Stonefish,

FleaFlickr02 is right. They filter feed. Consequently, when a rainbow trout reaches 16" in length, its gill rakers are spaced too far apart for it to effectively feed on them anymore, so they get no larger unless they have an alternative source of forage, like larger insects or baitfish.

If I remember correctly, daphnia are red clydocerans, the largest of the zooplankton, and critical to the food chain for fish.

Sg