Bullhead City Pest Abatement Manager Joe Iburg and at right, Lonell Crowther, owner of Crowther’s Freshwater Trout Company, a state-approved trout provider, stocked a total of 30,000 trout into the Colorado River in April and May ahead of the caddisfly spring hatch.

BULLHEAD CITY — Trout are taking a bite out of the population, but the caddisflies are still terrible, according to Bullhead City Pest Abatement Manager Joe Iburg.

“The spring numbers seem lower than last year, but it is not as drastic a change as I would like to see with the number of fish we put in the river,” Iburg said. “If we see a decrease in the fall population, we’ll know that the fish are actually playing a role.”

Iburg is working with scientists at the Grand Canyon Monitoring Research Center to help determine, based on water temperature and the nutrition requirements of the trout, how many times the trout are filling up on caddisflies.

“We’ll use my numbers of how many larvae are out there and within the next couple of months we should have a definitive answer if fish could significantly reduce the population enough to provide relief people are seeking,” Iberg said

Trout stocking is one of the mechanisms Iburg is exploring to combat a multi-year infestation of the caddisfly, an insect closely related to moths and butterflies that serve as a food source for many species of fish; the larvae live for six to eight months under the river surface and then emerge in major swarms roughly twice a year, with minor hatchings taking place throughout the spring, summer and fall.

Through a partnership with Millersville University, students are sorting through all the fish gut and larvae samples Iburg collected over the winter to determine what size insect the fish prefer to feed on, how many the fish eat and how the caddisfly develops in the river over time.

“There is no doubt trout are reducing the numbers of caddisflies, but it isn’t obvious to homeowners,” Iburg said. “It’s hard, because when you have 100,000 caddisflies, if you reduce the number to 50,000, people don’t really notice that.”

Iburg’s winter work included adjusting the black fly program to allow more competition to the area, test light traps, sample fish and work up and down the river to find where caddisfly larvae are most heavily concentrated — at the bend in the river.

“We wanted to not only put in these fish during the spring hatch, but target where we put them to get the most effect,” Iburg said. “Rusty Braun at the Riviera Marina has been a big help — we’ve gotten over 30 trout samples this winter and spring and that’s important because we’ve seen that the trout are feeding on all life stages of the caddisflies — they are packed with them — they eat the larvae, when they’re in diapause (dormancy), as pupae before they emerge as adults and the trout eat the adults as they dive in and out of the river.”

Last fall, Arizona Game and Fish Department approved a request from the Pest Abatement District for a spring stocking of an additional 30,000 catchable rainbow trout into the Colorado River, in addition to the already approved 35,000 trout stocked by AZGFD and 45,000 trout being stocked by U.S. Fish and Wildlife Service. 

“We’re probably still going to put in more trout in July,” Iburg said. “We’re going to shoot for sub-catchable fish after the Striper Derby and hopefully get some of the stripers — which eat trout — out of the system.”

In its battle against the insect, PAD is focusing on biological and mechanical controls; insecticides don’t work and pose environmental hazards to the fish in the river.

Iburg’s summer priorities include pheromone work to increase the effectiveness of light traps and putting oviposition substrates — basically tiles — into the river to determine how deep in the river the caddisflies lay their eggs.

“We are still hopeful that there might be some methods of control using river flow patterns,” Iburg said.

Iburg so far has unsuccessfully reached out to local groups to find help building bat houses, but encourages homeowners to install their own. He said he also plans to work with local birding groups on ways to increase insectivorous bird populations and is partnering with AZGFD to continue looking for other fish that also might be effective at eating caddisflies.

“We’re dealing with a couple things at once,” Iburg said. “If this were solely a research thing, I would be doing one thing at a time, looking at all my variables. If we have success, it could be because of the fish, it could be because of some of the light traps, it could be because the black fly program and in reality it would be because of all of those things. It’s probably going to take all of those things to make a noticeable difference.”

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