Besides the fact that the lake area will still be partially flooded, as other replies have said, fish are complicated and there are a lot of dam related issues that affect fish passage. Just a few:
- Downstream passage would have to be through spillways. Survival rates are pretty good (95%+) [1], much better than turbine passage, but not as good as not having a dam in the way.
- Conventional spillways open from bottom, which fish have a hard time finding. [2] Granted, if the water levels are reduced this could be less of an issue.
- Time spent in slow moving water increases predation on salmon. [3] If you visit a dam you'll see ospreys all around. Slow water can also favor some species over others.
- There are a bunch of other water quality issues that dams cause, most notably water temperature. The 2002 Klamath fish kill [4] (the largest in history and a major impetus for un-damming the Klamath) was caused by high water temperatures. [5]
- Even shadows can cause significant issues, from structures as small as docks or piers. [6]
- Upstream passage is not solved by opening the spillway and would still be an issue, requiring fish ladders or transportation. All the existing problems with fish ladders apply. [7]
- Besides salmon and trout, other important species get blocked by dams, including sturgeon and lampreys [8], and each are affected by dams in their own way.
[1] https://www.salmonrecovery.gov/Hydro/Structuralimprovements/...
[2] https://www.fisheries.noaa.gov/west-coast/endangered-species...
[3] https://www.fisheries.noaa.gov/west-coast/endangered-species...
[4] https://en.wikipedia.org/wiki/2002_Klamath_River_fish_kill
[5] https://www.waterboards.ca.gov/waterrights/water_issues/prog...
[6] https://wdfw.medium.com/casting-shadows-on-the-sound-3a9a555...
[7] https://medium.com/re-form/whats-the-dam-problem-3b5cd839ae7...
[8] https://e360.yale.edu/features/sea-lampreys-pacific-lampreys...