Rainbow Trout

A rainbow runs four cone types where a walleye runs two: ultraviolet near 365 nm, blue near 434, green near 510 and red near 565. A walleye has no blue channel and no UV channel at all. But the trout's UV channel is temporary — at smoltification thyroid hormone switches it off, and the gene signal disappears within about three days of that hormone rising. It is not quite permanent: a patch of retina looking down‑and‑forward keeps its UV cones for life, and a thyroid surge at maturity can regrow them elsewhere in the eye.

A trout is a drift feeder. In the classic study its stomach contents matched the composition of the drift — what floats past — and not the benthos, what crawls on the bottom. They eat what the current delivers. And in summer a large share of that delivery fell in from dry land: in headwater streams 40–50% or more of summer food can be terrestrial insects. That input peaks precisely when aquatic insect biomass hits its yearly low. The two are out of phase, and the hoppers fill the trough.

Adult rainbows hold in water that is much slower than anglers assume — a focal‑point velocity of roughly 0.33 to 0.46 ft/s, rarely beyond 0.72. But they hold that slow water immediately beside fast water. The fast lane is the conveyor belt; the slow pocket is the chair. Sitting in the current itself burns more than the current delivers, so the fish optimises net energy, not gross food. Boulder pockets are a cheaper lie than the average speed of the run suggests.

Whirling disease has been in Utah since 1991, first found in private hatcheries in the Fremont River drainage. Among the species living here, rainbow trout are the most susceptible — ahead of kokanee, golden, cutthroat, brook, brown and splake. Infected young fish may whirl, blacken at the tail, or die outright; survivors can carry twisted spines and deformed heads. DWR lists detections in Cache, Weber, Beaver, San Juan, Summit and Sevier counties.