Nearly five decades ago, triploid grass carp were introduced in the United States to control undesired aquatic vegetation. They have often done a great job meeting manager objectives and stakeholder goals. However, at times, they disrupt the ecosystem, initiating a chain of events that can adversely impact the waterbody and result in chronic cyanobacteria (also known as Harmful Algal Blooms or blue-green algae) issues, reduced water clarity, and the collapse of the existing fishery.
The larger the lake, the more challenging these unintended problems can be to resolve - both in magnitude and in cost. Depending on the waterbody, it can take years for the ecosystem to cycle through a natural progression before it begins to resemble its former self, before the introduction of the Grass Carp.
When these management failures occur, the opportunity costs to those living on the lake can be significant. In most cases, these failures result from poor decision-making, which typically derives from a lack of knowledge and experience or from trying to save money without understanding the long-term impacts of the decisions made.
Cause and effect.
Treating dense vegetation growth on a 500-acre lake with herbicides could cost $50,000-$100,000 or more, and the control may be short-lived. Meanwhile, spending $21,000 on Grass Carp one-time could control the vegetation for 5-15 years, depending on escapement and predation. Looking at those numbers, it becomes clear why grass carp are stocked. Although stocking may be the correct approach, additional management measures may be required. In some cases, these additional measures are expensive, although if done correctly, they can lead to a better, more sustainable outcome for the waterbody.
Throughout the process of properly utilizing Grass Carp, it is vital to understand that they have a long-term impact on the water body's habitat, and this disruption has far-reaching effects on the entire aquatic ecosystem. That's why it is so essential to understand and consider all the interconnected variables within the ecosystem that will be impacted before stocking these purposeful fish—aquatic vegetation species, water quality, water clarity, phytoplankton, zooplankton, invertebrates, fish, and waterfowl, to name a few.
Grass carp tend to either control very little vegetation within their dietary preference or control all of it. They are not a good tool if you expect long-term partial control of submersed aquatic vegetation. When grass carp have a dense enough population, they keep vegetation from going to seed and reduce the energy plants send to their roots. This leads to a decline in current vegetation biomass and a reduction in the potential for future vegetation growth. Even though the grass carp population will decrease annually due to natural mortality, vegetation typically becomes suppressed faster until it is eradicated from the ecosystem.
As with most things with Mother Nature, there are some exceptions. In areas where water is shallow and the growing season is nearly year-round, Grass Carp can have difficulty achieving control, requiring a larger carp population to achieve the same long-term effects.
When using grass carp as the sole means of vegetation control, it is wise to plan on them consuming all vegetation species within their diet. If carp do not escape or succumb to predation, those species will be under control for ten or more years. This long duration of control can be a positive attribute, but it requires proper planning.
Nearly five decades ago, triploid grass carp were introduced in the United States to control undesired aquatic vegetation. They have often done a great job meeting manager objectives and stakeholder goals. However, at times, they disrupt the ecosystem, initiating a chain of events that can adversely impact the waterbody and result in chronic cyanobacteria (also known as Harmful Algal Blooms or blue-green algae) issues, reduced water clarity, and the collapse of the existing fishery.
The most common unintended consequence of stocking Grass Carp are Harmful Algal Blooms (HABs) becoming prevalent as vegetation is eradicated. This undesired growth is fueled by phosphorus and nitrogen, which are no longer being used by aquatic vegetation and are reintroduced after they pass through the fish's digestive system. This issue typically lasts many years, as grass carp continually consume any vegetation attempting to reestablish.
Similar to H AB's, a decrease in water clarity is another common unintended consequence. This loss in clarity is usually a result of increased planktonic algae species, but in rare cases, it can be caused by suspended sediment. Harmful algae species do not always cause a loss in water clarity; in some cases, beneficial green algae and zooplankton species thrive. Although this can be great for the fishery, it can be highly undesired by stakeholders.
A third unintended consequence is that the forage base can become depleted due to both the lack of cover (i.e., protection from vegetation) and the loss of their food source, periphyton, and aquatic insects that live on or within aquatic vegetation. As the forage base collapses, predator fish growth slows, and predators become stunted. This leads to lower angler satisfaction.
A fourth unintended consequence is when midge populations increase and become problematic. Elevated midge populations tend to pester people, gravitate to lights at night, as well as cover the sides of houses and buildings. This increase in midge populations can occur when the forage base collapses, providing insects and insect larvae with higher odds of survival.
As you can see, this can be a chain of events that started as a good decision.
Avoiding these unintended consequences requires proper knowledge and foresight into how the waterbody may change once the carp successfully control submerged aquatic vegetation.
A few key data points to firmly understand during the process of drafting a Grass Carp Management Plan include phosphorus and nitrogen levels, water clarity, water depth, species of vegetation present, total surface acres of vegetation, species of fish present, the health of the existing fish community, and types and amounts of non-vegetative fish cover.
As mentioned, HABs and diminished water clarity due to phytoplankton are caused by nutrients redirected from plant growth into algae growth as the grass carp eradicate vegetation.
The potential for algae growth and the biomass of growth that occurs will depend on multiple variables, primarily the availability of phosphorus and nitrogen. Waterbodies with lower nutrient levels (oligotrophic or mesotrophic) are less likely to experience undesired plankton growth than waterbodies with higher nutrient levels (eutrophic or hypereutrophic).
Total phosphorus is the most critical variable to investigate when testing water quality. Remember that testing water for total phosphorus does not account for phosphorus bound within existing aquatic vegetation. As such, water quality data is subject to misinterpretation if not considering this variable.
Sediment phosphorus levels should also be tested, as they provide critical insight into the lake's internal nutrient load. In general, water bodies with elevated phosphorus in the sediment should anticipate an increase in cyanobacteria and phytoplankton growth once aquatic vegetation is eradicated.
It needs to shift into a less available form to prevent available phosphorus from fueling undesired growth. This can be done using a combination of methods, including promoting vegetation species that grass carp do not eat, maintaining sufficient dissolved oxygen at the sediment-water interface, and using products that bind phosphorus, such as alum, Phoslock, and EutroSORB.
Whether stakeholders are seeking to maintain clean, healthy water for boating and swimming or desire a thriving fishery, one of the best management strategies to consider is establishing a tolerance for vegetation species that grass carp won't eat.
Grass carp have dietary preferences, allowing some aquatic plants to coexist harmoniously. There are a variety of plant species they dislike, including most emergent vegetation species. There are quite a few species they can't eat because their soft, fleshy lips won't bite through thick stems and heavy leaves. White water lilies, spatterdock, pickerelweed, duck potato, and water willow are common plants to consider. Establishing them before stocking grass carp is a solid approach. Waiting until after submerged vegetation has been eradicated is significantly more complicated and requires fence material to create exclosures to protect plants. In addition to the grass carp, wildlife such as turtles, crayfish, waterfowl, muskrats, and deer feed on vegetation and seek out and eradicate juvenile plants installed, even if they are outside their preferred diet.
Although grass carp can get a bad reputation sometimes, they are an outstanding management tool when used properly. Their ability to provide long-term control over various vegetation species can significantly stretch budgets and maximize a waterbody's potential to meet stakeholder goals. With the correct data and foresight, management strategies can be implemented with a high probability of sustained success, allowing time and energy to be focused on enjoying the lake.
David Beasley is a Fisheries Biologist and the Director of Fisheries at SOLitude Lake Management, an environmental firm providing sustainable lake, pond, wetland, and fisheries management services. Learn more about this topic at www.solitudelakemanagement.com/ knowledge.
Reprinted with permission from Pond Boss Magazine
