Mike Maceina, Jeff Slipke, and John Grizzle

Increasingly, aquatic plant management activities are conflicting with the goals of anglers, duck hunters, and other parties interested in maintaining diverse fish and wildlife populations. AAES researchers have found that the use of sterile grass carp confined by electric barriers may help manage this issue.

The invasion and spread of introduced exotic plants, particularly hydrilla and Eurasian milfoil, has revitalized largemouth bass fisheries and duck hunting in reservoirs of the Tennessee River in Alabama and many other older reservoirs throughout the United States.

Submersed aquatic macrophytes, including hydrilla, are important to the ecosystem of waterways and lakes, but when they grow too prolifically they cause problems for boaters, swimmers, and other recreational and commercial users. Excessive aquatic plant growth can be controlled with mechanical harvesting or with herbicides, but these methods are expensive and labor intensive. In addition, the public generally expresses environmental concerns about the use of herbicides in public waterways. Today government agencies try to strike a delicate balance to maintain some plants in water bodies, yet provide for maximum benefits for all users.

Sterile (triploid) grass carp, which feed exclusively on aquatic vegetation, offer an inexpensive method to control plants. These fish are routinely stocked in small ponds throughout Alabama and the rest of the country. However, mass stocking of grass carp in larger multiple-use water bodies to control plants where fish and wildlife values are important is no longer an acceptable management option. These fish can completely eliminate aquatic plants for up to 20 years. In larger water bodies, it is nearly impossible to predict and obtain stocking rates of grass carp to reduce, but not completely eliminate submersed macrophytes. Climatic factors cause aquatic plants to increase and decrease naturally, which complicates intermediate control objectives with grass carp. Finally, once stocked into a large water body, mass removal of grass carp is not possible.

If grass carp could be confined or fenced in certain areas where fish and wildlife values were not a priority, this could provide lake managers with the option of “zoning” parts of lakes for certain uses. AAES researchers have been testing a way to use these fish in a specific area of a large reservoir to control their activity and movement. Results of that research indicate that an electric barrier fitted to an open fence is useful for keeping grass carp within a confined area.

Lake Seminole is a 32,500-acre reservoir located on the Alabama-Georgia-Florida border (see map) that is used regularly by anglers from Alabama and from throughout the Southeast. This multi-use Corp of Engineers reservoir provides excellent fishing for largemouth bass, bluegill, and crappie. Lake Seminole also is considered one of the best public waterfowl hunting locations in the region.

Hydrilla (Hydrilla verticillita) was introduced into the lake in the late 1960s and has increased dramatically during the past 10 years. Lake Seminole is a shallow water body (average depth 10 feet), annual water level fluctuations are typically less than two feet, and clear spring-fed water enters parts of this reservoir. These characteristics are conducive to aquatic macrophyte colonization. Submersed aquatic vegetation (primarily hydrilla) covered about 70% of the surface area of Lake Seminole in 1992, but declined to about 40% coverage by 1997 due to high rainfall events.

Typically, only about 1,500 acres per year have been treated with herbicides. Unconfined release of grass carp at stocking rates high enough to reduce hydrilla in Lake Seminole has been opposed by state conservation agencies and private interest groups.

If grass carp could be confined within certain areas (zones) where persistence of some aquatic vegetation is not a high priority, this approach could provide localized control of excessive aquatic macrophytes while maintaining plants in other regions of the reservoir. To test the feasibility of this concept, AAES scientists cooperatively worked with and received funding from the U. S. Army Corps of Engineers and from the Florida Department of Environmental Protection Bureau of Invasive Plant Management to test the success of confining grass carp with an electric barrier in a 900-acre embayment of Lake Seminole. This area contained about 720 acres of hydrilla that typically formed dense mats every summer.

A V-shaped funnel weir (a fence or enclosure set in a waterway to block fish) was constructed across a narrow opening at a bridge on an area of the lake known as Fish Pond Drain (see map). At normal reservoir elevations, the barrier was three feet above the water surface. The opening for the barrier was 12 feet and spanned about 320 feet across the mouth of Fish Pond Drain. The depth of the opening was six feet and the barrier at the opening came within three feet of the water surface

The V-shaped funnel weir was fitted with an electrical array constructed by Smith-Root Inc., that produced a low voltage (three to four volts) DC field to repel grass carp (see map). Power was supplied by a nearby power line adjacent to the bridge. The peak current output was 12 amps at a 10-millisecond pulse over a 500-millisecond duty cycle for an average current of 1.44 amps. Two motion detectors, placed at the entrance and exit of the electric array, turned off the system as boats passed through the barrier and the power stayed off for 30 seconds thereafter.

Boat passing through opening of funnel barrier with the electric array fitted to the opening.

The status of the system was continuously monitored by a computer modem that relayed the information to three different locations.

Radio tags with an expected life of three years were used to track grass carp. The tags also were fitted with a 24-hour delayed mortality circuit that indicated to researchers if a fish was alive, had died, or had expelled the transmitter. Each individual fish transmitted a unique radio frequency signal.

Surgical implantation of radio tags into grass carp.

In November 1997, radio transmitters were implanted in 84 triploid grass carp that weighed about four pounds (21-23 inches long). Fish were transported to Lake Seminole in December 1997 for release into Fish Pond Drain 2.5 miles upstream from the electric barrier.

Grass carp were tracked every other week from December 1997 until December 1998. Unless grass carp were within about 0.5 mile of the barrier, only approximate locations within the confined study areas were recorded. When grass carp were located close to the barrier, their approximate locations were marked on a map.

During the 13 months of tracking, all but one fish were accounted for, no verified escapes occurred, and 13 fish either died or expelled the radio tags.

Assuming this fish did escape, the average annual maximum potential escape rate based on remaining live fish was 1.3% and does not represent a threat of the fish infesting other areas.

Until mid-April 1998, little grass carp movement was detected. At this time, 96% of the grass carp were found in Ray’s Lake and most were in the upper region of this embayment (see map). After mid-April grass carp migrated and 55-68% of all fish emitting a live signal were found in Lewis Pond from mid-May to September 1998. During May and June 1998, 24 to 27 grass carp were within about 0.5 mile of the barrier. After this time, some of the grass carp found near the electric barrier moved upstream from the barrier. Only eight fish were within 0.5 mile of the electric barrier by December 1998. At the end of September 1998, 32% of the active grass carp were in Ray’s Lake, but by December 1998, 51% of these fish were found in the Ray’s Lake embayment because the grass carp tended to move upstream into Fish Pond Drain embayment between these time periods.

The electric array fitted at the opening of the funnel-weir barrier was effective at retaining grass carp in a 900-acre confined area for one year. In March 1998, 12 inches of rain fell in the Fish Pond Drain basin and water levels rose nearly three feet at the barrier, but the electrical array continued to work. During the one-year evaluation period, the electric barrier was nonoperational for only four days in June 1998 when air temperature within the control box exceeded 115^oF. A dehumidifier and shelter to provide shade were installed and the barrier was functional thereafter.

Results of this study showed that an electric barrier system can contain grass carp in specific locations in a water body, minimizing the potential for their escape, and thus adequately control vegetation. This barrier and confinement of enough grass carp allows managers the capability to “zone” water bodies, and permit complete elimination of submersed plants in certain areas while maintaining plants in other areas.

This method also permits aquatic plant managers to use a relatively inexpensive tool to control excessive aquatic vegetation, although initial costs can be high. The funnel-type barrier costs about $40,000 to build and the complete electric array system was about $32,000. Estimated average annual costs to control submersed macrophytes in Lake Seminole were $300 and $2,000 per acre per year for herbicides and mechanical harvesting, respectively. The estimated annual plant control* with grass carp using this barrier system would cost about $40 per acre per year in the confined area of Fish Pond Drain over 10 years.

* This estimate included the installation of the physical and electric barrier, the initial stocking of about 16 triploid grass carp per vegetated acre, and supplemental stocking and maintenance costs.