The water is 43°F and rising. Yesterday it was 39. You’re standing in a run you’ve fished all winter with a black-and-blue Intruder on a heavy sink tip, crawling it through the deepest slot at a pace that would bore a snail. It’s worked — or at least it’s produced the occasional grab in the dark months. But something has shifted. The light is different. There are caddis casings on the rocks. A pair of spawned-out fish rolled through the tailout twenty minutes ago, and for the first time since October, you’re wondering whether the fly you’re swinging is actually the right one.

A steelhead’s metabolic rate roughly doubles for every 10°F increase in water temperature — which means a fish holding in 43°F water is burning energy and processing stimuli measurably faster than the same fish at 37°F. That’s the biological engine behind everything that changes in spring. The deep, slow, dark-fly winter game worked because cold fish conserve energy, hold in predictable slots, and make calculated interceptions of things moving slowly through their immediate window. As temperatures climb past 40°F, those constraints loosen. Fish move into shallower, faster water. Their chase distance extends. Their response shifts from opportunistic to aggressive — especially as spawning hormones enter the picture and territorial behavior intensifies.

The flies, the swing speed, the water you’re reading — all of it needs to follow that metabolic shift, and the anglers who recognize the transition in real time are the ones who find April’s best fishing instead of grinding through winter tactics that stopped working two weeks ago.