I don't think you even need to invoke such a controversial argument as "all non-human lives don't matter". Drastic climate change is likely to impact every species. Considering the vast amount of cheap, safe, carbon-free power that's available from hydro, it's probably the right call for salmon to take one for the team here.

I live in the heart of the PNW and most of my power comes from COAL. The rest; natural gas. It's insane. We could power three states by damming a few rivers. I'm not an expert, but I also think it's more than possible to do that in a way that still let's salmon spawn, depending on how well things like fish ladders and run-of-the-river actually works. I'm pretty sure BC get's the majority of their power from hydro, and from how much time local fishermen spend up there, the salmon are doing just fine.