The grass carp (Ctenopharyngodon idella) is a large freshwater fish with a significant impact on aquatic ecosystems. Native to East Asia, it has been introduced worldwide, including the United States, primarily for aquatic plant control. This article explores the grass carp's diet, feeding habits, and its role in managing aquatic vegetation, considering both the benefits and potential drawbacks.
Identification and Origin
The grass carp is one of the largest members of the minnow family. It has an oblong body shape with a round belly and broad head. Key features include:
- Coloration: Silvery to dark grey back, lighter sides with a golden sheen, and a silvery-white belly. Fins are generally greenish-grey. Scales sometimes show a reddish tint.
- Scales: Relatively large and cycloid scales.
- Mouth: Dorsal mouths without barbels. Jaws are robust, with teeth arranged in 2, 4-4, 2 patterns.
- Pharyngeal Teeth: Teeth have been replaced by specialized structures called pharyngeal teeth located in the back of the throat.
- Body Shape: Torpedo-shaped, moderately compressed laterally.
The grass carp is native to large coastal rivers in Siberia (Amur River) and China that flow into the Pacific Ocean. They are extensively cultured for food in China, Malaysia, Singapore, Borneo, Indonesia, Thailand, Taiwan, Hong Kong, and the Philippines. In 1963, they were introduced to the United States by the Bureau of Sport Fisheries, which delivered 70 fish from Malaysia to Arkansas.
Habitat and Distribution
Grass carp were initially endemic to the waters of eastern Asia. As a result, they were initially cultivated and cherished by the Chinese for food. Grass Carp have since been stocked in many private lakes and ponds to control excess vegetation. Grass Carp began to be exported and stocked in the United States and Europe’s new waters as time went by. Grass Carp have been recorded from 45 states; there are no reports of introductions in Alaska, Maine, Montana, Rhode Island, and Vermont. It is known to have established populations in a number of states in the Mississippi River basin.
Typical habitat includes quiet waters, such as lakes, ponds, pools, and backwaters of large rivers. They commonly live in clear ponds, lakes, rivers, and streams and live within the middle to the lower layers of these water bodies because of their herbivorous nature. This enables them to hunt for what they can eat at any time. When they are fully grown, they migrate from the lower regions to higher regions of other transparent water bodies to continue their lives.
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The Grass carp can tolerate a wide range of temperatures, which plays a significant role in a relatively long life span. However, exceedingly low temperatures aren’t the most conducive for Grass Carp, as is the case during winters.
Diet and Feeding Habits
As the name implies, grass carp are primarily herbivores. Generally, Grass Carp can consume a lot of food, and they eat up to three times their body weight every day. For this reason, grass carp need waters full of aquatic vegetation to thrive. They feast mostly on aquatic vegetation at the bottom of the fishery. They rarely will consume small insects or worms in the water. This fish is known for favoring veggies such as cherry tomatoes and sweet corn. As such, they consume higher aquatic plants and submerged grasses (rooted macrophytes as opposed to algaes). A single Grass Carp can digest only about half of the approximately 4.5 kg of plant material that it consumes each day. They will also eat detritus, insects, and other invertebrates when necessary.
Grass carp feeding habits are fairly selective. They prefer softer, pliable aquatic vegetation. This desire for delicate vegetation reduces tendencies to eat most emergent and floating aquatic vegetation species. Although they have been reported to eat such greenery, it seems to be a low occurring anomaly if dealing with a stocking rate <15 fish per acre. Grass carp’s selective diet is significant to pond owners since it allows floating plants such as watermeal, and emergent species as pickerelweed, lilies and duck potato, to coexist with the fish. The reason this is so important is because it allows these floating and emergent species to both serve as refuge for fish and wildlife, while also helping to manage influxes of nutrients that occur over the growing season.
Reproduction and Life Cycle
Spawning of Grass Carp usually occurs between May and June. Grass Carp have particular requirements for reproduction, including a temperature of 68 to 86 degrees Fahrenheit; otherwise, their ability to multiply is greatly affected and reduced. Grass Carp reproduction cannot happen in stagnant water bodies; they require rivers with the current. Broadcast spawners, the females lay copious amounts of eggs in free-flowing rivers that are then fertilized by the male. Females usually lay their eggs in higher regions of the river after relocating from lower areas.
The eggs must remain suspended in the flow until they hatch or they will perish. After the carp eggs are laid, they begin to float downwards slowly and develop as this movement occurs. Considering that the waters are hardly ever in a state of complete rest, the turbulence keeps the eggs afloat as they drift downwards carefully and mature. If the eggs get to the water bed without the emergence of young fishes, they die.
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Grass Carp is a large member of the minnow family with a body which is moderately compressed laterally. Its mouth is terminally located on a wide head and eyes are small and low on the head. It lacks barbels. It is olive-brown on the dorsal side, with silver sides and a white belly. Scales are large with dark edging. The dorsal fin origin is anterior to the pelvic fin origin and it has a short caudal peduncle.
Use in Aquatic Plant Management
Because of their feeding behavior, grass carp are used in this country under special restricted circumstances to help manage aquatic plant problems. Many aquatic plants that pose problems in Florida are exotic and have few natural insect or disease controls, consequently they frequently create problems with access, navigation, flood control, irrigation and aesthetics. Since chemical herbicides and mechanical removal are costly alternatives that often create secondary problems, biocontrol agents, such as triploid grass carp can provide a beneficial tool.
Triploid Grass Carp
Triploid grass carp have three sets of chromosomes, instead of the normal two, which renders them functionally sterile. Triploids are produced artificially, by using hormones, stripping the eggs and mixing them with milt, then subjecting the fertilized egg to hydrostatic pressure, and finally suspending the fertilized eggs in well-aerated containers until they hatch. The resultant fish must be individually tested using a Coulter Counter, in a certification process developed by FWC biologists prior to being permitted for use.
Triploid grass carp are considered a natural method for controlling aquatic vegetation and macro-algae. They can live for many years (20 plus years in some cases). Their ability to live long typically makes them significantly more cost-effective than herbicides. Who’s opposed to saving dollars, long term? They are normally readily available year-round in several sizes and are legal to stock in many states (a permit is typically required in states where they are legal).
Triploid grass carp are unique, olive-green, cigar-shaped fish often observed basking in the sunshine in shallow water as they soak up warm surface temperatures. What sets these fish apart from others is their intense appetite for a wide variety of underwater plant life. If stocked at high enough density, these guys will eat plants faster than the greenery can grow. Many pond owners have witnessed grass carp work wonders, taking choked-out ponds and cleaning them up, restoring beautiful waterways. Other times people experience little to no vegetative control.
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Stocking Strategies and Considerations
Prior to stocking sterile triploid grass carp and fine-tuning your strategy, understand the pros and cons of them as a management tool. There is a risk/reward ratio that can be favorable with a clear understanding of tendencies and capabilities. In addition to the good and bad associated with stocking them, also understand why they sometimes work, but not at other times. That means John Q. Pondmeister needs to know what types of plants these fish eat, what they don’t, and then stock the right size fish at the proper rates.
Prior to stocking grass carp you should first be able to answer the following questions properly: Are they legal in your state? Do you have sufficient cover to help your predator-to-prey ratio stay in balance once your submersed vegetation is eradicated? Is your undesired vegetation a species that grass carp will eat? If you are using fish feeders and grass carp start eating the fish feed, will you take the necessary action to remove the grass carp? Or, will you change your feeding strategy? Do you plan to install beneficial emergent vegetation in the future?
One important factor when stocking grass carp is to stock the correct size. Far too often I see and hear of people stocking 8-inch fish. An 8-inch grass carp is a snack to most adult bass and catfish. To improve odds of success it is important to stock grass carp that are a minimum of 10 inches in length, but preferably 12 inches. These larger fish are hardier and can avoid predation more easily. Additionally, they have the ability to eat more aggressively and reach larger sizes more quickly, which will help reduce predation while also reducing their ability to escape through the outflow. If you have a bunch of bass 5-10+ pounds, you should ask your fish supplier for 2-pound grass carp.
The key to knowing when to stock grass carp depends on how easy the carp can escape the pond as well as the timing of fresh, succulent growth of your plants. If the outflow is set up so grass carp cannot fit through the openings, then you can get away with stocking them nearly any time of year. If the pond has the tendency to go over the emergency spillway regularly, or it is difficult to limit escapement through the outflow, then it will become much more important to stock in the spring right before the plants germinate. Additionally, scientific literature estimates the mortality rate on grass carp at approximately 20 to 30 percent the first couple years after you stock them.
When it comes to stocking rates, you can take two approaches. The first is to stock a low-moderate rate of grass carp to help reduce some vegetation, but not likely eradicate it. Stocking ~4-7 fish per vegetated acre is a good rate to meet this goal. The reality is that this low-moderate stocking rate does not seem to work very quickly, especially when dealing with invasive species. In many cases of low stocking rates the vegetation tends to maintain its dominant status for 18 months or longer. The second approach and, in my experience, the most successful strategy for controlling nuisance or invasive submersed vegetation is to stock what it will take to eradicate the plants. Stocking 10-15 grass carp per actual acre of vegetation typically works well. If vegetation is sporadic and not very dense, you can likely get away with only 10 fish per acre. With this approach it is critical that you provide an alternative source of dense cover for small fish to help the fishery maintain balance during this transition. If stocked at the correct size, time of year and at a high enough rate to eradicate the vegetation you will likely find that you do not need to restock annually. Although, after a few years, grass carp will die off annually. If you still have major vegetation issues by mid-summer following the carp’s first spring in the pond then you likely have too low a carp population to achieve control.
When it comes to restocking grass carp, you should stock based on the needs of the pond rather than restocking a set percentage annually. As identified above, the amount of vegetation going to seed is going to diminish annually if you initially stock enough carp, so a better restocking method is to stock based on the actual vegetation growth. If you observe vegetation growing, simply estimate the acreage of vegetation and stock additional carp.
Potential Drawbacks
If stocked at a high enough density early in the year before plants germinate, grass carp will consume all vegetation within their selective diet. This can be a negative because bare ponds leave young of the year (YOY) and juvenile fish with little refuge. If growing big largemouth bass is an important goal consider adding dense cover for small fish prior to eradicating undesired vegetation.
Another drawback for those managing fishing ponds is grass carp can get lazy and park themselves in front of your fish feeders, especially if there is no vegetation left in the pond. If you use fish feeders as a primary way to increase productivity, then understand grass carp will likely compete for fish feed at some point, typically as they get a few years under their scales. If they are observed eating fish feed you should harvest them. The most common method of removing grass carp is to shoot them or fish them out.
An additional drawback is that grass carp are a riverine fish and have the tendency to want to travel. They seem to gravitate to the sound of moving water, so depending on the pond; they may be difficult to keep from escaping downstream. To fix this issue plan to make an adjustment to the water bodies outflow so carp cannot escape.
While this fish may seem to be a magic bullet, be sure to think beyond the obvious. For example, one scenario to consider is a pond with elevated nutrient load and also choked with submersed aquatic vegetation, such as hydrilla or milfoil. In these situations, stocking grass carp at a high rate such as 10-15 fish per acre will often eradicate the vegetation and leave even higher nutrient loads, making the pond prone to dense algae blooms. If this is the case, you may need to consider applying algaecides to control the undesired algae and also implement management strategies that help to lower the nutrient level.
Environmental Impact
Grass carp individuals generally do not travel long distances except for the annual spawning migration. Nevertheless, there are reports of juvenile Grass Carp traveling as far as 1,000 km from their original spawning grounds.
In their overview, Chilton and Muoneke (1992) reported that Grass Carp seem to affect other animal species by modifying preferred habitat, an indirect effect. However, they also indicated that grass carp may directly influence other animals through either predation or competition when plant food is scarce. In his review, Bain (1993) stated that Grass Carp have significantly altered the food web and trophic structure of aquatic systems by inducing changes in plant, invertebrate, and fish communities.
Tolerance
Oxygen consumption (per gram of body mass) increases with higher water temperature and decreases with fish age and mass. The lethal low oxygen level for juveniles was <0.5 mg/L. The maximum pH for culture of grass carp was reported as 9.24. Egg hatching was delayed below pH 6.5 and increased mortality and deformation of larvae occurred below pH 6.0. Sensitivity to low pH decreased with age.
Grass Carp appears to be tolerant of low levels of salinity, and may occasionally enter brackish-water areas. Fry (32-50 mm TL) survived transfer from freshwater to a salinity of 12 ppt. Adults (2+ years) survived 10.5 ppt salinity for about 24 days and 17.5 ppt for 5 hours. However, Grass Carp acclimated to 3, 5, and 7 ppt had an upper tolerance of about 14 ppt. Maceina and Shireman (1980) showed that fingerlings reduce feeding at 9 ppt and stop feeding altogether at 12 ppt; thus, they predicted Grass Carp could inhabit brackish water bodies up to 9 ppt. Maceina and Shireman (1979) reported that the species can tolerate 14 ppt for as long as 4 days, but that the upper long-term tolerance of fingerlings to saline waters was lower, about 10-14 ppt.
Grass Carp as Food
In some countries, the grass carp is an integral part of fish culture, and fish flesh forms an important source of protein for human consumption. The fish have flaky white meat that is not too strongly flavored but tend to be very bony. Sometimes taken by anglers, especially those fishing for catfish or tilapia, but they must be released immediately and unharmed since those found in Florida were stocked deliberately for aquatic plant control purposes. The largest triploid grass carp taken in Florida was 15 years old, 56" long and weighed 75 lbs.