Contribution à l'analyse du cheminement du mercure dans les poissons des réservoirs hydroélectriques du moyen-nord québécois

Many applications of mercury bioaccumulation models to fish have been done in natural lakes for which they were first derived. These models were also used for reservoirs without taking explicitly into account the Specific environmental conditions known to exist in a newly created reservoir and which would exert a definite influence on the accumulation of mercury in fish. These were the reasons setting the objectives of the study reported here and which were: (1) To analyse characteristics of mercury concentration with time of non-piscivorous and piscivorous fish from data originating from an hydroelectric reservoir which reflected environmental conditions far from the ones observed in natural unperturbed lakes. (2) To evaluate the capacity of bioenergetic and mercury accumulation models, currently used for predicting evolution of fish mass and mercury burden in fish with time, to describe accurately the data available, and to make the necessary adjustments to the models, as seem fit, to ensure more accurate calculations. Whitefish (Coregonus clupeaformis) was chosen as the typical non piscivorous fish and Northern pike (Esox lucius) as the typical piscivorous fish in the Robert-Bourassa reservoir. Analysis of fish data by fish cohorts has permitted to characterise fish mass and mercury concentration in fish cohorts with time from the time they were born (i.e. having experienced all their life the evolving environmental conditions of the reservoir from year to year). For both fish species, very significant differences were observed in fish mass and mercury concentration in fish with time between fish in the reservoir and fish in the natural settings before flooding. Fish mass data as a function of fish age, used as an input to the bioenergetic model, permitted the calculation of the parameter of the model reflecting food availability with time. The results were coherent with the reported increase in organic matter available to fish in a newly created reservoir. They were also coherent with the antagonistic phenomenon of erosion of flooded soils with time. Accordingly, fish mercury burden (fish mass x mercury concentration in fish) as a function of fish age, used as an input to the bioaccumulation model, permitted the calculation of the fish food diet mercury concentration as a function of time since flooding of the reservoir. Available data from fish stomach content for both species has permitted to characterise the nature of the food components of the fish diet and the relative proportion of these components in the diet. For the whitefish, the major food groups considered were plankton, insects and benthos. For Northern pike, the diet was composed of non-piscivorous fish and piscivorous fish (surpredation). The results have shown the failure of the current version of the bioaccumulation model to produce accurate estimate of the fish diet mercury concentration. However, modifications made to the model, coherent with known physical phenomena, produced very satisfactory results when compared to available data