Major Trout Decline in Idaho’s South Fork Snake River

Idaho’s South Fork Snake is one of my favorite rivers, a place where you can hammer up to 50 native cutthroats a day when the fishing is good, and it’s also a place where you have a legitimate shot at catching a brown trout that stretches well past 24 inches, a fish that might weigh eight pounds or more. This is a river where dry fly aficionados and streamer addicts both get their due. But something happened between a trout population census taken in 2013 and one that was just completed—a lot of trout disappeared. That’s not good and the Idaho Department of Fish and Game doesn’t really have an explanation on why this happened. I’ll stay on this story, but you might want to read a little more here, words from the Idaho Statesman. The good news in all of this, and a message to take away from this read is that trout numbers were at an all-time high. So this decline puts the population back in the historical average range. No need, repeat no need, to cancel your trips to the Snake this year. You may not see as many trout as you did in the past few years, but there are still plenty of them out there.

Trout numbers in the South Fork of the Snake River are down, a puzzling development for Idaho Department of Fish and Game biologists.

“This year’s data is kind of an outlier, and it has us scratching our heads a bit,” said Brett High, the department’s point person on the South Fork. “We are trying to figure it out.”

The famed Eastern Idaho river attracts fly anglers from throughout the U.S. and is one of the most heavily guided trout streams in Idaho.

Annually, F&G biologists use electro-fishing gear near Conant and Lorenzo to track trout numbers.

At Conant, trout numbers dropped from 5,372 trout per mile in 2013 to 3,671 trout per mile this fall – a 32 percent decline.

At Lorenzo, the population dropped from 2,416 to 1,564 – a 35 percent decline.

In both cases, the trout population dropped from near record highs to closer to the long-term average for both locations.

“We shouldn’t paint the picture the sky is falling,” High said. “Populations ebb and flow, and we are in one of the downturns. Still, it is a bit puzzling.”

Fish and Game has collected population data at Conant since 1982 and at Lorenzo since 1987.

Traditionally, trout populations rise when there are good flows in the tributaries during the summer and in the main river during the winter.

Conversely, populations in the South Fork dip when tributaries are low in the summer and the main river is low in the winter.

That trend largely didn’t play out last summer and last winter, High said.

“Last year, we had relative good tributary flows in the summer and we had decent winter flows,” he said.

So what is happening?

High said it could be the result of a disease in the


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5 Responses to Major Trout Decline in Idaho’s South Fork Snake River

  1. SC says:

    The decline wouldn’t have to do with the fact that F&G are tagging rainbows and putting a prize on them would it? I am no fish biologist but the fact there is a no kill limit on rainbows, promoting to kill all rainbows in the system and flushing the river during Rainbow spawn, would in turn reduce the number of fish in the system…..

    • Greg Thomas says:

      Good thought and something I considered, but they’ve had that reg in place for many years and there hasn’t been a major dent put in that rainbow population. To think that only a decline in rainbows would drop numbers that significantly doesn’t make sense to me. I think they are leaning toward disease, which hit their whitefish population hard. Maybe it whacked the trout, too. I’m for IDF&G trying to lower the number of rainbows for the benefit of those wild cutts. Where do you stand? Thanks for the solid comment. GT

      • SC says:

        I’m a little torn on the subject to be honest. I grew up on this river and I’ll pick a healthy rainbow any day. Seems to me rainbows are stronger and smarter. I’ve sat over a pod of bows before and lost the fight. You stand over a pod of cutties and you can usually pick them all off and win every time. I know they are doing it for the right reason and I get it. It just seems crazy sometimes when you fool a giant rainbow who spools ya a few times and when you get him to the boat to think people don’t want them in the system….

        • Greg Thomas says:

          I understand your thinking, entirely. I think the department has made a stand and they are saying, OK, this is the best cutthroat river in the West and we’re going to keep this as a cutthroat stream. They’re sacrificing and keeping rainbows in other systems so people still have the chance to catch those big, cagey ‘bows elsewhere. But on the SF, they’re saying cutts are the way to go. I, too, like the mix of browns, cutts and ‘bows, but I do favor protecting a particular river for the benefit of cutthroat and the dry-fly opportunities they provide.

  2. SC says:

    Roger that..

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