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.
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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:

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“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.

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“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!