A Colorado Parks and Wildlife team led by aquatic biologist Jim White is making its biennial survey this week of trout in the Animas River.
“The goal is to assess the population and health of brown and rainbow trout,” White said Tuesday. “It also tells us how well our stocking of the river is working.”
White’s team looks at two stretches of the river – the renowned gold-medal fishery from the confluence of Lightner Creek south to the River Road bridge and from Lightner Creek north to 32nd Street.
The survey team uses electroshocking, a common scientific method to determine abundance, density and species of fish. Fish are stunned by the electric current but recover quickly.
Each stretch of the Animas is treated with electroshocks twice. The first day team members punch a tiny hole in the tail fin of stunned fish. They skip a day, then return to repeat the process and count fish with holes in their tail and nonmarked fish to determine the overall population.
“The holes in the tail grow back quickly,” White said.
Native flannelmouth suckers and bluehead suckers and nonnative white suckers aren’t counted, White said.
A gold-medal fishery must have 60 pounds or more of trout per surface acre and at least 12 fish that measure 16 inches or more, White said.
The lower stretch of the Animas surveyed perennially lives up to its reputation as a gold-medal fishery.
Gold-medal fisheries enjoy certain protection from in-channel projects that could affect water quality, White said.
The section of the river from Lightner Creek to 32nd Street has not reached the gold-medal standard for probably a decade, White said.
Reasons for the difference aren’t entirely clear, White said. The effects of 100 years of hardrock mining around Silverton could affect water quality.
The number of trout found north of Lightner Creek has been declining, White said.
The two sections of the river receive different management standards.
Above Lightner Creek, anglers may use bait and keep four fish per day with no size specified. Downstream from Lightner Creek, only artificial flies and lures may be used and only two fish longer than 16 inches may be kept per day.
Electroshocking started Monday and will end today. White said he will write a report on the survey results later this year.
daler@durangoherald.com
