MEDFORD, Ore. — Greg Huchko peers through the microscope at a slide that shows why Diamond Lake fly-fishers need to hang out at the lake’s south end this spring casting chironomid flies.
The slide reveals a handful of the 3,332 chironomid larvae plucked from a dinner-plate sized sample of muck from the lake bed just off the pizza parlor dock, and that number has everything to do with how well Diamond Lake’s trout will grow this year and what fly in the box likely will best catch them.
In 2015, a similar-sized wedge of the lake bed yielded 158 chironomids, meaning 2018 here could definitely be the year of the bug — perhaps even the healthiest ever recorded at one of Oregon’s most popular and productive lakes for big trout and lots of them.
“It’s kind of off the charts,” says Huchko, the Umpqua District biologist for the Oregon Department of Fish and Wildlife. “It’s excellent, a great problem to have.”
Biologists are midway through their annual benthic surveys — counts of bug larvae in lake muck — at Diamond Lake, and the bug levels might be the highest since scientists started counting Diamond’s insects more than 70 years ago.
Benthic levels were once as low as 17 pounds per acre during a massive invasion of tui chub in the early 2000s. In recent years, benthic levels have been coming in right around the 400-pounds per acre level, which is known scientifically as ginormous.
“I’d be real cautious to put numbers on it yet because it’s early,” Huchko says. “But if everything holds true, to what we’ve seen so far, it looks like we’ll have the same or more than the past few years.”
Benthic levels are more than just a sign of good lake health. They essentially define the lake’s carrying capacity for fish, thereby helping set stocking levels. They also determine how quickly stocked fingerling rainbow grow to catchable size, how big the big fish eventually get and what flies are the best to catch them.
So big benthic loads bode well for fish and those who stalk them.
“That’s very impressive, very high production,” says Joe Eilers, a Bend limnologist who studied Diamond Lake in the tui chub years. “I don’t think you can find a lake with better benthic production than that.”
At Diamond, the bugs in the muck run the gamut from aquatic worms, leeches, flatworms, scuds, mayflies, caddis flies, mosquitoes, water mites, snails and clams to dragonflies and beetles.
But about 80 percent of the benthic load are chironomids, also called midges or mouthless midges. Female chironomids lay eggs on the surface, and the eggs sink to the bottom, where the larvae hatch and burrow into the muck. They eventually leave the bottom and slowly reach the surface, where they emerge and fly away as adults.
Rainbow trout key on chironomids throughout the insects’ life cycle, but that transition from bottom to surface is when the feeding frenzy occurs.
At Diamond Lake, biologists each fall use a small dredge to collect samples from 23 subsurface sites, including that spot just off the pizza parlor, which happens to be a favorite for fly-fishers casting chironomids.
Originally a fishless lake that was first stocked with trout in 1910, Diamond Lake is rich in chironomids partly because it has natural sources of phosphorus and nitrates that help create algae on which chironomids feed, Eilers says. The soft lake bed allows for great burrowing and an escape from foraging trout and close to half the lake bottom supports aquatic vegetation that’s good for bugs, Eilers says.
That’s data ODFW biologists use to set annual stocking levels, which currently include 300,000 rainbow fingerling, as well as several thousand tiger trout added to prey on any tui chub and help ward off a chub infestation like the one in the early 2000s that led to the collapse of the lake’s trout fishery.
In 2006, the lake was chemically poisoned to kill off chubs, and since then benthic levels have risen annually.
As bug life increases, so does fishing success.
As recently as 2015, creel surveys showed anglers averaged just over one trout per trip here. Last year, it was 2.7 trout per trip, ODFW statistics show.
“There’s a direct relationship between benthic levels and the health and size of the fish,” Huchko says. “That’s why we still use the bugs to see if there are dramatic shifts one way or the other.”
Information from: Mail Tribune, http://www.mailtribune.com/