From what I have heard from guys who frequent the upper D and the W/E branches - where Didymo has been for a number of years(since 2007) - the fishing has been better than ever. It may look nasty and foul lines/flies/lures at peak growth times but I think overall trout pops will be fine. I have yet to read anything online about trout disappearing from any stream infected with it(knock wood).

And yes, I concur, kill birds starting with common mergansers who wander up nearly every trib. along the D.

FWIW -

I've been told by a couple of old timers on the delaware, that didymo has been in that river for a long time. It just recently has been identified and given a name.
Don't know how much truth there is to those claims though

Does the scientific community community have any idea how long it takes from the time of introduction until there is a sizable impact on a stream or river? I know this is a very open question but for instance, is there an estimate when Didymo was introduced into the Delaware? Was it a ten year process or twenty or fifty?