PLACERVILLE - The lunker of the morning came skittering to the surface near the center of Rapid Creek, prompting a cheer from Game, Fish & Parks Department seasonal fisheries aide Jim Rasmussen.

"Yeah, there's the one we've been looking for," Rasmussen hooted Thursday morning, as he helped net the wild brown trout and drop it into a plastic bucket.

The fish later measured almost 17 inches, a size once common in the previously thriving stretch of Rapid Creek from Pactola Reservoir down to Rapid City. But that was before an explosion of a troublesome alga called Didymosphenia geminata, the suspected culprit in the sharp decline in adult brown trout numbers in that stretch of creek during the past five years.

But this year, it seems to be Didymo - as it is often called without affection by biologists and anxiously-informed anglers - that seems to be declining in the creek. And GF&P biologists are seeing signs that some of the insect life choked out by the algae - and needed by baby trout to grow into adults - is beginning to come back.

"Overall, the creek looks great this year, compared to the last few years," GF&P fisheries biologist John Carreiro said. "Visually, we know that it's been a lot worse in past years. Based on the condition of the creek, there should be better invertebrate production."

Invertebrates include the aquatic insects that constitute such an essential food chain for trout. They apparently suffered with the onslaught of Didymo, as did the fish that eat them. Adult brown trout continued to produce their young, but the baby browns struggled to grow beyond 3 or 4 inches.

"We've lost some of those bigger adults, but the critical thing is we haven't been getting those trout to grow to the 6-to-9-to-10-inch range," GF&P fisheries biologist Greg Simpson said. "The fact that we haven't been getting those fish is the big thing."

That missing size class has been documented in regular stream sampling done by GF&P fish crews. They were doing just such a survey Thursday on a 100-meter-long stretch of Rapid Creek at the Placerville church camp, a picturesque facility along Rapid Creek between Pactola and Johnson Siding.

First, fish crew members blocked escape routes with nets strung across the creek at each end of the sampling section. Then, they used portable electro-shocking units to stun fish momentarily so they could be netted and placed in flow-through holding traps placed in the stream.

The trout were measured and weighed and recorded by species. Of the 373 taken Thursday, all but a few were wild browns. And almost all of those trout were 4 inches long or less.

Those are "young-of-the-year" trout that were spawned last fall, which continue to be plentiful in the creek each year. There were also a couple of larger browns, including the 17-incher. But Simpson also was encouraged to see a scattering of the missing size class of trout, generally between 5 and 9 inches.

That's crucial. Because when the 7-inch fish aren't there, it means the 12-, 15- and 18-inch fish won't be there in future years.

"We're picking up a few of those fish that are a little bigger," Simpson said. "It's starting to look like there might be some improvement in numbers there. It's hard to say at this point that it's for sure."

GF&P began a project immediately below Pactola this year to add a limited stream of nutrients to the water, hoping to fight the algae and spur production of invertebrates. There are indications that the project, which will continue next year, is having an effect.

"The fertilizer did increase the primary productivity in areas below where it was released," Carreiro said. "But we might not see the effects (on fish) until a year or two down the road."

It's possible that the Didymo surge has run its course and is now declining on its own, he said.

"We'd be ecstatic if it were part of a downward trend," Carreiro said. "Whether it's the natural cycle in combination with the fertilizer, we don't know, but it's all beneficial."

Contact Kevin Woster at 394-8413 or kevin.woster@rapidcityjournal.com