The number of brown and rainbow trout in the Animas River swimming through Durango has declined, according to an ongoing study.
In particular, a decline has been noted in fish from 32nd Street to the Lightner Creek confluence with the Animas, said Jim White an aquatic biologist with Colorado Parks and Wildlife, who worked on the fish survey.
The section of the Animas classified as Gold Medal, from Lightner Creek to Rivera Crossing Bridge (behind Home Depot), also saw a decline, but it was not as dramatic. The area maintained its classification as a prime fishing destination, but the researchers found a decline in large fish.
This is the first time the area hasn’t met the Gold Medal standard for large fish since 1996, White said.
Colorado Parks and Wildlife stocks the Animas with both kinds of trout, and the stocking practices have not changed in about 20 years.
However, the most recent survey in September revealed a worrisome decline in both young and large brown trout compared with prior years, White said.
“We’re concerned over the absence of these young brown trout,” he said.
White plans to start monitoring brown trout specifically and tracking their population fluctuation. He completes fish surveys along this stretch of the Animas every two years.
No clear reason has been identified as to why the fish populations may be declining, but there are many circumstances that might be playing a role.
“There’s a whole suite of negative environmental factors,” said Buck Skillen, president of Five Rivers Chapter of Trout Unlimited.
White plans to research the effect of dissolved heavy metals brought down the river by heavy runoff.
Zinc and cadmium from the mines near Silverton have had an adverse effect on the fish and the environment upstream of Bakers Bridge, said Peter Butler, coordinator of the Animas River Stakeholders Group.
Some other negative factors could include warmer water in the summer, sediment levels, low water levels and other possible factors.
An insect-population study done this fall may reveal more information about river health next year.
The Ecosphere Environmental Services study focused on the food supply for fish, and it will be compared to a similar study completed 10 years ago. The results will be released in the spring, Skillen said.
Anecdotally anglers have noticed a decline in caddisflies, pale morning duns, midges and blue-winged olives, he said.
Some of these insect populations may be affected by sediment flowing into the river because it fills in areas where they live, White said.