About the Author:
John Merwin lives in Vermont, where, when he's not tying flies, building lures, or digging up worms with his backhoe, he writes the monthly Fishing Column for Field & Stream magazine.

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September 08, 2008

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Merwin: A World-Record Brown Trout?

Last March, Brian Yamamoto, a dentist from Fairbanks, Alaska, caught what’s likely the largest brown trout ever taken on a fly while fishing Argentina’s Rio Grande River. The giant sea-run fish ate a muddler minnow and measured 46 inches long with a 25-inch girth. There was no scale available to accurately weigh the fish, which was released. Biologists working with this river’s trout estimated the mammoth fish’s weight at 41.5 pounds. If that weight had proved out on a certified scale, it would have surpassed the current IGFA all-tackle record for brown trout—a 40-pound, 4-ounce brown taken from an Arkansas tailwater in 1992.

This story got even better a couple of days ago when Dr. Yamamoto sent us a copy of an e-mail from Sarah O’Neal, one of the University of Montana biologists who has worked on the Rio Grande. (Understand that trout scales, like those of most scaled fish, grow in annular rings similar to those you’d see in the cross-section of a tree trunk, for example. Biologists can infer some aspects of an individual fish’s history by “reading” the scale rings.) O’Neal reports as follows:


“The first image is a photo of one of his scales.  This tells us a few things.  He was probably in his seventh year of life (age 6+), which is not very old for a Rio Grande sea trout (they live up to thirteen years or so).  And it appears he spent a bit longer than two years (2+) in the river before migrating to sea.  There's some question about those first two years; there could be a third year there.  Though judging from some data analysis along with consultation with my Argentine colleagues, that seems unlikely.  You can also see that near the end of his sixth year, there is a spawning mark...the scar that is evident particularly on the lower half of that annual ring.  With sea trout, the lack of a spawning mark does not necessarily indicate that the fish did not spawn (in other words, we can say with certainty only that this fish spawned at least one time).  But the occurrence of only one spawning mark combined with his remarkable growth leads me to wonder if this guy didn't just stay out in the marine environment for nearly four years without bothering to come upstream to reproduce. That's twice as long at sea as your average Rio Grande sea trout.”

O’Neal then graphically compared the apparent growth of the Yamamoto fish to that of other, measured Argentine sea-runs.


“The second image is a graph showing the average back-calculated growth of Rio Grande fish for resident browns (solid line) and sea trout (dashed line) relative to our friend here (the dotted line).  Wow. This is what's really got my head shaking.  It's important to note that these numbers [are only]…estimates. We can't know with 100% accuracy the exact size of this guy at each year of his life. But regardless, they suggest that he was a fast-grower from the get go. Even in freshwater you can see he was larger than the average sea trout.  And then he just went nuts out there at sea. The lower growth rate during his last year…is more evidence that he may indeed have remained out in saltwater until that first spawning mark at age six. Growth rate typically slows once reproduction starts.”

While Dr. Yamamoto’s catch is itself remarkable, this is the first time I can remember seeing such a detailed “biography” of a record-class fish. Good work all around, and thanks! 


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Dr. Ralph

I'm seriously offended that F&S makes this blog so hard to find... Pretzal, Hiss, Heavey, Hurteau, Romano and Deeter all fail to provide a link... I'm about to send them some week old chum and a great big F-you letter to show my appreciation. Keep it up JM, more people fish than hunt.


Dr. Ralph,

Let me prescribe some medicine you might not a have in your "dr." bag.

A chill pill.

Dr. Ralph

Wendy, I've been taking them for over a month patiently waiting. Now it's time for action... by the way Tim Romano at "Flytalk" has already said he's getting his link right. You can thank me later.

John Merwin

Don't worry. If "Dr. Ralph" gets too out of control, we can always call in "Dr. Phil" !!
Thanks for commenting!


thanks John. By the way does a sea run fish count as a world record?

John Merwin

Yes it would. The IGFA makes no distinction between steelhead and resident rainbows, for example, nor between resident and sea-run browns. Sea-run and resident are the same species. Records are tracked by species. That's just how it is.

John Merwin

Darn. Further to the above, I checked very quickly and was reminded of at least a couple of exceptions to the species/record thing. Striped bass and landlocked striped bass exist as separate IGFA records, as do Atlantic (sea-run) salmon and landlocked (Atlantic) salmon. In those two cases, a single species has dual records. I guess there's just no accounting for rules set by a committee!

Sd Bob

Record or not, that is one big beautifull fish! It'd be better though if it had been at the other end of my line!


The remarkable thing about the Rio Grande is that it gives-up so many other BIG brown trout. Most anglers in a week of fishing will land a sea-run brown in the 20 pound class. And about a half dozen times a season some lucky fisherman will land one in the 30 pound range. INCREDIBLE!