Research under way to improve Dillon Reservoir fishery
May 27, 2012
Don’t forget the shrimp cocktail sauce if you’re headed out to Dillon Reservoir.
There are millions of micro shrimp (a big one is the size of a pinkie fingernail) scurrying around the lake’s dark depths, put there in the 1970s to encourage brown trout growth. The tiny shrimp eyes are visible at night in a beam of light shone down from the docks. Fishery managers wanted to grow brown trout, and thought they’d mimic what was happening on a lake in British Columbia, where freshwater shrimp were the food source for massive brown trout.
The problem was, the currents differ in each lake. In British Columbia, the bottom-feeding shrimp were swept up into the trout’s daytime feeding path. In Dillon Reservoir, brown trout feed during the day while the shrimp hide, then they switch places at night. The currents don’t cause the two to interact. Instead, the shrimp are suspected to be competing with the brown trout for food – and winning.
“The feeling is the shrimp are really controlling the food web because they are consuming all the zooplankton, which is what trout eat,” Colorado State University professor Brett Johnson said.
He’s overseeing a project meant to improve Dillon Reservoir as a fishery, which could result in not only larger brown trout, but also a trophy fishery of Arctic char. Dillon Reservoir is already one of just two Arctic char fisheries in the lower 48 states, making it ripe to attract trophy fishermen seeking a four-pound, 20-inch, unique catch.
“There’s a huge benefit to Dillon and Summit County if we can answer these questions” of the food chain, researcher Douglas Silver said, to which Johnson added, “Dillon has the potential to be one of the state’s showcase fisheries because of its location, size and scenic beauty. The fishing has been lackluster for a long time … Our project finds what can be done within the constraints that exist,” such as keeping the reservoir clean. Fish thrive in dirtier water, Johnson said.
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The reservoir is currently considered sterile by most anglers, Silver said, even though the water houses kokanee salmon, rainbow trout and the white sucker in addition to the brown trout. Arctic char were also introduced to the food chain after biologists realized the shrimp were taking over, making it the only Arctic char fishery west of New York state.
Problems abound, though, even beyond the brown trout not growing. Arctic char catches are few and far between. Kokanee salmon compete with brown trout for spawning grounds. And so on.
As a result, an estimated 11 percent of the shoreline is being fished, and angler guides shy away from taking clients on the lake to fish. But Silver argues it’s not a sterile lake: Last summer, he caught 300 fish (he has an exemption permit for how many he can keep).
Several years ago, Silver approached Colorado Parks and Wildlife – then Colorado Division of Wildlife – to volunteer to perform and fund research on Dillon Reservoir. At the time, they didn’t have an aquatic research biologist, so Johnson, who specializes in coldwater reservoirs, became the main contact. Three years ago, Silver took his first shrimp sampling and two years ago, he and Johnson started a graduate study program, funded by Silver.
The goal is to understand the food web – the various producers and consumers and what’s eating what – and how it relates to growth rates and the temperature of the ecosystem.
“It’s a lot of fun,” Silver said of his research, which is gaining momentum. “It’s like going on a treasure hunt, only no one is telling you want to find, but when you find it, it’s really cool.”
Silver always wanted to study fish, but quickly realized it wasn’t very lucrative. Now that he’s accomplished in the mining industry and business world, he’s ready to give back. Helping the poor was already taken, he joked.
This summer, researchers are seeking permission from the Dillon Reservoir Recreation Area Committee to put enclosures in the lake to identify what’s living in the mud – and whether the white suckers are competing with trout for the same food.
There will be researcher-angler interaction, too, Silver said. A graduate student is tasked with doing a creel survey from May to August. The surveyor counts and interviews anglers and boaters to estimate catch, harvest and expenditures. It’s meant to estimate the current status of the fishery and its economic impact on Summit County.
Another graduate student will be focusing on studying the Arctic char population. If char eat the shrimp, more plankton is available to feed trout and kokanee salmon.
A benefit/cost analysis is also planned to evaluate various fishery management alternatives, such as enhancing the char, preventing brown trout from eating them as fingerlings, and boosting the growth of brown trout.
Researchers will continue to take fish samples, including pulling the ear bone to identify the fish’s age and taking tissue samples to determine what it’s eating. Silver wants any anglers who have a big catch to contact local officials or authorities to turn in the head and guts for research.
Shrimp will continue to be sampled at specific testing locations to determine their life cycle and food source. Researchers also test the reservoir’s temperature and dissolved oxygen content as well as collect zooplankton to get a full picture of what’s going on in the water.
Some research began last week and will continue off and on through August.
All the information gets put into a three-dimensional modeling system that “helps bring all the facts together in a way we can look at them,” Silver said.