NUMERICAL METHOD FOR MODELLING SPAWNING HABITAT DYNAMICS OF LANDLOCKED SALMON,SALMO SALAR

Abstract

The stability of habitat conditions determined by the hydrological regime of a river can play a major part in fish spawning success. Application of the classical IFIM approach (Instream Flow Incremental Methodology) does not usually take into account this dynamic aspect of the habitat. We developed a numerical method to simulate the rate of variation in spawning habitat conditions using two-dimensional hydrodynamic modelling coupled with a fish model based on habitat suitability indices (HSI). The approach assumes that, under natural conditions, fish spawning success is maximized under the most common rates of habitat variation. This method was applied during the feasibility study phase of a Hydro-Quebec hydroelectric project on Ashuapmushuan River (St-Jean Lake region, Quebec, Canada). The aim was to evaluate the environmental impact of different exploitation regimes on the rearing and spawning habitats of the landlocked salmon (Ouananiche) of this river. Only the spawning aspect is addressed here. After the hydrodynamic model was well established on a representative reference reach of the river, two spawning areas were identified within this reach and delimited for computational purposes. The per cent usable areas (PUA) were estimated on these two zones for 36 different flow discharges representing a wide rate of the usual hydrological conditions in summer and autumn. In addition, a time series of daily rate of variation of normalized PUA (dPUA′) were obtained from a 35 year habitat time series. Finally, some relevant statistics of dPUA′ (mean and exceedance probabilities of 50, 20, 10, 5 and 2%) were obtained for the natural flow regime. They were then compared with a hypothetical peak exploitation flow pattern currently 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 different options that were never seriously envisaged by Hydro-Quebec for this river.