The fry you might see now in a side eddy of a stream or pool of a small brook will be about 2.5 cm long. They are actively feeding on very small invertebrates. They will feed voraciously as long as the water temperatures are not too high and may attain a length of 10 cm by fall. This is by far the fastest growth rate which they will experience in their life. If there are too many fry for the available food, competition will be fierce and many will perish.
Predation can be a big factor.
They are probably where you might least expect them. You might call them trickles, but they are shaded by the forest canopy and are fed by groundwater or fens. They are cold and clean. This is microhabitat. If you were to look under some of the rocks or sticks in these small brooks you will find the fry.