He and his colleagues of the department of environment conservation, Ehime University, Japan, have conducted the study. During 1993 to 1996, pesticide profiles of river dolphins were measured through their blubber and the fish they ate. Since river dolphins are top predators in the riverine food chain, they are good indicators of contamination. At the same time, they are at great risk due to the presence of chemical contaminants in the rivers they inhabit.

The continuous degradation and pollution of the river is affecting the dolphin population and resulting in their rapid decline. Construction of dams, increased boat traffic, fishing and hunting of dolphins for meat and oil has led to declining numbers. The presence of various DDT manufacturing industries close to the holy river and their waste disposal in it has also resulted in high concentrations of DDT in the river.

DDT concentrations in the Ganga were found to range between 0.07 and 143 mg/l, with levels often exceeding 1mg/l, a safe limit proposed by the World Health Organisation. The study reports high levels of DDT in the blubber of river dolphins -- in the range of 30 to 120 mg/g. Exposure to high concentrations of these pesticides have been known to impair the reproductive and immunological functions in captive or wild aquatic mammals.

Senthil Kumar explains this elevated accumulation of DDT in the dolphins is due to the widespread use of DDT for malaria vector control and for the control of Kala-azar disease in India. The presence of high amounts of PCBs in dolphins also suggests local sources of PCB contamination. Compare this with the PCB levels in the North American Great Lakes region and the Baltic Sea, where concentrations of PCBs have been actually declining for almost 15 years. The study suggests that the close proximity of polluting industries near the Ganga, combined with the dolphins' own lesser capacity to metabolise contaminants may be the reason for the river dolphins to be more vulnerable to toxic effects.