Effects of Stream Poisoning Disturbance on the Benthic Invertebrate Fauna in a Mid Hill Stream in Nepal

  • S. N. Khanal Department of Biological Sciences and Envimnmental Science Kathmandu University, Dhulikhel, Nepal
  • O. Moog Department for Hydmbiology, University of Agricultura1 Sciences (BOKU) Vienna

Abstract

Although stream poisoning by local herbs 1ike Lyonia ovalifolia, Juglans regia and Euphorbia royleana is a common fishing practice in many parts of Nepal, the effects of this practice on the environment are more or less unknown. Quantitative study of the benthic invertebrate fauna in a middle sized mid hill stream of Nepal indicated that herb poisoning caused substantial reduction of taxa richness and diversity as well as a significant decrease of the individual and biomass density and composition of macroinvertebrates, irrespective of the systematic afiliation of the fauna. EPT (Ephemeroptera, Plecoptera and Trichoptera), Coleoptera and Diptera were among the most affected groups of macroinvertebrates that have been extirpated up to 95%. Ten core metrics with a significant discriminatory power to detect the herb effects on the benthic fauna have been found. Although this fishing technique mobilises the fine sediment in a significant way that leads to a severe turbidity of the water there were no effects of increased putrescible effects. The assessments based on NEPBIOS-ASPTindicated no change of the saprobic water quality by the disturbance activities. The major impairment to these fauna was narcotisation initiated by poisoning and mechanical disturbances of the river bottom habitats. Both effects lead to lethal effects as well as to an increased drift activity (400%) irrespectively from fauna's natural drifting character. Although only few percent of the standing crop of benthic invertebrates remained in the treated river sections, this type of impairment can be classified as of spatial and temporal importance if the procedure is undertaken within the re-colonisation capacity of the entire benthic fauna.