As I said in another post, I had the opportunity to hear Steve Moore, the Park fisheries biologist speak a couple of weeks ago. A large part of his presentation was on the drought. Here are some of the things I recall about rainbows:
There are very few 4 year old rainbows in the Park. Rainbow diets are strictly insects. The Park is not particularly rich in insects (try to convince me of that when the knats are flying in the humidity of mid-summer), therefore once they get to be a certain size and their food requirements increase, they burn more energy sitting in the feeding lanes than they can take in. And they die off.
That's essentially what happened to the 3 year olds and to a lesser extent, the two year olds during the drought. Their food source dried up and they died off. But that left what little food was available to the one year olds (YOY in his terminology) and they prospered. Bottom line is there should be just as many rainbows in the park as in prior years. But they will be smaller.