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    Trout population declining in Bighorn River near Thermopolis

    For the last decade, the Bighorn River near Thermopolis has been considered one of the premier trout fisheries in Wyoming and has routinely attracted anglers from hundreds of miles and many states.  In recent years, however, the trout population has declined from its all-time highs (2009-2016) and left many anglers wondering how this fishery is fairing and what to expect in the future.

    Cody Region Fisheries Biologist Joe Skorupski said fish populations are cyclical and react to extreme situations that impact habitat.   “The high and variable water years of 2015-2017 negatively impacted trout populations in the Bighorn River,” Skorupski said. “Recent population surveys, however, are encouraging and if over-winter survival is good, we will see an uptick in certain size classes of fish.”

    The Bighorn River is a tailwater fishery (fishery that is downstream of a dam) that supports a number of trout species including brown, rainbow and Snake River cutthroat trout.   Biologists manage this fishery to an objective, or target number, of 1,000 fish, greater than 12 inches, per mile.

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    “Tailwater fisheries in Wyoming have both benefits and limitations,” Skorupski said.  “They are typically stable and high in nutrients, which are good for supporting fish, but fine sediments and no peak flows result in poor wild fish production to the population.”

    Based on this, two strategies are used to manage this fishery; supplemental stocking of hatchery-raised fish and requesting annual flushing flows from the Bureau of Reclamation to improve spawning habitat for wild fish.

    Annually, 16,000 6-inch rainbow trout and 8,000 Snake River cutthroats are typically stocked in four locations along a 20-mile section of the river from Wedding of the Waters to the Skelton public access area.

    In 2009, regular flushing flows were scheduled on the Bighorn River; prior to 2009, flushing flows were sporadic.   “This is when the fishery really started to boom; trout estimates went from 1,000 fish per mile to between 2,100 and 3,000 fish per mile,” Skorupski said. “Coming off the low water years of the 2000s when the population was at all-time lows of around 600 fish per mile, the flushing flows were an important tool that improved spawning habitat for wild trout and increased production of invertebrates that fish depend on for food.”

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    For eight years, anglers enjoyed that boom in the trout population.  “There were big fish, and lots of them,” Skorupski said.

    Over the last few years, the population has dipped due to high and variable water conditions, which are factors out of human control.   Population estimates conducted annually through a mark-recapture effort documented poor survival of 6-12 inch fish from 2015-2017, and poor recruitment of wild fish in 2016 and 2017.  A total estimate of 1,623 trout per mile was documented in 2018. Of total fish documented, 665 of those were fish greater than 12 inches, which is below the management objective for this population.

    “High water conditions are not bad, however, three years in a row with very extreme conditions in 2017 negatively impacted the population,” Skorupski said.  “This caused poor survival of small fish and no recruitment of wild fish, which created “holes” in our population, resulting in a decline of larger fish numbers.”

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    Annual sampling conducted in October brought some encouraging news.  Good natural recruitment was documented as well as good growth and survival of stocked fish.  “In stocked fish, we documented one inch per month growth, which is as good as it gets,” Skorupski said.  “While fish greater than 12 inches continue to decline, we are optimistic that with improved survival and recruitment of 6-12 inch trout observed in 2018, we will see more big fish in the future.  Even though we have documented a decline, the Bighorn River is still a high-quality fishery, rivaling others throughout the region.”

    What can trout anglers expect next year on the Bighorn River?  Skorupski said there are many variables, but if over-winter survival is good, anglers can expect to see lots of 12-17 inch fish, but fish greater than 18 inches will be harder to come by.   If you happen to hook into a fish over 18 inches, the good news is that the average size fish in this size class has increased to greater than three pounds.

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