Arizona's waters kept clear by plant-eating specialist

When it comes to keeping some of Arizona’s most important water clean, local experts have turned to a non-native fish with a taste for greens.

Grass carp are a type of large fish native to East Asia known for their appetite for aquatic vegetation. They have been used across the state and country to limit plant interference to any closed body of water’s intended purpose.

Now the city of Litchfield Park is joining the likes of Salt River Project and the Central Arizona Project in deploying the first as a solution to its ongoing problem of aquatic weeds and algae growing in Tierra Verde Lake Park.

“Aquatic vegetation has been an issue for people operating the canals since the canals were built in the early days,” said Brian Moorhead, environmental scientist and engineer at SRP.

There are three main options for how to control aquatic vegetation: mechanical, herbicides or biological control.

“Mechanical can get pretty expensive, with long-reach excavators and cabling, it’s time consuming and labor intensive and only partially effective,” said Scott Bryan, water quality and biology administrator at the Central Arizona Project. “Chemical control can be really effective but then we’d be using chemicals, and obviously as a water conveyance system we’re transporting water to people who are showering and drinking in the water. So, we turn to grass carp as a biological control and they’re really effective grazer on different aquatic vegetation.”

When grass carp were first introduced in North America, the intention was to control invasive aquatic plants, just as the city of Litchfield Park plans.

But as effective in reducing these plants as they are, their feeding can also lead to unintended consequences, such as the depletion of native aquatic vegetation, which many species rely on for habitat and food. As a result, integrating them, or any other nonnative species has government regulation.

In a statement from Arizona Game and Fish, the organization said  managing nonnative species is everyone’s responsibility. Without control, aquatic invasive species can have devastating effects on native aquatic wildlife through disease introduction, predation and competition.

Arizona requires that for the fish to be stocked the water body has to be a closed system so the fish cannot escape to natural waters or streams, and the fish had to be sterile or triploid.

“Triploid fish, in the egg stage, are exposed to heat or pressure stress and it creates a third chromosome in the fish that stops the fish from reproducing,” Bryan said.

Keeping the fish in canals and away from naturally occurring aquatic habitats is made possible through grate systems in the canals.

In Litchfield Park, the Tierra Verde Lake is a standalone, man-made feature that holds about 4 acres of water with a depth of 12 feet at the center. The community lake is generally stocked every two years with rainbow trout, tilapia, catfish and small mouth bass.

“(The lake) is a staple for the city,” said Litchfield Park Director of Public Works Kyle Ames, “it’s a destination enjoyed by a lot of residents, who walk the lake lines and enjoy community fishing. A few weddings have been hosted there, and we get lots of photo shoots for graduations and businesses on the lake dock.”

Aimes says in the past Litchfield Park used chemical solutions to limit invasive plant growth on the water to keep fishing accessible to residents and to help manage the habitat.

Arizona Lake and Pond Management, an aquatic company contracted by the city, uses a blue pond dye and two rounds of chemical solutions in the summer to manage the growth of aquatic plants stimulated by still water and rising temperatures.

“We’ve always treated it with chemicals, which is inevitable but there are solutions to avoid dumping chemicals into our beautiful lake and doing it a more natural way,” Aimes said.

In seeking that natural alternative, Arizona Lake and Pond Management recommended stocking Tierra Verde Lake with 750 to 1,000 pounds of grass carp to control the aquatic weeds.

“(Grass carp) eat all of the weeds and those other fish will need places to hide,” Moorhead said.

In Litchfield Park, the supplementary fish habitats left behind after the grass carp eat the rooted aquatic weeds are Christmas trees collected by the Litchfield Park Christmas tree disposal program. Sunk to the bottom of the lake, the trees provide hiding spots for all of the young and small fish.

The grass carp are expected to be stocked during winter months, Aimes said.

“We need the water to come down in temperature so the fish can acclimate to the heat,” he added. “Even humans would hate jumping into 115 degrees straight from 65 degrees.”