Competition Response
Competition response, while rare, happens enough that it's worth mentioning. When a pod of steelhead moves into a pool at the same time, there is inevitably competition for the primary holding spots. As fly fishers we call these spots "buckets" because fish seem to rest in these spots time and time again. There might be a boulder, a depression, a ledge, or something that causes steelhead to consistently stop and rest there.
A run often has multiple buckets, but there is typically one primary holding lie. In my experience, no other river defines this phenomenon like Oregon's Deschutes. The buckets are incredibly reliable. When I was a guide, I was never surprised when one of my clients hooked a fish in a known bucket . . . after all, the fish was supposed to be there!
However, my antennae would go up when we hooked a fish outside of a bucket. A fish that has taken up a station in a run is much more likely to eat than a moving steelhead. Subsequently, when we hooked a fish outside of a known holding lie, the first thing through my mind was why wasn't it in the bucket? More than likely the steelhead were displaced to a secondary lie because there was multiple fish in the pool.
As steelhead move upriver, they often do so in groups or "pods." This is especially true of summer steelhead. As the fish move into a new pool, there is often a hierarchy that plays out. While there may already be fish resting in the run, the most dominate fish seek out the primary bucket. Smaller, less dominant fish may jockey for position, but ultimately they are forced to find secondary holding lies or to keep moving. This competition for real estate can make steelhead extremely aggressive—so aggressive they sometimes lash out at anything in their space, including your fly.
A number of years ago I was guiding four longtime clients on the Lower Deschutes. We were camped on a spectacular piece of water. As dawn broke, just about the time the coffee was kicking in, I put two of the guys in camp water and shuttled the two others in my jet boat to a run directly across the river. It was the kind of morning that just felt "fishy."
The air was still and filled with the sound of songbirds, always a good omen when chasing steelhead. To go into the full details of that morning's events could be an article in and of itself. I will tell you, however, that it was pure magic. The fish were coming easy...really easy!
At one point, I noticed that one of my guys had barely moved despite the fact that I had put him in the run an hour before. I saw him land a couple of fish while I was helping one of his cohorts across the river, so I motored over to have a chat.

Profanities were the only thing he could say as I approached. He had just hooked his sixth fish of the morning, and it was only 7 A.M. We both stood there with incredulous grins on our faces as he told me what happened. What really blew my mind was that he still had 20 yards of water to cover before getting to the best holding water in the pool.
Our good fortune continued well after the sun had illuminated the canyon. It was fun to hear the whoops and hollers of my clients echoing down the river. It was like the sound of boys playing in a distant schoolyard. All of them were experienced anglers, but none of them had ever seen fishing like that morning. It was a day earned. Four friends, four lifetimes of faithfully chasing anadromous fish. That day, all of the ass-kickings of the past paid off. All told, the crew hooked more than 40 fish before lunch. Like I said, magic.
The lower Deschutes is notorious for pulling up big pods of fish when the water temperatures run cooler than the Columbia River. Many of these fish are destined for other Columbia tributaries upriver from the Deschutes. They pull into the Deschutes to avoid the warm water in the big river, only to turn around and head back out once the weather cools.
That day we landed on a run where a massive pod of fish decided to park. The competition response was in full effect.
Although there's no way to ever predict when you'll find yourself swinging through a giant pod of competing steelhead, pay attention to where you have hooked fish in the past. Make a mental or written note of the location where you got the grab. Over time, you will see a pattern evolve.
Knowing where the buckets are in a run is worth its weight in gold in the game of steelheading. It's a small slice of clarity when trying to decipher the mysterious world of steelhead. If you're finding fish outside of the bucket, there are likely multiple fish in the pool. Consequently, it's almost always worth making a second pass.

I've always believed the best fly fishers are astute observers of the animals they pursue and the environments in which they live. The really good anglers also have a knack for using that information to develop a planned approach that goes beyond "huck and hope."
In the end, these theories of why steelhead eat our flies are just that, observations from my time on the water. I have no scientific data to back it up, just a lot of hours watching. While these notions are wonderful educated guesses, we'll never truly know why a steelhead eats a fly, but that's part of the game. It's the mystery of steelhead that captivates us as anglers.