**3. The method**

On the other hand in some temperate regions, in spite of grass carp maturing at the same time as in their native distribution, their gonads do not mature. This is possibly related to a lack of nutritional, photoperiod and water temperature requirements for grass carp [12]. A well‐marked and limited spawning season occurs in temperate latitudes. On the other hand, in tropical areas, the breeding season expands and becomes less distinct, and as a result of

In their native areas, grass carp begin migration to spawning areas when water temperatures reach 15–17°C [6]. Water temperature and its level play key roles for inducing spawning, and it varies with latitude. Water temperature required for the stimulation of sexual maturation and spawning ranges between 20 and 30°C. Optimum spawning temperature is generally thought to be between 20 and 22°C. In addition, increases in water level exceeding 122 cm within a 12‐hour period are required for spawning [6]. If water levels do not rise during the spawning season, females with small reserves of body fat will either release no eggs or release

Grass carp spawn in the rivers and canals during high water. Spawning usually takes place in spring and summer in the upper part of the water column over rapids or sand bars [5]. Preferred spawning habitat is found in turbulent water of the junction of rivers or below dams [14, 15]. Grass carp prefer to spawn in water currents ranging from 0.6 and 1.5 m/sec, but spawns generally occur in currents as low as 0.2 m/sec or even in ponds where the current is absent [15].

Fecundity is directly proportional to length, weight and age of the females and ranges from 0.001‐ to 2‐million eggs but generally averages to 0.5 million for a 5‐kg broodstock [5, 6]. Grass carp eggs are 2.0–2.5 mm in diameter when released but quickly swell to a diameter of 5–6 mm as water is absorbed [6]. The eggs are semi‐buoyant and nonadhesive, requiring well‐oxygenated water and a current to keep them suspended until hatching [6, 15, 16]. Eggs

Grass carp feed almost exclusively on aquatic plants. They can eat 2–3 times their weight each day and may gain 2–4 kg in a single year. The larger they get, the more plant material they consume. Cultured grass carp may reach up to 1 kg in the first year and grow approximately

Grass carp prefer soft and low fibre aquatic vegetation such as duckweed and various underwater plants. If the more desired plant species aren't available, they feed on plants above of the water surface. Grass carp even have been observed to feed on terrestrial plants that are hanging over the water. Triploid and diploid grass carp seem to consume similar quantities of aquatic plants and to have similar feeding habits and prefer succulent young plants. Because of its strong preference for aquatic vegetation, the grass carp is being widely used to control

The five most‐preferred species in order of preference are hydrilla, musk grass, pondweeds (*Potamogeton* spp.), southern naiad (*Najas guadalupensis*) and Brazilian elodea (*Egeria densa*) [17]. Grass carp are not a good control method for filamentous algae, Eurasian milfoil (*Myriophyllum* 

only a portion. Non‐released eggs are subsequently absorbed in the body [13].

may travel along the downstream, that's about 50–180 km [14].

2–3 kg/year in temperate areas and 4.5 kg/year in tropical areas [5].

**2.4. Feeding behaviour of grass carp**

aquatic vegetation in lakes and ponds [7].

this, multiple spawning can occur in a year [5].

32 Grasses - Benefits, Diversities and Functional Roles

Intensive use of chemical fertilizers in agriculture and also human and industrial pollution causes eutrophication. This situation causes growing of plants quickly and as a result of this, plant control cannot be solved mechanically or chemically. The most obvious solution in these cases is the introduction of grass carp to these waters covered with plants.

Some several thousand hectares of large ponds covered with overgrown macrophytic vegetation can be cleaned by introducing of grass carp. Grass carp is one of the optimal species for controlling of aquatic plants in water reservoirs. At this point, several parameters such as stocking density of grass carp, plant and plankton composition, water quality, and also the structure of the benthos should be noted.

## **3.1. Stocking of grass carp for controlling of aquatic vegetation**

The grass carp number required to control aquatic plants varies depending on the degree of plant infestations, plant types, pond sizes and the size of fishes stocked. A number of different methodologies have been used to determine the suitable number of grass carp to stock. The most precise method is to determine the weight of aquatic vegetation in the pond and knowing the consumption rates of the fish.

In spite of investigation of different stocking rates, there is no guideline that will fit all situations for grass carp. Each aquatic reservoir is different because of its own combination of fertility, water clarity, shallow water and chemical makeup. So, each of these variables affects the number of grass carp required to achieve the plant level to the desired control. Stocking rates may vary as low as one to as many as 20 grass carp per acre, depending on the amount and types of vegetation.

reason, the speed and extent of macrophyte removal by the grass carp affect the phytoplank-

Importance of Grass Carp (*Ctenopharyngodon idella*) for Controlling of Aquatic Vegetation

http://dx.doi.org/10.5772/intechopen.69192

35

Zooplankton consumption is necessary for juvenile and adult grass carp, but the consumed amounts are negligible in case the stocking density is not extremely high [28]. In lakes stocked with herbivorous fish, the growth of zooplankton and zoobenthos is enhanced through consumption of macrophytes by the fish and subsequently increased nutrient remineralization rates. The overall result can be also demonstrated through an increase of fish production [29]. Finally, the zooplankton communities shifted from copepod and copepod‐cladoceran‐dominated communities to rotifer and small cladocerans. Changes in zooplankton abundance and community structure were due to an increase in phytoplankton and shifts in planktivore pre-

The effects of grass carp on plants and water quality are highly variable and often inconclusive due to the lack of proper control sites. The proportion and rate of plant removal by the grass carp is crucial. Changes in water quality as a result of plant removal by the grass carp mostly occur in small, non‐flowing water bodies and least occur when only a small proportion of plants is removed from large, relatively deep, flowing reservoirs. In this concept, decreases can be observed in oxygen concentration of water following grass carp stocking, depending on the disappearance of macrophytes [31]. Primary producers such as phytoplankton and

which results in an increase in water pH. Changes in oxygen concentrations following grass

Higher stocking densities of grass carp or their longer impact can increase concentrations of nutrients in the water, but these increases are mainly dependent on the water‐body characteristics. These changes result from sediment resuspension during feeding and faecal matter deposition by carp as well as collapse of mechanisms responsible for maintenance of the vegetated state due to removal of macrophytes. Changes in benthos corresponded closely to changes in aquatic vegetation which stabilize sediments and provide additional substrate in the form of root masses and decaying material. Zoobenthos also responded to changes in

The rate of aquatic plants elimination determines the magnitude of impact [30, 34]. These changes in water quality are often followed by algal blooms [35] which in most lakes signal a shift to an alternative stable state [36]. Increasing rates of nutrient cycling following resuspen-

In conclusion, grass carp can be effective in controlling of aquatic plants, but its potential adverse effects to aquatic ecosystems may be severe. In this concept, changes in plant

during photosynthesis,

dation on zooplankton by fish after macrophyte removal [30].

aquatic macrophytes not only release oxygen but also consume CO<sup>2</sup>

carp stocking were positively correlated with the changes in pH [32].

water quality following removal of aquatic macrophytes [33].

sion of sediments lead to decreases in ecosystem stability [37].

**4. Conclusion**

**3.3. Changes in water quality and benthos**

ton production.

Stocking rates need to be increased as temperature decreases (as indicated by daily temperature units (DTU) decrease) because grass carp plant consumption and growth decrease. Stocking densities need to be based on the standing crop (biomass) of aquatic vegetation. This is estimated by multiplying plant distribution by average plant density; therefore, the higher the vegetation biomass, the higher the required stocking rate.

It should be well known that "overstocking" is followed by complete removal of all vegetation, while "understocking" of a water body causes either selective reduction of vegetation [21] or it can also result in no vegetation [22]. Low stocking densities can maintain intermediate plant control. On the other hand, plants rejected by the grass carp are left and may grow vigorously [23].

The amount of aquatic plants consumed by grass carp and its selectivity depends on many factors such as stocking density, age, temperature conditions, the length of time the fish have been in the pond and the quality and quantity of food present.

Initial plant density is an important indicator for the biological control. Biocontrol is effective if grass carp is stocked prior to the beginning of the rapid vegetation growth. Water level fluctuation should be estimated and taken into consideration. A dramatic decrease of water level could cause overstocking of the grass carp, and it is extremely difficult to remove fish from lakes. For this reason, stocking density of grass carp should be calculated for the lowest water level.

In addition to these, grass carp age and size are also important due to the possible predation on them, which can markedly reduce their initial stocking density. Grass carp should be larger than 30 cm when stocked; otherwise, they are very vulnerable to predators. In some areas, the otter can capture grass carp of about 2.7 kg (length of 60 cm), causing serious problems for fishpond management [24].

#### **3.2. Changes in aquatic plant pattern and plankton composition**

Grass carp can continuously control preferred aquatic plant species. Their impacts have been observed for 15–20 years at higher stocking rates. It is assumed that elimination of aquatic plant species preferred by the grass carp results in reduction of the diversity of the aquatic macrophyte community [25].

The stocking density and controlled plant area affect the extension of phytoplankton production in the ponds or lakes. In case of slow controlling of plants by grass carp, the indirect consequences of grass carp stocking on phytoplankton are negligible. It was determined that changes in the concentration of chlorophyll‐a in the water were non‐significant at low stocking density (30 kg ha−1) [26]. Cassani et al. [27] also determined that in case of suppression of macrophytes, annual mean chlorophyll‐a concentration remained stable in the ponds.

Primary production of the water reservoirs depends on light and nutrient availability. These two factors affect unstable equilibrium between macrophytes and phytoplankton. For this reason, the speed and extent of macrophyte removal by the grass carp affect the phytoplankton production.

Zooplankton consumption is necessary for juvenile and adult grass carp, but the consumed amounts are negligible in case the stocking density is not extremely high [28]. In lakes stocked with herbivorous fish, the growth of zooplankton and zoobenthos is enhanced through consumption of macrophytes by the fish and subsequently increased nutrient remineralization rates. The overall result can be also demonstrated through an increase of fish production [29]. Finally, the zooplankton communities shifted from copepod and copepod‐cladoceran‐dominated communities to rotifer and small cladocerans. Changes in zooplankton abundance and community structure were due to an increase in phytoplankton and shifts in planktivore predation on zooplankton by fish after macrophyte removal [30].

#### **3.3. Changes in water quality and benthos**

the number of grass carp required to achieve the plant level to the desired control. Stocking rates may vary as low as one to as many as 20 grass carp per acre, depending on the amount

Stocking rates need to be increased as temperature decreases (as indicated by daily temperature units (DTU) decrease) because grass carp plant consumption and growth decrease. Stocking densities need to be based on the standing crop (biomass) of aquatic vegetation. This is estimated by multiplying plant distribution by average plant density; therefore, the higher

It should be well known that "overstocking" is followed by complete removal of all vegetation, while "understocking" of a water body causes either selective reduction of vegetation [21] or it can also result in no vegetation [22]. Low stocking densities can maintain intermediate plant control. On the other hand, plants rejected by the grass carp are left and may grow

The amount of aquatic plants consumed by grass carp and its selectivity depends on many factors such as stocking density, age, temperature conditions, the length of time the fish have

Initial plant density is an important indicator for the biological control. Biocontrol is effective if grass carp is stocked prior to the beginning of the rapid vegetation growth. Water level fluctuation should be estimated and taken into consideration. A dramatic decrease of water level could cause overstocking of the grass carp, and it is extremely difficult to remove fish from lakes. For this reason, stocking density of grass carp should be calculated for the lowest water level.

In addition to these, grass carp age and size are also important due to the possible predation on them, which can markedly reduce their initial stocking density. Grass carp should be larger than 30 cm when stocked; otherwise, they are very vulnerable to predators. In some areas, the otter can capture grass carp of about 2.7 kg (length of 60 cm), causing serious prob-

Grass carp can continuously control preferred aquatic plant species. Their impacts have been observed for 15–20 years at higher stocking rates. It is assumed that elimination of aquatic plant species preferred by the grass carp results in reduction of the diversity of the aquatic

The stocking density and controlled plant area affect the extension of phytoplankton production in the ponds or lakes. In case of slow controlling of plants by grass carp, the indirect consequences of grass carp stocking on phytoplankton are negligible. It was determined that changes in the concentration of chlorophyll‐a in the water were non‐significant at low stocking density (30 kg ha−1) [26]. Cassani et al. [27] also determined that in case of suppression of

Primary production of the water reservoirs depends on light and nutrient availability. These two factors affect unstable equilibrium between macrophytes and phytoplankton. For this

macrophytes, annual mean chlorophyll‐a concentration remained stable in the ponds.

the vegetation biomass, the higher the required stocking rate.

been in the pond and the quality and quantity of food present.

**3.2. Changes in aquatic plant pattern and plankton composition**

and types of vegetation.

34 Grasses - Benefits, Diversities and Functional Roles

vigorously [23].

lems for fishpond management [24].

macrophyte community [25].

The effects of grass carp on plants and water quality are highly variable and often inconclusive due to the lack of proper control sites. The proportion and rate of plant removal by the grass carp is crucial. Changes in water quality as a result of plant removal by the grass carp mostly occur in small, non‐flowing water bodies and least occur when only a small proportion of plants is removed from large, relatively deep, flowing reservoirs. In this concept, decreases can be observed in oxygen concentration of water following grass carp stocking, depending on the disappearance of macrophytes [31]. Primary producers such as phytoplankton and aquatic macrophytes not only release oxygen but also consume CO<sup>2</sup> during photosynthesis, which results in an increase in water pH. Changes in oxygen concentrations following grass carp stocking were positively correlated with the changes in pH [32].

Higher stocking densities of grass carp or their longer impact can increase concentrations of nutrients in the water, but these increases are mainly dependent on the water‐body characteristics. These changes result from sediment resuspension during feeding and faecal matter deposition by carp as well as collapse of mechanisms responsible for maintenance of the vegetated state due to removal of macrophytes. Changes in benthos corresponded closely to changes in aquatic vegetation which stabilize sediments and provide additional substrate in the form of root masses and decaying material. Zoobenthos also responded to changes in water quality following removal of aquatic macrophytes [33].

The rate of aquatic plants elimination determines the magnitude of impact [30, 34]. These changes in water quality are often followed by algal blooms [35] which in most lakes signal a shift to an alternative stable state [36]. Increasing rates of nutrient cycling following resuspension of sediments lead to decreases in ecosystem stability [37].
