Effets des peches intensives sur les concentrations de mercure dans les poissons de lacs naturels du nord quebecois

We have conducted an intensive fishing experiment in five small natural lakes of Northern Quebec, Canada. Three of these lakes were intensively fished in 1998. In the first two lakes, approximately 60% of the total fish biomass was removed. In the third, removal represented approximately 25% of total fish biomass. The two other lakes served as reference and only characterization fishing took place during the course of the study. Three years after the intensive fishing, we find that Hg concentrations in some fish species for a given length have decreased significantly in the two most fished lakes (up to 60% fish removal). In the first lake, Hg concentrations have decreased by about 33% in walleye and by about 20% in whitefish. In the other lake, Hg concentrations in walleye and northern pike have decreased by 20%. We tested three possible mechanisms to explain these observed Hg concentration decreases: (1) the overall removal of MeHg from the aquatic system through fish biomass removal; (2) faster growth rates in the surviving fish; (3) shifts in diet in surviving fish.; The first hypothesis proposed that by removing an important part of the fish biomass from a lake, a significant amount of methylmercury (MeHg) would be eliminated, therefore reducing the Hg available to the remaining biota, at least in the short term.; The second hypothesis proposed that fishing could result in biodilution of Hg in fish due to faster fish growth rates. Growth rates were found to increase only in walleyes of the two most fished lakes. Biodilution can be defined as a lower cumulative exposure to mercury in a given time resulting in lower concentration for a same-size standard fish. Biodilution can explain part of the Hg concentration decreases only for walleyes of the most sensitive lake after fishing.; The third hypothesis proposed that Hg levels in fish changed, at least in part, because of a shift in trophic level, in diet or at least in Hg intake.; Finally, in the last part of this thesis, we tested the effects of intensive fishing on Hg concentrations in fish using two mechanistic models. The first model applied was a bioenergetics-based model for Hg bioaccumulation in individual fish (Onefish). The second model applied was a larger-scale whole-lake model, the Dynamic Mercury Cycling Model (D-MCM). The D-MCM is designed to help understand and predict the cycling and fate of the major forms of Hg in lakes, including the transfer of methylmercury through a simplified food web and bioaccumulation in fish. (Abstract shortened by UMI.)...