Brigham City battles algae on Mantua Reservoir, hopes to avoid another trout kill

An experimental approach to managing harmful algal blooms in Mantua Reservoir is underway Tuesday, after October’s application of the test product improved the water quality but resulted in 300 dead trout.

Erin Mathis has been fishing and organizing tournaments on Mantua Reservoir for around seven years. She said Mantua is "the top largemouth bass fishery in northern Utah."

"We have seen really, really nasty algae in the years prior, that blue-green stuff," Mathis said. "It can kill dogs; it can make people and kids sick."

The blue-green sludge found on many water bodies, while called algae, is actually an aquatic bacteria that photosynthesizes like a plant, according to the Utah Department of Environmental Quality.

The cyanobacteria can produce dangerous levels of toxins that damage human organs and neurologic systems and can be harmful to pets. Jennifer Mickelsen, a biologist working for Brigham City, said the bacteria "love to grow when there is nutrient pollution," like nitrogen and phosphorus.

While fertilizer runoff from farmland often feeds algae blooms, "there is agriculture all around the reservoir, but they're already limited on what they can use because we have drinking water wells all around," Mickelsen said. "So we don't think that that's a big part of it."

Brigham City, which manages the reservoir, has increased testing in the last five years and found that the natural springs coming out of the mountains above the reservoir have high enough nutrient pollution to support the blooms.

"We were starting to have the algae bloom on top of all the plant life that's already underwater in the reservoir," said Chris Penne, northern region aquatics program manager for the Utah Division of Wildlife Resources. If those blooms "get big enough and last long enough," Penne says the reservoir flips, becoming dominated by algae, which literally overshadows the plant life below.

In that state, the DWR wouldn't be able to maintain as many fish in the reservoir, and the quality of fishing would decrease, Penne said.

So beginning last October, Brigham City used a new product to treat the high levels of harmful bacteria colonies. The tiny granules , when applied to cyanobacteria, release hydrogen peroxide, which causes the mass of bacteria to collapse on itself and die.

"There were definitely measurable differences. (The reservoir) looked way better," said Mickelsen after the treatment.

But even though cell counts dropped, DWR added 10,000 rainbow trout to Mantua within days of the treatment, and 300 of those fish, or 3%, ended up dying, with lack of oxygen as the likely culprit.

"What caught us off guard," Penne said, "was the treatment had basically killed enough of the algae that, as it was decomposing, it lowered oxygen levels a lot lower than we thought."

Even though the reservoir temperature was within range, the algae was consuming a lot of oxygen while it decomposed, which is especially difficult for sensitive trout species. The kill was "highly visible with all the clear water," Penne said, which made many anglers and recreators believe the new chemical treatment had poisoned the fish.

"We learned something in that event," according to Penne, increasing communication with Brigham City going forward.

"They started doing these treatments," said Mathis, "and yeah, we did see a lot of dead trout."

The fish that were native to the fishery upstream were not affected.

In the spring, DWR conducted a survey of the fish population. Penne said, "The fish population looked really healthy, with good number of bluegill, largemouth bass, yellow perch and rainbow trout."

So Tuesday, Brigham City used the same product, but "on a lot smaller basis," Mickelsen said. The city hopes to see what kind of measurable impacts the treatment has on the algae and aquatic life, without the added variable of a fish stocking event.

Penne believes the city’s approach is “more proactive perhaps, and more experimental than other water managers. And that’s a great thing.”