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

¹ EDF R&D LNHE 6 quai Watier 78400 Chatou France
² Cemagref, U.R. Biologie des Ecosystèmes Aquatiques Laboratoire d'Hydroécologie quantitative 3 bis quai Chauveau, CP220 69336 Lyon cedex 9 France
³ ECOGEA 26 rue dels Pitbouls 31120 Lacroix Falgarde France
⁴ Conseil Supérieur de la Pêche - Délégations Régionales de Dijon et de Montpellier : 23 Bd Doc. Jean Veillet 21000 Dijon France et 55 chemin du Mas de Matour 34790 Grabels France
⁵ 21 avenue de l'Europe BP 41 38040 Grenoble cedex France
⁶ ENSAT, Laboratoire Environnement Aquatique Avenue de l'agrobiopole BP 107 31326 Castanet Tolosan cedex France
⁷ EDF Pole Industrie Mission Technique Hydraulique 1 place Playel 93283 Saint-Denis cedex France

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.