The first scenario depends on the usage of the term "evolution". In the common idea of change from one species to another, no it is not as both populations of fish belong to the same species. If it's in vein of change in general, even within a species, then yes as there has been a genetic change within that population of fish and thus evolution.
As for the second scenario it depends on lake conditions. If the smaller lakes are simply miniatures of the old lake then we can postulate that, provided genetic dominance doesn't favor one allele over the other, conditions are equally favorable for both populations and thus both will flourish in the new environment. However, if conditions favor one allele over the other, the more favorable trait will grow to be the dominate, if not only, expression of that species in the new lake.
