Surveys show significant fish declines in Blue River upstream of Dillon Reservoir
DILLON — Fish populations are seeing a significant decline in the Blue River. That is according to a 2019 fishery survey report published by Colorado Parks and Wildlife, which found a precipitous drop in trout for a stretch of Blue River upstream of Dillon Reservoir.
Since 2007, Parks and Wildlife has conducted a biennial fishery survey of the reach with assistance from Summit County, the U.S. Forest Service and the Colorado Water Quality Control Division. After evaluating the latest survey, Parks and Wildlife aquatic biologist Jon Ewert found “an obvious and significant decline occurring in this fishery.”
That conclusion is based on the steady drop in total biomass of surveyed fish collected during each survey since 2011. The 2011 estimated trout biomass was 228 pounds per surface acre. Since then, the figure has dropped by more than 50%.
“The 2019 survey yielded the lowest estimate to date, which is less than half of the peak values observed in 2009 and 2011,” the report said. “The consistency and repeated observations of this downward trend over a period of several years makes it a virtual certainty that this is not an artifact of sampling error.”
The survey was conducted on a 581-foot stretch of the Blue River, named the Fourmile Bridge reach, that is 2.7 miles upstream of the Dillon Reservoir.
The survey was conducted by using electrodes that safely stun fish for collection and measurement by workers who trawl the stretch with nets to gather the trout.
While the report does not make any conclusion as to what might be causing the decline, it does urge action and study to discover the root causes for the fishery depletion and address them to improve the health of aquatic wildlife in the Blue River.
Ewert said the report is meant to jumpstart that investigation.
“The purpose of the report is to highlight the fact that there’s this situation developing where we can observe a steady decline in fish,” Ewert said Wednesday. “It means to say, ‘Let’s come together and figure out how to improve the situation.’”
Though no cause has been established, the report does speculate as to the possibility that the stretch’s habitat has changed so that it no longer supports a high density of fish. That could include contamination from abandoned mine runoff, obstructions in the water placed by humans or other disruption caused by human activity.
Richard Van Gytenbeek, Colorado River Basin outreach coordinator for freshwater habitat conservation nonprofit Trout Unlimited, said that areas worth exploring include the health of the aquatic food chain, which starts with algae.
“Aquatic invertebrates need algae to graze on, they are really dependent on that food source,” Van Gytenbeek said. “You need more phosphorus and nitrogen in the water to get the algae. Without that, you don’t have the bugs fish feed on, which puts that population under stress, as well.”
The other area Van Gytenbeek believes is worth exploring is the characteristics of the current fish population.
“If you have a bad spawn year, and not many fish go upstream to spawn, you’re going to have very low numbers in that year’s class,” Van Gytenbeek said. “Two, three, four years down the road, when that year’s class of fish get sexually active, there’s not as many spawning.”
Van Gytenbeek also suggested that high elevation environments might have a part to play, with lower water temperatures than at sea level.
Ewert pointed out in the report that despite the decline, the surveyed stretch of river is still a healthy fishery.
“It is important to note that, despite the ongoing decline that we have observed, this is still an excellent fishery that exceeds the criteria set for gold medal designation in Colorado,” the report said.
The gold medal designation is applied to waterways that have at least 60 pounds per surface acre of biomass with at least 12 fish larger than 14 inches per surface acre. The 2019 survey of the Fourmile Bridge section found a density estimate of 21 brown trout of more than 14 inches in length per surface acre.
Ewert said an upcoming fish population survey for Dillon Reservoir could be useful to see whether there is a relationship with the upstream fish decline, along with more temperature monitoring and closer study of human activity between the surveyed stretch and the reservoir.
The Blue River Watershed Group, a nonprofit dedicated to conserving the Blue River, is also drafting an integrated management plan to help with fish supplies downstream to Green Mountain Reservoir, a stretch of river that lost its vaunted gold medal fishery status.
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