Much peer reviewed research today will tell you that beaver dams increase fish species richness, especially in slow water streams. They are temporary and partial barriers, they do have some negative effects, but a net positive impact on the fishery... the biggest issue is that they have to be left to fill on their own. In areas where beaver dams have been managed, you'll have to keep managing the dams... you've created a man made issue that can only be fixed by letting nature take its course- beaver dam succession.
You will see a false positive when dams are removed- you've temporarily increased flows (flushing flows), you've instantly changed the temperature, you forced forge fish (fish food) into marginal habitat> increasing prey availability to trout, you uncover coarse stream beds (flushing downstream and up), and you've forced trout to redistribute in the stream. But to get that affect you have to allow beavers to build a dam and then remove it. A stream not allowed to change will stagnate, diversity will drop, and anglers will leave.
I'm not saying it is all bad, I'm just making a point, that, there can be a net benefit if the system is allowed to naturally grow.
We tried to stabilize banks here with log cribs... now it is a worse problem because they are all failing. We have had (and still have) beaver dam removal programs... those streams are starting to stagnate.