By Scott Bryan, CAP Senior Biologist
Aquatic weeds and water delivery are typically not a good combination. When weeds are growing in CAP’s 336-mile closed system, they tend to slow water flow by causing excess friction.
When the vegetation dies, it floats to the surface and eventually restricts water flow to the pumps by clogging the trash racks. If our rake systems can’t keep up, the pumps will shut down due to vibration, cavitation, or just a lack of water. In many water conveyance systems, drying out the canal or using a good dose of aquatic herbicide is used to control or eliminate weed growth. However, CAP delivers water to municipal and agricultural customers 365 days a year, so drying out the canal isn’t possible and chemical treatment can’t be used. So at CAP, we invite a special guest to the salad bar!
That guest is the grass carp, or white amur. Grass carp are a non-native fish species included in the Asian carp family. Although other members of the Asian carp family (including silver carp and bighead carp) have wreaked havoc in rivers of the Midwestern United States, fish managers have figured out how to take advantage of the grass carps’ voracious appetite. When stocked in the proper densities, they are very effective at controlling or eliminating aquatic vegetation from a body of water. However, they can very easily over-populate and create an imbalance in an aquatic ecosystem. Because of this, most states, including Arizona, require that only triploid grass carp are stocked. Triploid means that they have an extra set of chromosomes which prevents these fish from reproducing in the wild (sterile). The process of sterilizing the fish starts early in the life of the fish, when either heat or pressure is applied to the fertilized egg. Fish are intensively tested by the U.S. Fish and Wildlife Service prior to being sold for stocking to ensure that each individual is triploid.
At CAP, we have been stocking triploid grass carp in the canal since 1990 to control vegetation. Grass carp live about 10 years before dying off naturally, but are only effective at vegetation control until about age 7, so they must be re-stocked to maintain an effective population. We base our stocking rates on historical weed growth, water surface area, and estimates of mortality and effectiveness. Because of this “complex” equation, our stocking numbers will vary from year to year, but is usually between 2,500 and 3,000 fish per year. In March, fish are trucked in from Arkansas, which is about a 14 hour trip, and distributed throughout the canal based on areas of need.
All fish stocked have a head width equal to or greater than the space between the grates on our trash racks, which is about 2.5 inches. That means that the grass carp stocked into the canal average about 18 inches in length and weigh about two pounds. They grow fast and they grow large, consuming almost 20 percent of their body weight each day. Fishing isn’t allowed in the canal (it is fenced to provide security for people and wildlife) and our system is closed so they cannot escape. Which means most of us that have peered into the water from the deck of a pumping plant have seen large grass carp that range from 30 to 60 pounds!
And they are definitely doing their job. In recent years, we have perfected our stocking quantities, virtually eliminating weeds throughout our closed system.