USING GRASS CARP FOR AQUATIC PLANT CONTROL

Grass Carp (Photo source: Gettys et al. 2009)

1. Aquatic plants, their importance and impacts to the ecosystem

     Aquatic plants are important to aquatic ecosystems because they provide food for various fish species. This energy is consumed by herbivorous fish and used for growth and reproduction (Madsen 2009). Aquatic plants produce oxygen required by all fish for survival through carbohydrates (sugar) via photosynthesis (Madsen 2009). Aquatic plants also provide diverse habitats for fish particularly covers for small fish protecting them from predators. Plants also directly impact the water quality and sedimentation rates of suspended matter (Madsen 2009).  More than 80% from 38 Florida lakes are highly productive and they were classified either as eutrophic or hypereutrophic (Hanlon et al. 2000). Meanwhile, Canfield and Hoyer (1988) classified that 52% and 35% out of 165 lakes in Florida are mesotrophic and eutrophic lakes respectively.

     Aquatic macrophytes can create crucial problems in Florida’s water bodies due to their excessive growth resulting from their shallowness and high water temperatures (Hanlon et al. 2000). Non-native plants can have significant negative effects in water bodies and can threaten the function and diversity of these aquatic ecosystems (Hanlon et al. 2000). Hanlon et al. (2000) examined the effective mass of grass carp in controlling aquatic vegetation in 38 Florida’s lakes which has biologically had problems with excessive aquatic macrophytes, mainly from the introduction of invasive species.

     Non-native aquatic plants can dramatically change the functionality and structure of aquatic ecosystem and can cause economic loss from reduced recreational use and increase cost to manage these ecosystem (Madsen 2009). Exotic plants directly impact water bodies by reducing the growth of native plant species, changing the fish habitat which can result in smaller game fish species being produced, increasing the sedimentation rate within these systems, and lowering oxygen content in the water due to lack of lake circulation due to high densities of exotic vegetation (Madsen 2009). 

Hydrilla dominated a lake (Photo source: Gettys et al. 2009)

     The number of exotic aquatic plant species that significantly contributes to problems in Florida water bodies is hydrilla (Hydrilla verticillata). In a study of 38 Florida lakes, 27 lakes were colonized by hydrilla, with 7 to 10 percent of the area of these lake covered by aquatic plants (Hanlon et al. 2000). Hydrilla has resulted in high and costly damage to ponds and lakes in the United States, and it is believed that this species will continue to expand its distribution (Richardson 2008). As such, aquatic plant management needs to be implemented when the aquatic communities reach nuisance levels (Hanlon et al. 2000) or when the introduction of non-native plant species and depleting growth of the native aquatic plant occurs (Richardson 2008).

2. Grass carp as aquatic plant control

     Aquatic plants management is often needed because of the introduction of exotic species and the excessive growth of native aquatic plants (Richardson 2008). Current management practices include removal of aquatic plants using mechanical tools (e.g., weed harvester and dredges) and the use of herbicides (Richardson 2008). Biological control, however, is the best management practice because it is more efficient, cost less, and controls plants without using chemicals (Hanlon et al. 2000).

     The best biological management technique to control invasive aquatic plants, namely hydrilla, is by using grass carp (Ctenopharyngodon idella ) (Hanlon et al. 2000; Richardson 2008; Colle 2009; Madsen 2009). The use of grass carp can be effective, cost efficient (Hanlon et al. 2000), and can remove aquatic plants for the long term (Colle 2009). Hanlon et al. (2000) collected data from previous Florida lake studies and suggested that the best biological control for the overpopulation of aquatic plants in lakes is by using grass carp.

     Grass carp mainly originated from China and has been introduced to many countries around the world including in the United States because of its ability to control aquatic plants. In Florida it was introduced in 1970’s for experimental purpose to control the hydrilla (Sutton et al. 2012). Grass carp are herbivorous species that prefer certain types of aquatic plants. Some of the most preferred aquatic plants are hydrilla (H. verticillata), muskgrass (Chara spp.), southern waternymph (Najas guadalupensis), Brazilian waterweed (Egeria densa), watermeal (Wolffia spp.) and duckweed (Lemna spp.) (Sutton et al. 2012).

     Grass carp will consume almost any aquatic plant available in an aquatic system if the desired plants are depleted (Colle 2009). They even observed to consume terrestrial plants that hanging at the surface of the lake water such as grasses and banana leaves (Sutton et al. 2012). This species can survive for at least 25 years in Florida lakes and can grow up to ten pounds per year (Colle 2009). Only triploid grass carp can be used in Florida, with almost guarantees that there will be no reproduction of grass carp after stocking them into natural water bodies (Hanlon et al. 2000; Colle 2009). Diploid grass carp cannot successfully reproduce in standing water (lakes and pond) but have been documented to reproduce in large rivers such as the Mississippi River.

Just imagine how big a grass carp can grow (Photo source: Gettys et al. 2009)

     Many lakes have been stocked with low stocking rates of grass carp to control aquatic plants (Hanlon et al. 2000). The stocking rate that is most suitable for closed systems generally ranges from 5 to 120 fish per hectare (Colle 2009). In South Carolina, Richardson (2008) cited from previous papers that stocking rates of 20 to 30 grass carp per hectare reduced the vegetated area from 17,000 ha to only about 500 ha. The optimum stocking density is dependent on the quantity of aquatic macrophytes available in the area, and there is no ‘magic number’ of stocking rates for grass carp (Colle 2009). Furthermore, balancing the number of grass carp and the plant growth is different among water bodies and difficult to achieve (Colle 2009).

     Another successful method in controlling aquatic plants in aquatic ecosystem is the integrated approach (Hanlon et al. 2000; Cuda et al. 2008). Lakes are treated with herbicides to reduce the biomass of aquatic plants prior to stocking grass carp (Hanlon et al. 2000). For example, the integrated approach was conducted in 38 lakes in Florida where the lakes were treated with certain level of herbicides to reduce the starting biomass of aquatic plants before stocking the fish (Hanlon et al. 2000). The optimum sizes of grass carp for stocking are 12 inches or bigger to avoid predation and to ensure that aquatic plant species are controlled successfully (Colle 2009).

3. Effects after stocking

     Aquatic plant management can be successful using biocontrol agent such as grass carp, especially if long- term control is required (Colle 2009). Hanlon et al. (2000) believe that grass carp take several years to control the production and biomass of aquatic plants. Grass carp can maintain the aquatic plants for 10 to 15 years without complete elimination (Colle 2009).

     Stocking with a high density of grass carp can effectively eliminate all submerged aquatic plants (Hanlon et al. 2000; Colle 2009). Hanlon et al. (2000) found that the stocking of 24 to 74 fish per hectare resulted in the total loss of submersed aquatic plants, because the consumption rates of the fish exceeded the growth rates of the vegetation. On the other hand, Hanlon et al. (2000) suggested that stocking rates of 25 to 30 grass carp per hectare gave some management advantages in that grass carp would selectively control some types of aquatic plants species while leaving others alone.

     Hanlon et al. (2000) also indicated that within 3 to 10 years of grass carp stocking, the average vegetated area decreased from 57% to 24%. They concluded that if the percentage of vegetation area left in a lakes is 14% or greater, that grass carp would not totally eliminate the entire aquatic plant community. Stocking grass carp at 25 to 30 individuals per hectare of aquatic vegetation is considered an excellent method if the control of vegetation in a lake is matched with the goals of the managers. 

     On the other hand, there are some negative effects resulted from the stocking of grass carp. Water quality changed as a result of total elimination of aquatic plants by grass carp, as systems may change to being dominated by phytoplankton (Colle 2009). Water clarity decreased because of abundance of phytoplankton and due to wind driven currents that stir the sediments from the lake bottom up to the surface (Colle 2009). For instance, Colle (2009) stated that some of the fish species in two Florida lakes are no longer available as a result of the consumption of aquatic plants by grass carp for 15 years. Deterioration of spawning areas, nursery grounds, and food sources might be the results of the loss of the fish species.

     Many debates arise pertaining to the stocking of grass carp to control aquatic vegetation (Hanlon et al. 2000). After introducing the grass carp into a water body, they can be extremely difficult to remove (Hanlon et al. 2000; Cuda et al. 2008; Colle 2009). For example, it took a few years of effort by fishermen to remove grass carp from a lake in Louisiana (Colle 2009). However, according to Colle (2009), users generally admit that grass carp could not be removed once they were stocked into the water body.

     In general, Cuda et al. (2008) reported that the abiotic factors, biotic factors, predation, parasitism, diseases, and other technical factors limit the success of biological control of aquatic macrophytes. Colle (2009) mentioned that 7% to 70% of grass carp that were stocked into ponds in Florida were lost due to predation after one year of stocking. Predation is problematic when large fish such as striped bass and largemouth bass become predators to the stocked fish (Colle 2009). According to Hanlon et al. (2000), it is difficult to maintain 25 to 30 grass carp per hectare as a stocking rate because of different mortality rates of the stocked grass carp. The consumption rates of the grass carp were lower than the growth rates of the aquatic plants, thus little control of the aquatic macrophytes was achieved with higher mortality rates (Hanlon et al. 2000).

Literature cited

Canfield, Jr., D. E., and M. V. Hoyer. 1988. Regional geology and the chemical and trophic state characteristics of Florida lakes. Lakes and reservoir management 4(1): 21-31. 

Colle, D. 2009. Grass carp for biocontrol of aquatic weeds. Pages 61-64 in L.A. Gettys, W.T. Haller, and M.Bellaud, editors. Biology and control of aquatic plants, Aquatic Ecosystem Restoration Foundation, Marietta, Georgia.

Cuda, J. P., R. Charudattan, M. J. Grodowitz, R. M. Newman, J. F. Shearer, M. L. Tamayo, and B. Villegas. 2008. Recent advances in biological control of submersed aquatic weeds. Aquatic Plant Management 46: 15-32. 

Gettys, L.A., W.T. Haller, and M. Bellaud. 2009. Biology and control of aquatic plants, Aquatic Ecosystem Restoration Foundation, Marietta, Georgia.

Hanlon, S. G., M. V. Hoyer, C. E. Cichra, and D. E. Canfield Jr. 2000. Evaluation of macrophyte control in 38 Florida lakes using triploid grass carp. Aquatic Plant Management 38: 48-54.

Madsen, J. 2009. Impact of invasive aquatic plants on aquatic biology. Pages 1-8 in L.A. Gettys, W.T. Haller, and M.Bellaud, editors. Biology and control of aquatic plants, Aquatic Ecosystem Restoration Foundation, Marietta, Georgia.          

Richardson, R. J. 2008. Aquatic plant management and the impact of emerging herbicide resistance issues. Weed Technology 22: 8-15.

Sutton, D. L., V. V. Vandiver Jr., and J. E. Hill. 2012. Grass carp: A fish for biological management of hydrilla and other aquatic weeds in Florida. EDIS University of Florida.

Acknowledgement
I would like to thank you Dr Charles Cichra for the draft editing