When at last I close the gap, everything comes clear: the line over his back, the pale something, the unrivaled power.

This taimen is just a handbreadth short of five feet long and has the heft and shoulders of an adult tarpon. There’s an unmistakable gouge in the right side of his head, gleaming like bone and apparently healed. Jeff ’s streamer flies like a pennant from the ridgetop of his rippling back, a few inches behind the dorsal fin.

Here’s how Jeff would later describe the realization in a Facebook post: “I had just foul-hooked the fish that I’d wanted to catch more than any other. I was crushed. I still am a little bit, but I am coming to terms with it.”

Back in the moment, our first concern is for the taimen. It’s not easy to pull a large creature with hydrodynamic fins backwards or sideways, even with the heavy tippets we use in Mongolia. It would be a shame for this fish to suffer through an unnecessarily long fight; winded is one thing, weakened another.

While I slip into the river and wade into the current, Jeff maintains a steady, solicitous tension. The last thing we want is to provoke another long run. Our approach is earnest but unthreatening, a mutual nonaggression pact. I extend my right arm beneath the river’s surface and, without hesitation, grasp the hard wrist above the broad red tail with the firm grip of a handshake.

I’m in the cool water up to my thighs, and I can feel the taimen’s immense strength like an electric current coursing through my arm and shoulder. He doesn’t exactly relax, but he doesn’t bolt either. I am unquantifiably grateful.

LIKE MOST HUMAN ENDEAVORS, fly fishing is a profoundly moral pursuit. This doesn’t imply that any particular version of morality is applied, only that morals come into play in what is, essentially, an exercise in play. Fishing is absorbing and relaxing because it is fun. Sometimes that fun derives, at least in part, from adopting a moral system (such as catch-and-release, no-kill). This imposition of rules, ostensibly for the fish’s benefit, sometimes accrues to our own betterment as well.

Scarface the movie declares its allegiance to two fundamental precepts. As described by Frank and Elvira (two more fine names for taimen), they go like this: Rule No. 1—Never underestimate the other guy’s greed; Rule No. 2—Don’t get high on your own supply.

By film’s end, both rules have been broken repeatedly. The result? Fear and betrayal, paranoia and delusion, violence and death. Writer Oliver Stone admits that, before finishing the script, he and his wife decamped to Paris in order to escape his cocaine habit. Al Pacino confesses that his nose hasn’t been the same since.

That afternoon at the river, Jeff and I have no such worries. The only thing that we cannot escape is our admiration for Scarface, the fish. We can’t know what he is thinking, of course, but his behavior is surprisingly accommodating, almost cooperative. We cradle him in the current and take turns with the camera, savoring our brief connection with the kind of hugeness and wildness that few would associate with a trout.

As a group, taimen and lenok split off from the rest of the salmon family in the early Pleistocene, nearly 2 million years ago, during the time of the cave bear and the saber-toothed cat, the woolly mammoth and the dire wolf. Because of their geographic isolation, the two species are more closely related to each other than they are to any of the other species of trout and char.

Catching and releasing a big lenok is undeniably entertaining, but doing the same with a big taimen feels altogether different. Taimen are not like other fish: they are too long-lived, too individually distinct, too fearless. Some might argue that we didn’t really catch this fish, but Scarface himself wouldn’t render that opinion. Judgment, like gratitude and self-respect, is a human trait. While it is true that he was not fairly hooked, it is also true that we devoutly wished that he had been. There can be no shame in our moment of attachment—and no glory either.

As Jeff writes, “Even though this fish will forever be accompanied by a giant asterisk, it is an exceptional fish that I will remember until I can’t remember things anymore.”

After we release him, I scramble back up the bank to check on his whereabouts, craving another look. I want to replay that initial surge of excitement when we first spotted him, to relive that sense of wonder. From my vantage atop the dike, I can inspect nearly the entire pool, from the deeper run at its head to the wide shallow tailout. There are no fish in view, not even a grayling. It’s as if the river has temporarily cloaked its inhabitants with invisibility.

THE NEXT MORNING, a very large taimen swims past camp, just as the guests are preparing for their first day of fishing. Two anglers step off the gravel bar between the drift boats and cast, but the monster refuses both flies. Because he is swimming upstream and the observers are standing on his left, nobody notices the scar.

While most of the boats—including mine—hoist anchor and set off downriver, one stays behind. Zolboo, the guide in question, is a former national fly casting champion and an innovative fly tier. Zolboo’s signature invention is a pattern we call Chewbacca because of its resemblance to an oversized extraterrestrial, with yak hair and eyes. Chewie’s only shortcomings, from the caster’s perspective, are its weight and wind resistance. On the plus side, the angler can pick it up and toss it like a softball, if necessary.

I’ll admit here that I thought about Scarface long into the previous night, reviewing that excruciating instant when I knew I’d botched the hookset, wondering if perhaps he hadn’t opened his jaws at all, if maybe my fly had slid blamelessly across his closed mouth or hard gill plate. I thought, too, about the release and that glimpse of him moving hugely and purposefully, without panic, into the current—and how strange it seemed that a fish that big could disappear so quickly and in such clear water.

I’ll also confess that I was concerned about his welfare. What, for instance, might have inflicted that scar? No eagle or crane—no sane bird of any sort— would attack a fish the size of a Labrador retriever, and brown bears seldom venture into this part of the drainage. Which leaves only humans: either local herders (this seems unlikely) or rogue visitors like the ones Jeff and I encountered paddling downstream in a raft. I’ve seen the ancient fish-harvesting implements in the museum in Ulaanbaatar, so I know that a multipronged iron spear is a possibility, although one that seems even less probable than an osprey with a god complex. Most modern-day poachers use set lines or nets or explosives.

Nevertheless, something or someone wounded him—and somehow he made a complete recovery. When the guides gather for a shore lunch, Zolboo tells me that when he met Scarface, the taimen was in excellent form and, in fact, ate Chewbacca on three separate occasions—before the angler finally managed to set the hook.

Peter W. Fong is the author of the award-winning novel Principles of Navigation. He has been the head guide at Mongolia River Outfitters for more than a decade.