You are completely wrong about mercury in salmon. Pregnant women are not told to abstain entirely from fish. Long lived fish species bio accumulate mercury; salmon, being relatively short lived, collect very little. Where do you get this info? Dams are and have always been the beginning of the end for salmon. Take a little time in the evening and read up on depression era dam projects built in the Northwest on the Columbia and tributaries, follow the decline of the greatest wild salmon runs in North America in years following (here's one simplified source http://www2.kenyon.edu/projects/Dams/bbe02har.html). Reservoirs lead to warmer water, among other things, and they by nature block the ability of salmon to migrate upstream. Both of these effects are bad for salmon.

No dams on salmon streams or their tributaries!