Designing a fish assessment program for high pesticide use agricultural areas in Sri Lanka

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University of New Brunswick


The increased use of limited land resources in tropical countries used for agricultural activities requires development of ecosystem-based assessment tools to ascertain system sensitivities to protect those ecosystems from impacts. A highly fragile agricultural ecosystem in the upper Mahaweli River basin was selected to address the concepts of "multiple stressors" and '"cumulative effects" in the Sri Lankan context. The agricultural catchment exhibited extensive exploitation, which prevented the location of reference sites within the catchment. Furthermore, an interpretation of data from an alternate forested catchment was challenged by the presence of cryptic fish species and shifted reproductive development. The community composition in agriculturally-exploited areas revealed loss of prominent species and/or species groups relative to the forested catchments. Seasonal reproductive patterns of three fish species ( Garra ceylonensis, Devario malabaricus and Rasbora daniconius; Teloestei, Cyprinidae) were monitored from 2009 to 2011. The following characteristics of G. ceylonensis were most suitable for assessment: numerical abundance, wide distribution, single spawner, lower trophic food preference, and peak reproductive exposure to stressors. The response to agricultural stressors by G. ceylonensis primarily followed a eutrophic gradient, such as that this species exhibited increased density, larger body size of individuals, and increased male and female gonad size in response to increasing agricultural land use. In the middle elevations of the catchment, there were sub-optimum gonad investment and fecundity of females suggesting that fish are exposed to chemical stressors, and/or physical stressors. Results of residue analyses of organophosphate (OP) and N-methyl carbamate (NMC) pesticides in water and sediments were related to acetylcholinesterase enzyme activity in three stream-dwelling fish species; total OPs and NMCs and fish responses were higher during drier periods relative to the wet season. The overall objective of this dissertation research was to integrate current understanding on fish biology and the effects of possible agricultural stressors by linking theoretical and observational information, identifying data gaps, and recommending an overall approach to population risk assessment related to non-point, multiple stressor impacts from agriculture.