Recent CPW surveys examine trout populations in the Animas River
Colorado Parks and Wildlife
Colorado Parks and Wildlife conducted a survey last year as part of the normal every-other-year cycle. But because of the Silverton mine waste spill in August, CPW aquatic biologists decided to perform a survey again this year. The survey in Durango was conducted Aug. 27-30 the survey in Silverton was done Sept. 8-10.
Neither survey showed any acute effects on fish from the mine spill.
To do the survey, biologists use a technique called electro-fishing to deliver a mild shock to the water which temporarily stuns the fish. They are then scooped up, measured, weighed and identified for species type.
In Durango, the survey was conducted in two segments: from behind the La Plata County Fairgrounds to the 9th Street Bridge and in the section from Cundiff Park to the high bridge.
Compared to last year, the overall biomass -- a measurement of the total weight of fish per surface acre -- increased in both sections of the river. Much of the increase, however, can be attributed to routine stocking by CPW — about 40,000 fingerling trout per year, explained Jim White, aquatic biologist for the agency in Durango.
In the section above 9th Street, biomass -- weight of all trout combined per surface acre -- increased from 27 pounds in 2014 to 66 pounds this year. In the lower reach, overall biomass increased from 75 pounds to 110 pounds. Both sections meet the biomass standard of 60 pounds of fish per-surface-acre set for Gold Medal water status.
But the number of large fish in the river remains low. The number of fish greater than 14 inches improved slightly from last year, from nine to 11 fish per acre. There must be at least 12 fish or more per acre longer than 14 inches for the river to achieve Gold Medal status.
White is most concerned about the lack of trout in the 7-to-12 inch size in the river. Those are the fish that are two- or three-years-old that should eventually grow to reach 14 inches in a couple more years.
"We've been seeing these gaps in age class for the last six years," White said. "We'd like to see more of the young fish we stock recruit into the overall population."
The survey also found little evidence of any natural reproduction by trout. The number of fry – very young trout – found in the river has been in decline since the late 1990s.
White explained that the Animas River in Durango is negatively affected by several factors: the flow of tainted water from area mines and run-off from hillsides, golf courses, lawns, city streets and parking lots that add sediment, contaminants and nutrients to the water. Also, in the last decade there has been less water in the river due to declining snow fall. With less water there is less habitat available and the increase in water temperature is detrimental to trout.
In the river near Silverton, only a few fish were found in the canyon below the confluence of Cement Creek to Elk Park. CPW surveys show that the decline in the number of fish in that section of the river occurred between 2005 and 2010. A few miles east of Silverton above Howardsville, small brook trout were found in the river as they have for many years.
"The bottom line is that trout need cold, clean water," White said.