Couplea other early season food sources Zen didn't mention
are Snails and Dragonfly nymphs.
Dragonfly nymphs go through 4-5 instars and most live as nymphs
for 2 or more years before hatching, so some are present year around.
Trouble is they are not that prolific except for in a couplea lakes I know
of (which I'm keeping under my belt) that have lots of woody stuff in
the water. But still always worth an early season try.

Snails on the other hand are one of the commonest of food sources and
are present in nearly every lake I know of. They are NOT widely imitated
by flyfishermen and that's a mistake. The trout relish snails year
around, but by early spring mainly large adults are still alive and they tend
to be toooooo big.....gives a normal sized trout baaaaad constipation.
But still, they are worth fishing in early spring and trout will eat them
in the smaller sizes even if no small ones present.