Colorado Parks and Wildlife strives to manage these reservoirs as trophy trout fisheries. Anglers can play a part in maintaining the quality fisheries by harvesting all northern pike caught.
Northern pike were first introduced into Eleven Mile Reservoir by the former Colorado Division of Wildlife (DOW) in the mid-1960s to try and combat the white sucker and common carp populations that were competing with trout for resources.
“In the 1960s when we would set nets at Eleven Mile, we would catch about 50 white suckers and common carp to each trout,” said Jeff Spohn, CPW Senior Aquatic Biologist for the South Platte Basin.
When the pike were first introduced, water levels at Eleven Mile Reservoir fluctuated significantly for water use in Denver. When the pike tried to spawn in the spring, the water was so low that they did not have adequate spawning habitat, so they could not reproduce. However, by the late 1970s water operations at Eleven Mile changed and water levels were kept high enough during the spring to create suitable spawning habitat for northern pike, allowing them to reproduce.
The northern pike reduced the white suckers and carp, but once the pike started naturally reproducing they began to pose a threat to the trout fishery.
“By the early 1980’s, northern pike accounted for nearly 60 percent of the fish we caught in our standardized surveys,” said Tyler Swarr, CPW Aquatic Biologist for the Upper South Platte Basin. “The low-point for the trout was in the mid-90s when they only accounted for eight percent of our total catch. Pike are good at what they do, but have a tendency to eat themselves out of house when left unmanaged.”
Spinney Mountain Reservoir was completed in November 1981, only a few short miles upstream of Eleven Mile. When the dam at Spinney Mountain Reservoir was nearly complete, the agency did a chemical reclamation treatment on 22 miles of the South Platte River upstream of the newly constructed reservoir to remove all northern pike that were upstream of Eleven Mile Reservoir.
“At the time the DOW thought they successfully reclaimed 22 miles of the river upstream of Eleven Mile before Spinney’s gates were closed,” Spohn said. “However by the early 1990’s it was clear that the reclamation project was either not successful or someone illegally moved pike into Spinney because we found northern pike in there.”
Roughly 10 years after the reclamation, Spinney Mountain Reservoir had a robust pike population like Eleven Mile that also became naturally reproducing. Eleven Mile, Spinney Mountain and Tarryall are the three reservoirs in South Park that have northern pike in their waters.
“Today we have problems with pike predation on our sportfish in South Park,” Swarr stated as a reason for concern with northern pike being in these waters. “To manage around the pike we have to stock larger trout, and it is more expensive to raise larger fish in our hatcheries.
“Northern pike can consume prey that is up to half of their body length. This makes managing waters with naturally reproducing northern pike populations challenging because pike can consume nearly any size of fish that is stocked at most life stages.”
To reduce losses of the stocked trout to pike predation, the stocking regimes at Eleven Mile, Spinney, and Tarryall were changed from “sub-catchable” (less than seven inches long) trout to “catchable” (ten inch long) trout in the early 2000s. In 2010, additional work determined that stocking the trout through the ice in winter would also increase their survival in the presence of northern pike.
“We are having to stock the trout in the winter through the ice when the pike’s metabolism is lower,” Swarr said. “Fish are cold-blooded animals, and when it is cold, their metabolism decreases and they don’t need to eat as much as they do when the water is warm. Trout are more cold tolerant than pike and can grow a few inches under the ice before the pike metabolism increases. We had to change up the management of trout and stocking regime to get through the situation with the northern pike. Seeing them get out of hand in the 80s and 90s is why we encourage the harvest of northern pike that are caught.”
“The bottom line is when we stock these larger trout it takes away from the limited resources that could be used elsewhere,” Swarr added.
This winter, the Mt. Shavano Fish Hatchery out of Salida will stock 115,000 cutbows (rainbow trout crossed with cutthroat trout) into Eleven Mile Reservoir and 45,000 at Spinney Mountain Reservoir.
The trout cost the hatchery about $1.15 per fish to raise them to 10-inches in length over 14-15 months (costing roughly $132,250). Economic studies show that each fish generates about $36 for the local economy through the purchase of gas, tackle, food, hotels, etc. by anglers (so pumping $5,834,687 into the economy from Eleven Mile alone).
“The economy gets a good return on our investment, but we get a far better return on investment when we are able to manage fisheries by stocking sub-catchable fish instead,” Swarr said.
Trout fishing is the bread and butter of anglers in most of Colorado, both in streams and reservoirs. Thus, CPW manages high mountain fisheries to accommodate the demand of anglers.
These three South Park reservoirs offer up prime trout fishing. Spinney Mountain is even classified as a Gold Medal water, which is given to the highest quality cold-water habitats that have the capability to produce many quality size (14 inches or longer) trout.
Spohn says anglers are the top solution in helping maintain a good equilibrium between the pike and trout in these waters.
“We have a decent balance right now and that is thanks to the anglers that are harvesting pike, but if it were ever to get lopsided again, CPW would have to resort to going in and mechanically removing some of the northern pike,” Spohn said.
Colorado Parks and Wildlife continues to ask anglers to harvest all pike caught at Eleven Mile, Spinney Mountain and Tarryall Reservoirs.
“We just need to balance them and hope the anglers can help us do that,” Swarr said.
Spinney Mountain State Park is closed for the season and will reopen after the ice is off in the spring. Anglers enjoy fishing year-round at Eleven Mile and Tarryall reservoirs – you can see hundreds of ice huts on the weekends at Eleven Mile State Park.
Colorado Parks and Wildlife is celebrating its 125th Anniversary throughout 2022 to honor the legacy of our agency and the talented staff who make fulfilling CPW’s important mission possible. For more stories like this, please visit the Colorado Parks and Wildlife’s 125th Anniversary web page!
Written by Jason Clay. Jason is a public information officer for the Colorado Parks and Wildlife northeast region.