Environmental flow regimes based on natural flow regime: myth or reality for trout population dynamics?

Before and after relicensing of Pont-Haut hydropower station on the Roizonne (in the French Alps), fish monitoring surveys were conducted over a fifteen-year period, complemented by measurement of discharge and characterization of habitat. A model of the dynamics of the trout population (MODYPOP) was calibrated for the period to analyze the fluctuations in the different age classes upstream and downstream of the power plant. Several hydrological scenarios were developed over a twenty-year period on the basis of random picking from the available daily natural discharge time series. For the bypassed reach, four management modes were reconstituted, with MIF(i) constant throughout the year, (ii) varying in accordance with the discharge upstream of the water intake, and (iii) varying in accordance with the season (high in winter and low in summer, or the opposite). The corresponding habitat time series were reconstituted and compared. The response of the population to these different scenarios and dam operation options was tested with the aid of MODYPOP imulations, combining two hypotheses of population regulation as a function of the habitat: i) density-dependent mortality, which tends to push the population toward a size compatible with the local carrying capacity in a hydrologically typical year under low-flow conditions, and ii) adjustment of the adult biomass to the local carrying capacity using a one-month time step. The results of the simulations of population dynamics are compared with the results obtained on the upstream reach under natural flow conditions. This comparison is helpful to a discussion of the natural flow regime paradigm adopted in global instream flow methodologies that are based essentially on hydrology.