Not a big specimen, by anyone's standards, but look how pristine and healthy she is. Steelhead from Lake Superior seldom have marks from commercial nets, deformed mouths from previous run-ins with fishermen, or scars of any kind. No sea lions or sharks to worry about. No muskies, very few pike. Since so few anglers know how to find them out in Superior, ospreys might comprise the most troublesome predatory influence for steelhead in one of the world's largest freshwater lakes (I learned this week that Superior holds something like 9% of the world's surface supply of fresh water.)
The largest predators in Lake Superior are lake trout. To eat the specimen I'm holding, a laker would have to weigh 25 pounds or more. Plenty of lakers in Superior exceed 25 pounds, but few would be fast enough to make a steady diet of onchorynchys mykiss.
Just how fast is a steelhead? Rick Hammer asked that question one year as we sat around a fireplace sipping cocktails at our annual Steelhead Camp. Mike "McTrout" McLeod said, "Twenty six feet per second." Hammer thought that sounded ridiculous. The next morning, McTrout and I stood on the bank, comparing hooks, and Hammer hooked up directly below us. The silver missile leapt, pointing downstream. Within a few seconds it leapt again, about 50 feet upstream. It happened so fast, Hammer's line was still pointing downstream. The line then ripped out of the water, sizzling upriver like a fast-burning fuse just as the fish leapt again — several yards downstream of Hammer, whose head was on a swivel, snapping left-right-left. Over the next minute or so, I don't believe the line was ever pointing to the spot where the fish actually was. When his line finally snapped it sounded like a gun shot.
Hammer slumped. He turned to us, mouth still agape. "What was that?"
"Twenty six feet per second," McTrout drolled. I still don't know where he got that statistic, but I can tell you this: The fish Hammer hooked was moving exponentially faster than the 23 mph attributed to rainbow trout by WikiAnswers. Info.com is even worse, somehow clocking "trout" at 24 km per hour. Another site claims salmon can swim at 45 km per hour (technically, rainbow trout are pacific salmon, as indicated by the designation onchorynchus, but based on my experiences with each, I tend to believe steelhead are faster than kings, silvers, chums, etc. And quite a bit faster than brown trout.)
In-Fisherman ran a comparative chart years ago that claimed steelhead could swim at 35 mph, but that's probably an educated guess from a steelhead fisheries manager that tried to clock one with a speed gun. Personally, I think the only way to know for certain is to have an orca chase one past a radar station.
Dolphins are, of course, mammals, not fish, but they're probably the fastest things in the sea. Certain species of shark are among the fastest fish in the sea. Like tuna, they can purportedly achieve bursts of 50 mph or so. (Some sites say bluefins can hit 80 mph for short bursts.) Dolphins are known to attack sharks by circling underneath to ram them in the belly. Sharks are more likely to fear dolphins than the other way around.
Anyway, it's all conjecture. I don't think anyone has adequately clocked a truly motivated steelhead — one being chased by a huge predator. But 35 mph, underwater, is plenty fast. Put it this way: Whatever the top-end speed steelhead can achieve is, it's fast enough to give Hammer whiplash.