Brown Trout decline in some southwest Montana rivers

Posted 6/16/21

The other day when I met Galen for an evening of fishing, he handed me a book containing a note that outlined a serious decline in brown trout populations within some Montana rivers. This is what …

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Brown Trout decline in some southwest Montana rivers


The other day when I met Galen for an evening of fishing, he handed me a book containing a note that outlined a serious decline in brown trout populations within some Montana rivers. This is what that note said:

Big Hole River, Melrose to Brownes Bridge:

  • Brown trout population lowest in 50 years.
  • 2014, 1,800 trout per mile to 400 trout per mile.

Ruby River:

  • Last six years, from 1,500 brown trout per mile to 600 to 700 per mile.

 Beaverhead River:

  • 2,000 brown trout per mile to 1,000 brown trout per mile.

 Upper Clark Fork River:

  • In 2013, 1,800 brown trout per mile. In 2019, 82 brown trout per mile.

Upon return home, I immediately began an internet search to see what additional information might be available about this, potentially, very serious problem. Those of you who read this column know I have fond memories of the years I spent in “the Big Sky.” So the information I received from Galen was more than a little troubling.

The fact that brown trout populations are in decline in several Montana watersheds further complicates an already complicated situation. If it were just one river system, it might be easier to pinpoint the cause. But because brown trout are in decline in so many rivers, covering a large geographical area, finding the cause could be all the more difficult. As an interesting side note, this problem, whatever it is, does not appear to be affecting other salmonoid fishes like rainbow, cutthroat and Dolly Varden trout.

Right now, there are several theories as to the cause of the brown trout decline in Montana. So, before expounding on them, I made a call to Montana Fish Wildlife and Parks, where I spoke with a fisheries biologist. From what I learned during our conversation, fisheries professionals are just as concerned about the brown trout decline as everyone else. The biologist told me that low flow and warming water temperatures during some years are considered an issue. Low flow and warming water temperatures, based on the research I conducted, definitely affected the aquatic insect population in the Madison River and Henry’s Fork River in past years. So, it seems likely that temperature and flow are two parameters that are at least part of the cause of the brown trout decline.

The biologist went on to explain that, even during good water years­—years when brown trout populations rebounded, with higher flows and lower water temperatures—the trout did not respond. When I asked him about the possibility of disease being the cause, he said that disease had been ruled out.

I asked him about the recruitment (transition of very young trout to the next life stage) of young brown trout from a previous fall spawning. He explained that while recruitment was down in some rivers, it did not appear to be a factor in the overall brown trout decline. At the same time, others indicated that recruitment of young browns was poor in all the rivers noted. I mention recruitment because it can be an issue here in the Catskills for those river systems that are dependent on natural reproduction to maintain trout populations. During the fall spawning season, when brown trout deposit their eggs, low flows in tributaries can prevent adults from reaching spawning grounds. Based on poor recruitment here in the Catskills, and based on the comments of some Montana fisheries professionals, recruitment of young trout may be a significant factor in Montana’s brown trout decline. Fortunately, both rainbow and cutthroat trout are spring spawners and, therefore, not subjected to low stream flow associated with fall. The biologist then spoke with me about the increase in fishing pressure on Montana rivers. For example, on popular sections of the Big Hole River, angler trips exploded, increasing from 30,000 in 2001 to approximately 90,000, in 2019. Whether or not angling pressure is an issue remains to be determined.

Based on what I’ve read, and from my discussion with the Montana fisheries biologist, it’s important to try and put a perspective on why brown trout populations are declining in some Montana rivers. Assuming the brown trout decline is related to warming water trends and lower stream flows, why are rainbows and cutthroat’s not affected? Cutthroat’s in particular are warm water intolerant.

There are no definitive answers, just some possible causes of Montana’s brown trout decline. If I were a biologist dealing with this issue, and I’m glad I’m not, I would definitely be looking at water temperature and streamflow, along with the lack of recruitment, as probable causes of this decline. I would want to know and compare the numbers of young brown trout recruited each year, over time, into every river. That information itself might either eliminate recruitment as a possible cause or confirm it as the cause of the brown trout, decline. I would also want to look at streamflow and water temperatures over time in relation to the brown trout decline. I don’t know if the United States Geological Survey has that data for Montana or not, but it’s a place to start. For now, there are lots of questions and not a lot of answers. There are just some thoughts on a problem that’s occurring 2,000 miles away.

In the meantime, with the salmon fly hatch that is only a week or so away, fly fishers, from all over will flock to Montana rivers. The Big Hole River, in particular, will top the list of angler destinations. Hopefully, rainbow and, to a lesser degree, cutthroat trout will offset the losses associated with the brown trout decline.


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