**2. The grass carp**

Grass carp (*C. idella*), also known as the white Amur, is one of the most important farmed freshwater fish species with an annual global production of 5,537,794 tons in the year of 2014 [4]. The grass carp is one of the largest members of the family *Cyprinidae* and is the only member of the genus *Ctenopharyngodon* [5, 6]. It shouldn't be confused with other carp species such as silver carp (*Hypophthalmichthys molitrix*), bighead carp (*Hypophthalmichthys nobilis*) or mud carp (*Cirrhinus molitorella*) These carp species are not good biological control agents for aquatic vegetation because they feed on different components of the pond ecosystem.

Grass carp is native to southeastern Russia and northwestern China. This herbivorous species has been deliberately introduced into many countries for vegetation control purposes. In addition, the grass carp is an integral part of fish culture and forms an important source of protein for human consumption.

#### **2.1. Distribution of grass carp**

Controlling and eliminating aquatic vegetation from ponds are often confusing and frustrating tasks. The selection of a vegetation control program depends on local conditions of the pond. For this aim, there are three approaches including mechanical, chemical and biological control. First one is the mechanical control which involves physical removal of the vegetation and is often more difficult in water than on land. Second, chemical vegetation control is often unsuccessful, and retreatment may be needed. Also, chemical vegetation control can become expensive, and the selection of a chemical depends on the plant species involved. In addition, chemical vegetation control is short lived due to most of aquatic herbicides that do not persist

For these reasons, the ideal aquatic plant management tool should provide cost effective control with long‐term impact, a high level of selectivity and if possible have minimal or no negative side effects. Another alternative control method to mechanical or chemical vegetation control is biological control which involves using of fishes to control the aquatic vegetation. Biological control has many advantages over the other vegetation control means. For instance, it takes much less human work effort than most of mechanical control means and does not require using expensive and hazardous aquatic herbicides. In addition, using fish species provides longer term control than other control mechanisms due to fishes that usually have a

Fish used for aquatic vegetation control include several species of tilapia (*Tilapia* spp.), silver carp (*Hypophthalmichthys molitrix*) and the grass carp (*Ctenopharyngodon idella*). Of these fish, only the grass carp is able to consume large quantities of aquatic macrophytes [2]. Under suitable conditions, adult grass carp can consume more than its own weight of plant material on

From this point of view, controlling aquatic vegetation with grass carp is one of the available options for pond owners with aquatic plant problems. In many situations, using grass carp is

Grass carp (*C. idella*), also known as the white Amur, is one of the most important farmed freshwater fish species with an annual global production of 5,537,794 tons in the year of 2014 [4]. The grass carp is one of the largest members of the family *Cyprinidae* and is the only member of the genus *Ctenopharyngodon* [5, 6]. It shouldn't be confused with other carp species such as silver carp (*Hypophthalmichthys molitrix*), bighead carp (*Hypophthalmichthys nobilis*) or mud carp (*Cirrhinus molitorella*) These carp species are not good biological control agents for

Grass carp is native to southeastern Russia and northwestern China. This herbivorous species has been deliberately introduced into many countries for vegetation control purposes. In addition, the grass carp is an integral part of fish culture and forms an important source of protein

aquatic vegetation because they feed on different components of the pond ecosystem.

more than a few months.

30 Grasses - Benefits, Diversities and Functional Roles

life‐span of several years.

an economical, long‐lasting and effective option.

a daily basis [3].

**2. The grass carp**

for human consumption.

Grass carp is a sub‐tropical‐to‐temperate species and is native to large rivers and lakes in eastern Asia. Its native range extends from southern Russia southward to northern Vietnam and in large rivers like the Amur (border of China and Russia), Yang Tze (northern China), Yellow River (central China), and the Min River (crosses the border from Vietnam into China) [5].

In addition, grass carp have been introduced to many countries around the world including Taiwan, Israel, Japan, the Philippines, the United States, Mexico, India, Malaysia, the Netherlands, Switzerland, Czechia, Slovakia, Denmark, Sweden, Romania, Poland, Italy, West Germany, France, the United Kingdom, Argentina, Venezuela, Fiji, New Zealand, Australia and South Africa [5].

Grass carp are considered uncommon in their Amur basin native range, relative to other species of Asian carps. There is a broad range of climatic conditions within the native range of the grass carp. The mean annual air temperatures range from 25°C (in the southernmost part of the hemisphere) to −60°C (in the northernmost part of the hemisphere) [5].

#### **2.2. Biological features of grass carp**

Grass carp is characterized with a wide and scale‐less head, sub‐terminal or terminal mouth with simple lips which do not include barbels, protracted upper jaw and a very short snout [7–9].

The body is slender and rather compressed with a rounded belly and slightly decurved lateral line [9]. Dorsal fin origin is above or just in front of the pelvic fin origin and the dorsal and anal fins do not have spines [5, 10]. Cycloid scales are dark‐edged with a black spot at the base, and the gill rakers are short, lanceolate and widely set [7, 9]. Pharyngeal teeth are bi‐serial and are 2.5–4.2, 2.4–4.2, 2.4–5.2 or 1.4–5.2 [5]. Diploid chromosome number is n = 48 and biochemical analysis of five tissues revealed an estimated 49 loci [9]. The colour of adult grass carp is dark grey on the dorsal surface with lighter sides (white to yellow) that have a slightly golden shine. Fins are clear to grey‐brown colour [7].

#### **2.3. Reproduction of grass carp**

Mature grass carp require approximately 1500 to 2000 days within a year for gonadal development and maturation [11]. Maturity occurs at earlier ages and smaller sizes in tropical climates [5] which is between the ages of 1 to 8 years in the introduced and cultured grass carp populations. Grass carp males generally mature 1 year earlier than females at 50–86 cm in length [5].

In grass carp, the external sexual dimorphism appears in adults at the onset of maturity with the appearance of tubercles on the dorsal and medial surfaces of the pectoral fins in males. Temporary tubercles may develop in females, but they are not as highly developed as in the males. Females exhibit soft, bulging abdomens and swollen, pinkish vents at onset of maturity [5].

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

*spicatum*), spatterdock (*Nuphar advena*), fragrant water lily (*Nymphaea odorata*), sedge (*Cladium* spp.), cattail (*Typha* spp.) or other large aquatic plants [18]. Factors such as age, size, temperature, availability of plant species, size of waterbody and stocking density (in pond cultures) may

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

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While active feeding begins at 7–8°C, intensive feeding occurs only when water temperature is at least 20°C [16]. Three or four days after hatching, larval grass carp begin feeding on rotifers and protozoans, moving up to larger cladocerans at 11–15 days after hatch [9, 14]. By 2 weeks after hatching, grass carp feed on larger prey such as daphnia and insect larvae [9, 14]. After 3 weeks, the occurrence of plants in the diet increases with the appearance of filamentous algae and macrophytes. Macrophyte feeding begins from 1 to 1.5 months after hatching [9]. However, juveniles consume other items including chironomids, cladocerans, copepods,

Studies indicate that grass carp lose weight when kept in unvegetated ponds with sufficient animal food sources [19]. When the supply of macrophytes is low, adult grass carps are able to utilize other food sources including benthos, zooplankton, water beetles and crayfishes [16]. Lopinot [20] indicated that grass carp feeds on almost anything when vegetated food is scarce including small fishes, worms and insects, but in pond culture, they seem to prefer pelleted

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

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

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 know-

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

cases is the introduction of grass carp to these waters covered with plants.

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

influence grass carp feeding strategies [9].

structure of the benthos should be noted.

ing the consumption rates of the fish.

food to vegetation.

**3. The method**

insects and their aquatic larvae, crustaceans and small fishes [6].

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

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

#### **2.4. Feeding behaviour of grass carp**

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

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 aquatic vegetation in lakes and ponds [7].

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*  *spicatum*), spatterdock (*Nuphar advena*), fragrant water lily (*Nymphaea odorata*), sedge (*Cladium* spp.), cattail (*Typha* spp.) or other large aquatic plants [18]. Factors such as age, size, temperature, availability of plant species, size of waterbody and stocking density (in pond cultures) may influence grass carp feeding strategies [9].

While active feeding begins at 7–8°C, intensive feeding occurs only when water temperature is at least 20°C [16]. Three or four days after hatching, larval grass carp begin feeding on rotifers and protozoans, moving up to larger cladocerans at 11–15 days after hatch [9, 14]. By 2 weeks after hatching, grass carp feed on larger prey such as daphnia and insect larvae [9, 14]. After 3 weeks, the occurrence of plants in the diet increases with the appearance of filamentous algae and macrophytes. Macrophyte feeding begins from 1 to 1.5 months after hatching [9]. However, juveniles consume other items including chironomids, cladocerans, copepods, insects and their aquatic larvae, crustaceans and small fishes [6].

Studies indicate that grass carp lose weight when kept in unvegetated ponds with sufficient animal food sources [19]. When the supply of macrophytes is low, adult grass carps are able to utilize other food sources including benthos, zooplankton, water beetles and crayfishes [16]. Lopinot [20] indicated that grass carp feeds on almost anything when vegetated food is scarce including small fishes, worms and insects, but in pond culture, they seem to prefer pelleted food to vegetation.
