataract-like condition reducing the economic value of the fish, said Punjab University researchers who carried out the study. The study was initiated after fish farmers from Patiala and Sangrur districts took their complaint to the researchers. The farmers said the fish started going blind during monsoon a few years ago.
Carp are reared in ponds across the country. To increase their body size and weight, supplementary feeding is done but the blind fish are unable to locate the food and remain underweight. "This is affecting farmers' income," says M S Johal of the department of zoology, Punjab University, and one of the authors of the study. During the monsoons the water from the rice fields flows into the nearby ponds bringing the pesticide from there, which affects the fish, he adds.
To confirm their suspicion that monocrotophos was causing the blindness, the researchers exposed the fish to different levels of monocrotophos and analysed their eye lenses. They found that with an increase in pesticide levels, production of the enzyme, acetylcholinesterase, reduced. This enzyme is essential for the normal functioning of the nervous system in many species, including human beings. The pesticide belongs to a class of pesticides, organophosphates, that is known to inhibit the action of the enzyme. Besides, earlier studies on wild salmon found that fishing nets treated with pesticides have led to blindness in their population. Hence, measuring the level of acetylcholinesterase is a commonly used test of organophosphate accumulation in animal tissues, the researchers said in the study published in the May 25 issue of Current Science.
Concentration of glutathione, another important enzyme that is essential for maintaining the transparency of eye lens, was also tested. The enzyme, in a non-oxidized state, keeps the eye lens clear of calcium crystals, ensuring no cataract-like formation. Monocrotophos promotes formation of the oxidized form of the enzyme leading to deposition of calcium crystals, the study says. The researchers measured the non-oxidized glutathione levels and found that it decreased with increase in monocrotophos levels.
J Venkateswara Rao of the Indian Institute of Chemical Toxicology, Hyderabad, says the link between monocrotophos and cataract formation is new. "It has not been reported earlier. However, the pesticide is unlikely to be the sole cause of cataract. Perhaps it is working in synergy with some other compound. More study is needed to establish a definitive connection," he says.
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