NUMERICAL-METHOD FOR MODELING SPAWNING HABITAT DYNAMICS OF LANDLOCKEDSALMON, SALMO-SALAR

The stability of habitat conditions determined by the hydrological reg ime of a river can play a major part in fish spawning success. Applica tion of the classical IFIM approach (Instream Flow Incremental Methodo logy) does not usually take into account this dynamic aspect of the ha bitat. We developed a numerical method to simulate the rate of variati on in spawning habitat conditions using two-dimensional hydrodynamic m odelling coupled with a fish model based on habitat suitability indice s (HSI). The approach assumes that, under natural conditions, fish spa wning success is maximized under the most common rates of habitat vari ation. This method was applied during the feasibility study phase of a Hydro-Quebec hydroelectric project on Ashuapmushuan River (St-Jean La ke region, Quebec, Canada). The aim was to evaluate the environmental impact of different exploitation regimes on the rearing and spawning h abitats of the landlocked salmon (Ouananiche) of this river. Only the spawning aspect is addressed here. After the hydrodynamic model was we ll established on a representative reference reach of the river, two s pawning areas were identified within this reach and delimited for comp utational purposes. The per cent usable areas (PUA) were estimated on these two zones for 36 different flow discharges representing a wide r ate of the usual hydrological conditions in summer and autumn. In addi tion, a time series of daily rate of variation of normalized PUA (dPUA ') were obtained from a 35 year habitat time series. Finally, some rel evant statistics of dPUA' (mean and exceedance probabilities of 50, 20 , 10, 5 and 2%) were obtained for the natural how regime. They were th en compared with a hypothetical peak exploitation flow pattern current ly in use on a neighbouring river. According to our model, this regime would induce a 10-20-fold increase in the rate of variation compared with the most extreme natural conditions (dPUA exceeded 2% of the time ). This regime was considered to obtain a contrasting image of differe nt options that were never seriously envisaged by Hydro-Quebec for thi s river.