Triploid Grass Carp
Throughout the Hydrilla Issues Workshop the use of grass carp as a tool for controlling hydrilla was mentioned many times. All participants were aware that grass carp are used extensively in Florida, but some felt it was necessary to reevaluate their use, especially in some of Florida’s larger public lakes (e.g., KCOL) containing abundant populations of Fluridone resistant hydrilla. This concern was largely driven by the reality that Fluridone resistant hydrilla is consuming a large portion of the management budget. As these fluridone resistant plants spread they will consume larger portions of the budget and therefore the need to consider the cost-effectiveness of grass carp balanced with the potential environmental impact.
Diploid grass carp arrived in Florida and other parts of the country in the early 1970s. Their arrival caused an explosion of research on their use as an aquatic plant management tool. Today only sterile triploid grass carp can be used for aquatic plant control. An excellent summary of the majority of grass carp research for aquatic plant management can be found in the Proceedings of the Grass Carp Symposium held in Gainesville, Florida (U. S. Army Corps of Engineers 1994). There were two major consensuses that came from the grass carp symposium that were also parallel from participants in the Hydrilla Issues Workshop: 1) when stocked in sufficient densities at a large enough size to avoid predation (approximately > 12 in) grass carp are very efficient at controlling all submersed aquatic vegetation, and 2) when grass carp are stocked into a lake at densities high enough to control all submersed aquatic vegetation they are extremely hard to remove from a system. Thus, if the elimination of submersed aquatic plants is an acceptable management objective for a lake system, then grass carp are an efficient cost effective means of aquatic plant control.
Less clear from the Proceedings of the Grass Carp Symposium (U. S. Army Corps of Engineers 1994) and the Hydrilla Issues Workshop is the potential for grass carp to be used after herbicides have decreased aquatic plant biomass to selectively control regrowth of hydrilla and maintain moderate levels of native aquatic vegetation. Florida lakes Deer Point, Miccosukee, and Conway have all been used as success stories at maintaining some submersed vegetation with low levels of carp controlling hydrilla. However, with the exception of Lake Conway (Leslie et al 1994) there has not been consistent evaluations and publication of data confirming the successes of these examples. With a quick literature review we found some more current evidence suggesting that stocking low levels of grass carps can achieve control of palatable problem species while maintaining moderate levels of native aquatic vegetation in ponds (Pipalova 2002) and small impoundments (Blackwell and Murphy 1996).
Contrary to the above findings, Hanlon et al. (2000) examined long-term macrophyte control in 38 Florida lakes using triploid grass carp and found an apparent break point suggesting an all or nothing control response. These efforts suggest that stocking grass carp between 25 to 30 grass carp per hectare of vegetation can result in a lake having some plants left (Hanlon et al. 2000). However, the remaining vegetation will be unpalatable submersed, floating leaf, and emergent vegetation. Hanlon et al. (2000) also noted that while stocking rates based on the amount of submersed vegetation provides a “narrow window of opportunity” to control nuisance vegetation like hydrilla while maintaining some submersed plants it is extremely difficult to achieve a stocking rate of 25 to 30 grass carp per hectare of vegetation. These rates are difficult to maintain in individual lakes because of varying grass carp mortality rates, fish mobility, and changing limnological conditions. When grass carp were stocked at densities greater than 25 to 30 fish per vegetated hectare the complete control of aquatic vegetation was achieved and lakes stocked with less than 25 to 30 fish per vegetated hectare tended to have the same or higher amounts of aquatic vegetation as when the lake was originally stocked. Similar results were found in Washington State when Bonar et al. (2002) evaluated grass carp stocking in 98 lakes and ponds. Nineteen months after stocking in the Washington lakes and ponds, submersed macrophytes were either completely eradicated or not controlled at all with a small percentage (18%) maintaining an intermediate level of submersed aquatic vegetation. Bonar et al. (2002) recommended against stocking grass carp in lakes where eradication of submersed vegetation cannot be tolerated.
Grass carp have been used to control hydrilla in several large systems including Lake Conroe in Texas (Klussmann et al. 1988), Santee Cooper Reservoir in South Carolina (Kirk et al. 2000), and Lake Guntersville in Alabama (Webb et al. 1994). In lakes Conroe and Santee Cooper almost complete control of all submersed aquatic plants was achieved while in Lake Guntersville hydrilla was reduced for a one to two year period while maintaining moderate levels of other submersed plants. In Florida, Lake Istokpoga (surface area approximately 12,100 ha) was stocked with 125,000 grass carp with little or no impact on the abundance of aquatic macrophytes. The hypothesis is that the grass carp left Lake Istokpoga through outlet canals before they could impact the hydrilla in the system, however insufficient data were collected to support this hypothesis. Lake Yale is another large Florida lake (surface area approximately 1,635 ha) where grass carp were used in an attempt to control hydrilla. Initially, the carp were stocked at low levels (approximately 7/ha or 3/acre) but additional stocking brought that density to approximately 17/ha (7/acre) where the consumption rate of the carp exceeded the growth rate of the submersed vegetation. The final result was the complete elimination of submersed and emergent vegetation (primarily grasses) for several years. It took 4 to 5 years of intense electro-fishing and other carp removal strategies to bring carp densities to a level where revegetation efforts once again established littoral vegetation. The cost of these efforts was high and would suggest that current technologies for grass carp removal are not cost-effective or practical. Thus, the large lake case histories show a similar trend to the pond and lake studies listed above. Some large lakes experienced total elimination of submersed aquatic vegetation, some had no reduction in aquatic plants and there was limited evidence that some level of temporary hydrilla control can be achieved while maintaining other species of submersed aquatic vegetation.
With little hard evidence that submersed aquatic plant control can be achieved with low density stocking of grass carp while maintaining some submersed aquatic vegetation, a common warning in the grass carp literature is the statement that “unless complete elimination of submersed aquatic vegetation can be tolerated, grass carp stocking is not recommended.” Thus, the key to universal use of grass carp for plant management is to have the ability to develop a cost-effective strategy to remove the fish from a system if the amount of plant control exceeds target amounts. Historically, managers have experimented with several methods for removing grass carp from lake systems including: herding, angling, attracting, use of lift nets, and toxic fish baits (Schramm and Jirka 1982; Bonar et al. 1993; Mallison et al. 1994). Unfortunately, all techniques used in the removal studies were time consuming, labor intensive, sometimes quite expensive and in each case failed to remove a major portion of the carp population. This is especially important in light of evidence suggesting that it may take only 0.5 carp per acre to maintain complete control of submersed vegetation regrowth after complete control of submersed vegetation is achieved (Moxley et al. 1993).
Recently, grass carp have been trained experimentally to come to sound (Willis et al. 2002) and then after training placed in ponds to evaluate recapture after attracting with sound (Duncan 2002). Another new method for the removal of grass carp from lake systems is the use of biodegradable capsules containing fish toxicant that could be implanted in the carp and used to euthanize the fish after a given period of time (Thomas 2004). These and other new methods for manipulating carp densities could make the grass carp a more useful and acceptable aquatic plant management tool.
Much more could be written on the volumes of grass carp literature that exists but the brief summary above covers the major issues brought up at the Hydrilla Issues Workshop.
Recommendations
Recommendation 2: Throughout the literature review, Grass Carp Symposium and the Hydrilla Issues Workshop it is clear that if there was some cost-effective and selective method of removing grass carp from a lake system before complete eradication of submersed aquatic vegetation was accomplished then triploid grass carp would be an excellent method of hydrilla control for large and small lakes. Therefore, we recommend making funds available for more research on new techniques for removing grass carp from lakes. Research on this and other methods may be expensive but a successful method would pay great dividends to aquatic plant management in Florida lakes.
Comments on the first draft of this report echoed warnings from previous studies suggesting that if total elimination of aquatic vegetation is unacceptable then the use of grass carp to control vegetation in large or small lakes should not be considered. However, if research provides an efficient method to remove grass carp from a lake then it is recommended that this method be evaluated in a Florida lake requiring aquatic plant control.
Justification: With the onset of resistant hydrilla there are limited tools with which to manage large infestations of hydrilla that are cost effective and selective. Thus, increased use of grass carp will likely be a major alternative. Because of the fear of complete removal of submersed aquatic plants from lake systems, it is imperative that some means of predictably removing grass carp from systems be obtained.