The “Guaranteed Flow Working Group”: A French evaluation of microhabitat component of IFIM based on habitat and brown trout population monitoring

Abstract

In the middle of the 1980s, the relicensing of numerous hydropower-plants led to widespread studies of the influence of discharge on trout populations. In particular, specific efforts were made to adapt and validate the PHABSIM microhabitat component of the Instream Flow Incremental Methodology (IFIM) to French rivers. IFIM is now widely used in France in impact studies for hydropower installations, to determine the instream flow to be recommended for the bypassed sections to meet the requirements of water resource legislation for salmonids. This paper presents the studies presently under way to validate this approach in biological terms. Initial feedback showed that the change from the original discharge in the bypassed section to the recommended value often allowed for a significant improvement in habitat. However, the population response in terms of change in biomass or population structure is not easy to identify. The problem is to determine the ultimate impact on a fish population due to an increase in the potential habitat. Our 4-year study of fish population dynamics on three different streams allows a better understanding of changes in populations. The results have illustrated the role of several factors which actually control the population density.