Effectiveness of Three Barrier Types for Confining Grass Carp in Embayments of Lake Seminole, Georgia

Abstract

Three types of barriers were evaluated in Lake Seminole (13,158 ha) to determine the success of confining triploid grass carp Ctenopharyngodon idella in two embayments (250 and 350 ha) that were almost entirely covered with submersed macrophytes. In 1995, two different physical barriers that permitted boat passage were constructed. One had tandem V-shaped weirs placed at the entrance of a cove, and the other had two gated barriers that confined an embayment connecting two arms of the reservoir. Grass carp were radio-tagged, stocked into the confined areas (N = 119 for the V-shaped barrier and N = 69 for the gated barrier), and tracked from December 1995 through Sep 1997 to estimate escape rates. In addition, 18,000 triploid grass carp fitted with coded wire tags were stocked in December 1995 into the two confined areas. A low-voltage (3–4 V) electric barrier (Smith-Root, Inc.) was installed in December 1997 at one of the V-shaped funnel barriers, and an additional 84 grass carp were radio-tagged and tracked for 13 months. Based on verified locations outside the confined areas, an average of 9% of the grass carp escaped through the V-shaped, and 23% escaped through the gated barriers each year. However, based on missing fish, tag functioning rates determined from dead fish or expelled tags, and locations of fish before becoming missing, potentially up to 42% of the grass carp escaped from the V-shaped barriers and 35% escaped from the gated barriers each year. In addition, electrofishing surveys conducted in summer 1998 downstream of the tailrace in the Apalachicola River, Florida, indicated that 68% of the grass carp were escaped fish (coded wire tag present) that were stocked nearly 3 years earlier into the confined areas. After the V-shaped barrier was fitted with an electric barrier, no verified escapes occurred and with the exception of one fish, every radio-tagged grass carp was found within the confined area. Therefore, the maximum escape rate was only 1.3% per year, if this fish did indeed escape. Thus, the electric barrier and confinement structure have the potential to provide managers with a tool to confine grass carp in specific areas of large water bodies. Over many years, control of excessive aquatic macrophytes with this system is about 10% of the cost of herbicide treatments or mechanical harvesting.