After whirling disease, new strain of trout gaining a foothold
By Bob Berwyn
SUMMIT COUNTY, COLO. An electroshock fish count early Tuesday in the Blue River near Silverthorne yielded plump browns up to 15 inches long, along with a slew of sleek young rainbows ranging between about four and eight inches.
The rainbows are a hybrid of rainbows from the Colorado River and a whirling disease-resistant Hofer strain. They were stocked in the same reach of the Blue River — the Gold Medal catch and release waters just below Dillon Dam — as fry a couple of years ago and many have grown several inches to catchable size since then.
About halfway through the session, the catch was running about 50-50 browns and rainbows — a hopeful sign that rainbow trout could be establishing themselves along with the hungry browns.
Electroshocking only stuns the fish for a few seconds, just long enough to scoop them into a net so they can be weighed and counted and quickly released unharmed. It’s very rare for a fish to die from electrofishing. Improper handling by inexperienced anglers is a much greater threat. It’s the best way to get an accurate count of fish.
CDOW aquatic biologist Jon Ewert and a team of techs caught every fish they could, taking measurements and marking the tails to try and get an accurate count before placing them back in the chilly water. They’ll return Thursday (Aug. 26) and survey the same reach, when a comparison between the number of marked and unmarked fish will give Ewert an accurate idea of the total number of fish in the Blue.
The count is not only important for rainbows, but because this part of the river is a Gold Medal stretch, where the total biomass has to measure up to strict standards. During the past few years, CDOW biologists have said the fishery could be faltering due to low flows and poaching, but habitat improvements done by Silverthorne and Trout Unlimited have helped create at least some pockets where fish can find cover. Better outreach, signage and enforcement have also helped reduce the poaching.
But food and water temperature for trout in the Gold Medal reach are still a question mark. The water comes from the bottom of Dillon Reservoir. the temperature stays constant all year, which is good in the winter, but the 40 to 42 degree range is not optimal for growth.
And since the reservoir is managed for domestic water supplies, the emphasis is on keeping nutrients out. That makes it cheap for Denver Water to purify the supply, but limits nutrients for animals at the bottom of the aquatic food chain, especially the mysis shrimp, upon which the trout in the Lower Blue depend.
Ewert said the last time they sampled for mysis shrimp in the reservoir, the numbers were down.
Summit Voice will have a more detailed story on the fish count later. The team was still working in the river through the factory outlet area and toward the bridge near the 7-11 as I posted this video.
For an in-depth background story on whirling disease and efforts to restore Colorado’s rainbow trout population, click here.
Filed under: Dillon Reservoir, Environment, rivers, Summit County Colorado Tagged: | Blue River, Colorado Division of Wildlife, fishing, Hofer trout, Rainbow Trout, Summit County Colorado, Summit County News, whirling disease