Now, Utah wildlife officials are taking back the state's namesake lake, paying commercial fishing businesses about 20 cents a pound to remove 32 million pounds by 2018.
"The idea is to get them to a manageable level. They were 91 percent of the lake's biomass. When you have something so heavily skewed like that, it's an unhealthy ecosystem," said David Tinsley, a Division of Wildlife Resources biologist.
To gauge the impact of this million-dollar-a-year removal campaign, Tinsley supervises annual fish surveys of the lake. Boats working in pairs pull the ends of 200-yard purse seines together and researchers count whatever turns up.
The carp, some as long as 2½ feet, go in one boat and the rest, mostly imported sport fish such as walleye, bluegill and channel catfish, go in the other, according to Jamie Reynolds, a Utah State University graduate student in the Department of Watershed Sciences.
Unlike commercial rigs that ply waters where carp congregate, researchers spread their efforts to nine areas on the lake, dropping seines in randomly selected spots. The study is in its fourth year and has expanded its scope from 27 to 45 net deployments, according to Tinsley. Deployments typically yield 200 to 300 fish.
"That's nothing compared with what the commercial fishermen take. They will catch tons of fish," Reynolds said. "We aren't biasing anything."
After the fish are sorted and counted by species, the weight and length of a subset are recorded before they are all tossed back — even the destructive carp.
"A big criticism is that you can never remove enough fish to make a difference," Tinsley said.
The study could help prove naysayers wrong and fine-tune the carp removal program. Once carp are knocked down to a certain threshold, competition from other species will keep their numbers in check, according to Eric Ellis, executive director for the Utah Lake Commission. The target is a 75 percent reduction in the 7 million adults that swarmed the lake when the removal project began in 2010.
More than 18 million pounds of carp have so far been sent to compost yards and mink farms. Ecological improvements already are being documented.
"Clarity has improved a couple inches," Ellis said. "As carp go down, some of the plant life can come back and that holds the sediments at the bottom."
A key goal of carp removal is to make the lake safe for young June sucker, an endangered native that is the subject of intensive recovery efforts. It is one of just two native species to still inhabit the 90,000-acre lake, which also suffers from high nutrient loads that are fueling algal blooms.
Bonneville cutthroat remain in the watershed, but are confined to tributaries.
"None of the larval fish transition to adult fish because they are eaten by predatory fish. What is lacking is refuge habitat. That is where carp go to feed. They rip out the vegetation," said Mike Mills, the Central Utah Water Conservancy District's June sucker recovery coordinator.
Without vegetation there is nowhere for tiny fish to hide and nothing to anchor sediments, which are easily disturbed by wind thanks to the lake's shallow depth.