Green River trout estimates break records

Wyoming Game and Fish Department
Posted 4/15/21

The year 2020 is one many of us will remember.

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Green River trout estimates break records


PINEDALE – The year 2020 is one many of us will remember. The year will certainly be remembered for many things, but will it be remembered as the “glory days” of fishing on the upper Green River in Wyoming? Recent assessments of the Green River trout fishery by Wyoming Game and Fish Department biologists in Pinedale indicate the trout population is at or near an all-time high.

Every year, biologists monitor the status of the fishery by completing an electrofishing assessment within a designated reach of the river. Darren Rhea, a fisheries biologist in Pinedale, directs all of the work conducted on the Green River.

“We maintain a fairly rigorous monitoring strategy for the Green River,” said Rhea. “It’s an extremely valuable resource in the region and supports a lot of angler use.”

“It’s important that we keep a close eye on the fishery to ensure that it is not being over-utilized, or impacted from other factors,” he continued.

In the Upper Green River (above Fontenelle Reservoir), six different reaches are monitored on a three-year rotation, with two reaches monitored each year to maintain a robust dataset going back several decades.

The process can be fairly complex. Large rafts and drift boats, specifically designed to carry generators and other specialized sampling equipment, will carry a crew of two to three people down the river producing an electrical field in the water, which allows for the effective capture of adult trout. The process is repeated over a period of three to four days to generate an estimate of the total number of fish in the river based on a mathematical model that uses the number of captured and re-captured fish over the course of the sampling event. The resulting population estimate, along with size-structure and species composition, provides managers with empirical data to evaluate the current status of the fish population and compare to previous years to monitor trends or long-term changes.

The effort in 2020 focused on two areas of the river, including one reach of the river near the Daniel Fish Hatchery and Forty-Rod Creek, and another below the Five-Mile Bridge area near Big Piney. Both areas have been sampled regularly for a period of 25-plus years and provide a good baseline for monitoring changes over time.

Work conducted within the “Forty-Rod” reach in September 2020 generated an estimate of trout greater than any estimate ever obtained within this section of the river. A total of 719 trout/mile were estimated to inhabit the river with brown trout accounting for over 80 percent of the population. Rainbow trout made up the majority of the remaining population. This was the first time in over 20 years that the number of trout in the Green River exceeded 700/mile in this section.

Historical data, dating back over 30 years, indicates that more trout used to inhabit similar areas of the Green, though most of them were small rainbow trout (less than 9 inches) that were the result of widespread stocking has since been discontinued. Wild trout now make up the vast majority of fish in the river and their numbers have never been higher.

This section of the river continues to be popular among anglers and supports some of the highest angler use in the region. Recent monitoring data indicates that fishing pressure, especially boat-angler pressure, has been increasing on the Upper Green.

“It’s encouraging to see such a robust population of wild trout persisting within this section of the river,” says Rhea, “especially considering all of the angling use the river supports right now.”

One reassuring phenomenon Rhea has witnessed over the course of his career is a strong shift in voluntary catch-and-release angling on the Green and other popular river fisheries.

“During the summers of 2019 and 2020, we interviewed over 300 anglers fishing the Green River and not a single angler reported harvesting a trout, despite catching and releasing hundreds of fish.”

Similarly, the portion of the river sampled near Big Piney smashed historic records. Trout numbers were estimated at a fairly pedestrian 560/mile.

“We have seen estimates of trout numbers higher in this reach,” says Rhea, “but we have never seen a biomass estimate (the total number of pounds of fish) anywhere near what we recorded in 2020.”

Biomass estimates account for the total pounds of fish in the water. Despite similar numbers of fish in the river compared to previous years, greater biomass estimates indicate that the trout population is comprised of much larger individuals than years past.

By all accounts, 2020 may well have been the “glory days” of fishing on the Green River. Unfortunately, Rhea doesn’t expect the phenomenon to persist.

“Last year was among the most ideal conditions for sampling trout in the Green River. Lower than average flows added to our capture efficiency and contributed to very precise estimates,” he said.

However, persistent drought conditions and low flows do not bode well for fish populations in the future.

Lower than average discharge means less over-winter habitat and reduced survival for many fish in the river.

“I suspect we will see a drop in overall fish numbers in 2021, and without significant snowfall during the winter, we may see dramatic impacts that will last for a period of years,” Rhea said.

Despite the current circumstances and short-term prospects of the river fishery, Rhea says he is largely encouraged by the outcome from the 2020 sampling effort. “It indicates, that overall, the river is healthy and resilient to annual changes in climate and flow.”

“For close to 20 years, I have witnessed the ups and downs in the river fishery as a result of dry and wet periods, but if we continue to care for our water and land, and make the best use of our resources, I am certain the Green River fishery will persist for many generations to enjoy.”