China has a vast freshwater system, with inland rivers, lakes, reservoirs, and ponds throughout the country. Because of this, China is rich in fishery resources. There is a wide variety of fishes, of which more than 50 per cent are carps. Until now, about 30 species have been cultured in the ponds of integrated fish farms. This introduction focuses on four well-known carps: silver carp, bighead, grass carp, and black carp. Other good stocks, i.e., common carp, crucian carp, Chinese bream (Wuchang fish), mud carp, tilapia, etc., are also discussed. Understanding the habits of fish, mastering the laws of their growth, development, propagation, and feeding, and satisfying their ecological requirements will be of great practical significance to fisheries production, the application of farming techniques, and the improvement of fish yields.
Silver carp (Fig. 1.1)
Silver carp belongs to family Cyprinidae, subfamily Hypophthalmichthyiane. Body: compressed. Scales: small. Mouth: in front, with lower jaw slightly slanting upward. Eyes: comparatively small, situated below horizontal axis of body. Gill membrane: not connected to isthmus. Gill rakers: dense, interlaced, connected, and covered with a spongelike sieve membrane. Abdominal keel: extending from the base of pectoral fins to the anus. Pectoral fin: terminal tip does not exceed the base of the ventral fin. Pharyngeal teeth: one row in 4/4, with fine lines and tiny grooves on surface. Intestinal length: 6–10 times body length. Colour of body (alive): silvery white; dorsal colour, very dark brown. The largest specimen found so far was 20 kg.
Fig. 1.1. Silver carp (Hypophthalmichthys molitrix).
Bighead carp (Fig. 1.2)
Bighead is similar to silver carp in shape, and belongs to the same subfamily, Hypophthalmichthys. Head: bigger than silver carp. Snout: short and blunt. Eyes: small, situated below horizontal axis of body. Gill membrane: not connected to isthmus. Gill rakes: dense and separated; without spongelike sieve membrane. Abdominal keel: between the bases of ventral fins and the anus. Pectoral fin: terminal tip reaches one-third to two-fifths of the base of the ventral fin. Pharyngeal teeth: one row in 4/4, surface flat. Intestinal length: about 5 times body length. Colour of body (alive): dorsal and upper sides, light black, scattered with irregular yellowish black spots; ventral surface, silvery white. The largest specimen found so far was 40 kg.
Fig. 1.2. Bighead (Aristichthys nobilis)
Grass Carp (Fig. 1.3)
Grass carp is a large fish, belonging to subfamily Leuciscinae, family Cyprinidae. Body shape: almost cylindrical, with flat head and round abdomen. Scales: big. Mouth: in front; lower jaw, shorter. Gill membrane: connected to isthmus. Gill rakes: small and short, in scattered arrangement. Pharyngeal teeth: two rows in 2,5/4,2, compressed like combs. Intestinal length: 2.3–3.3 times body length. Colour of body (alive): dorsal, grey; abdomen, yellowish white; sides, greenish yellow; fins, a lighter colour. The largest specimen found so far was 35 kg.
Fig. 1.3. Grass carp (Ctenopharyngodon idellus)
Black carp (Fig. 1.4)
Among Cyprinidae, black carp is similar to grass carp. Body shape: like grass carp, but with a pointed head. Scales: big and circular. Mouth: arc-shaped in front. Eyes: medium size, situated in the middle part of head sides. Gill rakes: short. Gill membrane: connected to the isthmus. Pharyngeal teeth: one row in 5/4; big, short, and molarlike; surface smooth. Intestinal length: 1.2–2 times body length. Colour of body (alive): dark grey dorsal, darker; abdomen, light grey; fin, black. The largest specimen found so far was 70 kg.
Fig. 1.4. Black carp (Mylopharyngodon piceus)
Common carp (Fig. 1.5)
China has a long history of culturing common carp, which have a wide distribution and strong adaptability. There have been a lot of morphological variations through the artificial breeding and natural selection of this species: e.g., scale carp, mirror carp, Wu Yuan red purse carp, Xing Guo red carp, etc. Body: compressed. Dorsal: projected in an arc shape. Abdomen: round. Mouth: slightly downward with a long blunt snout and with two pairs of barbels on upper jaw; lower pair a little longer. Dorsal fin: long. Scales: big and thick. Pharyngeal teeth: three rows in 1,1,3/3,1,1; molarlike teeth on inner sides. Intestinal length: 1.5–2 times body length. Colour of body (alive): varying with different living conditions, usually dark grey or yellowish brown on dorsal; sides, golden yellow; lower part of caudal fin, red. The largest specimen found so far was 40 kg.
Fig. 1.5. Common carp (Cyprinus carpio)
Crucian carp (Fig. 1.6)
Crucian carp is close to common carp among Cyprinidae. Body: compressed and relatively thick. Abdomen: round. Head: small and short. Snout: blunt. Mouth; arc-shaped in front. Lip: thick without barbels. Pharyngeal teeth: compressed, one row in 4/4. Intestinal length; 2.7/3.2 times of body length, some even reaching 5 times. Colour of body: silvery grey (alive); darker on dorsal and lighter on abdomen. The largest specimen found was 1.5 kg.
Crucian carp has a wide distribution and a strong adaptability. It can live in different water bodies such as rivers, lakes, ponds and ditches with some variations and differentiations in characteristics.
Fig. 1.6. Crucian carp (Carassius auratus)
Chinese bream (Wuchang fish) (Fig. 1.7)
Chinese bream belongs to subfamily Abramidinae. Body: high, compressed and lozenge-shaped. Head: small and short. Mouth: slanting. Abdominal keel: extending from the base of pelvic in to the anus. Pharyngeal teeth: 3 rows in 2,4,5/4,4,2. Intestinal length: 2.7 times of body length. Colour of body: dark grey (alive), darker on dorsal; scale, dark grey in the middle and lighter on its edge. The largest body found was 3 kg.
Fig. 1.7. Chinese bream or Wuchang fish (Megalobrama amblycephala)
Mud carp (Fig. 1.8)
Mud carp belongs to family Cyprinidae, subfamily Barbinae. Body: long and compressed. Abdomen: round and slightly flat. Snout: short, round and blunt. Mouth: inferior and transverse, with two pairs of barbels; snout barbels, strong and thick; jaw barbels, small and short. Caudal fin: deeply separated with upper part a little longer than the lower part. From the upper part of the pectoral fin around the lateral line, there are 8–12 scales with dark dots at their bases that form lozengeshaped spots. Fins: dark grey. Pharyngeal teeth: three rows in 2,4,5/5,4,2. Intestinal length: about 14 times body length. The largest specimen found so far was 4 kg.
Fig. 1.8. Mud carp (Cirrhina molitorella)
Tilapia (Figs. 1.9 and 1.10)
Tilapia belongs to order Perciformes, family Cichlidae. This genus consists of more than 100 species and subspecies. At present, 15 species of Tilapia are being cultured around the world. In China, mainly Oreochromis mossambica and O. nilotica are reared.
Oreochromis mossambica (Fig. 1.9) — Body: short and compressed. Dorsal: a little higher. Shape: similar to crucian carp. Mouth: bigger. Lip: thick, with lower jaw a little longer than upper jaw. Scales: circular. Lateral line: disjointed. Intestinal length: about 7 times body length. Colour of body (alive): dark grey; during spawning stage, male fish is dark green; edges of dorsal, anal and caudal fins, obvious red; female, grayish yellow. The largest specimen found so far was 0.5 kg.
Fig. 1.9. Oreochromis mossambica
Oreochromis nilotica (Fig. 1.10) — Colour: changing with external conditions, light black; abdomen, white, with nine longitudinal black stripes on body surface, of which seven are below dorsal fin and two are on the peduncle; caudal fin with 10 clear vertical black stripes for life. Scales: ctenoid. The largest specimen found so far was 2.5 kg.
Fig. 1.10. Oreochromis nilotica
Nuptial colour can be found on male and female tilapia during the reproductive period. External genital organs of the male and female are different in appearance. The male has two pores: anus in front and urinogenital pore in the rear (Fig. 1.11 A). The female has three pores: anus in front, oviduct opening in the middle, and urinary pore in the rear (Fig. 1.11 B).
Fig. 1.11. A: male, B: female.
Although silver carp, bighead, grass carp, black carp, common carp, and mud carp all belong to Cyprinidae, they have their own methods of food intake and food chain positions at different stages of development. This is due to their longterm adaptation to ecological conditions. The method of food intake and the structure and function of the digestive organs improve as the fish grows.
Silver carp and bighead carp
At the larval stage, silver carp mainly feed on zooplankton. As they mature, their feeding turns to phytoplankton. Bighead, throughout their life, feed mainly on zooplankton. This difference in feeding is due to the differences in structure and density of gill rakers (the filtering organ) between the two species.
The gill rakers of silver carp and bighead are situated in the operculum with four pairs on either side; the fifth pair is on the gill arch, which is specialized into the inferior pharyngoskeleton. The gill rakers and gill filaments are attached to the gill arch bone. The gill rakers of bighead are delicate and sabre-shaped, each consisting of a neck, a stem, and a base. The neck is narrow and short. The stem is the principal part of the gill raker, the dorsal side being thicker than the ventral side. On each side, there is a row of “wartlike”, lateral protuberances (Fig. 1.12). Each lateral protuberance of a gill raker is interwoven with the next. Occasionally, there are opposite protuberances; their triangular bases cling to the gill arch bones. The gill rakers of silver carp, unlike those of bighead, are interconnected by minute bony bridges that are covered with spongy sieve membranes (Fig. 1.12). Gill raker density is higher in silver carp than in bighead.
Fig. 1.12. Structure of the gill raker of silver carp.
There are broad and narrow gill rakers (Fig. 1.13); one broad gill raker occurs for every three to six narrow gill rakers.
Newly hatched silver carp and bighead fry nourish themselves with egg yolk for the first 3 or 4 days. After this, when they are 7–9 mm long, they begin to take in plankton. Until body length reaches 15 mm, the food-filtering organs are imperfect, with short and sparse gill rakers. During this time, silver carp and bighead eat the same food. The major groups of zooplankton that silver carp and bighead eat are rotifers, nauplius of copepods, and tiny cladocerans.
The shape and structure of the filtering organs of fry about 20–30 mm long are generally the same as that of an adult fish. Because silver carp fry have nearly 200 gill rakers - 1 mm in length each, with minute bony bridges between them and sieve membranes covering the bridges to form a fine net - their main food is phytoplankton. The short and sparse gill rakers of bighead fry are separated by larger spaces and, therefore, the tiny phytoplankton are more difficult to detain. As a result, their main food changes from tiny zooplankton to all sorts of zooplankton as they mature.
Fig. 1.13. (A) Broad gill raker. (B) Narrow gill raker.
In addition to the filtering organs (gill rakers), silver carp and bighead have accessory organs: palatine folds. Palatine folds, located at the apex of the mouth cavity, consist of nine vertical ridges of mucous membrane, four on either side and one in between. The middle ridge is short and resembles an inverted “Y”. Palatine folds act in coordination with gill rakers in filtering the food.
Under culture conditions, silver carp and bighead can also eat commercial feeds such as cakes, brans, and dregs.
Grass carp and black carp
The fry of both grass carp and black carp under 15 mm in length feed mainly on zooplankton. However, the feeding habits of fry larger than 20–30 mm are different. At this time, the fry of grass carp begin to consume tender aquatic plants and the fry of black carp start to feed on benthos such as snails and Corbicula spp.
Grass carp is a typical herbivorous species, consuming all sorts of aquatic and terrestrial grasses; hence, the pharyngeal teeth are well developed, tough, and strong (tooth formula: 2,5/4,2). The teeth are shaped like choppers with saw-toothed edges. Pharyngeal teeth at both sides are interlaced and are against the callous pad of the basioccipital, grinding food into pieces for digestion in intestines. Grass carp are voracious eaters, but cannot digest plant cellulose.
Black carp is a carnivorous species and usually feeds on molluscs such as snails, clams, and Corbicula spp. By using its pharyngeal teeth and callous pad, they crush the hard shells, spit them out, and swallow the meat. Their pharyngeal teeth are strong, tough, and molar-shaped (tooth formula: 4/5).
Under culture conditions, both species are omnivorous, feeding on oil cakes, brans, dregs, and animal feeds such as silkworm pupae, earthworm, and animal entrails.
Common carp and crucian carp
At the larval stage, the feeding habits of common carp and crucian carp are basically similar. They chiefly feed on rotifers, cladocerans, copepods, chironomid larvae, and other insect larvae. Common carp and crucian carp about 50 mm in length are omnivorous. Common carp are inclined to be more carnivorous, whereas crucian carp are more herbivorous. Both species have feeding habits of phagotrophy.
Common carp has relatively well-developed, molar-shaped pharyngeal teeth in three rows, with transverse grooves on the rest of the inner row except the first tooth, which is smooth. This species feeds on a wide range of foods. Their common natural foods are benthos such as snails, young clams, Corbicula spp., cladocerans, copepods, chironomid larvae, shrimps, and insect larvae. They can also consume the detritus of higher aquatic plants and plant seeds. The nasal bone of common carp is well developed, enabling them to project their premaxilla and mandible like a tube and dig in the mud for organic detritus.
Crucian carp chiefly feeds on large amounts of detritus, diatoms, filamentous algae, aquatic grasses, and plant seeds. Other foods include cladocerans, copepods, chironomid larvae, and water earthworms.
Under culture conditions, common carp and crucian carp also consume commercial feeds such as oil cakes, brans, crops, and silkworm pupae.
Chinese bream (Wuchang fish)
At the larval stage, Chinese bream feeds mainly on zooplankton such as cladocerans and copepods. At the adult stage, it feeds mainly on aquatic grasses such as Vallisneria spiralis, Hydrilla verticillata, and Potamogeton malainus. Secondary food include Potamogeton crispus, Myriophyllum spicatum, Spirogyra, and plant detritus. With a small mouth and small and weak pharyngeal teeth and callous pad, their ability to search for food and the intensity of food intake are much less than those of grass carp.
With a small mouth, transverse in inferior position, under natural conditions, mud carp uses the bony edges of its upper and lower jaws to scrape diatoms, green algae (Chlamy domo nas), and filamentous algae off stones. Mud carp also commonly feeds on the detritus of higher plants, bottom humus, and zooplankton.
Under culture conditions, mud carp consumes commercial feeds such as oilcakes, dregs, brans, and animal manure.
Tilapia is omnivorous with a tendency to be herbivorous. At larvae stage, it feeds mainly on zooplankton. The scope of food widens as the fish grows. Common foods include all kinds of planktonic, benthic, and epiphytic algae, tender higher aquatic plants, all organic detritus, and some animal material (earthworms, small shrimps, and aquatic insects).
Oreochromis, possessing denser gill rakers (24–31), is more likely to feed on phytoplankton and could utilize some of green algae, Chlorophyta, and bluegreen algae, Cyanophyta, which can hardly be digested by other fishes. However, because O. mossambica has only 14–19 gill rakers, it consumes mainly detritus. Under culture conditions, both species can feed on all kinds of vegetable leaves, tender grass, animal manure, bran, oil cake, and pelleted feeds. Their ability to search for and consume food is dependent on temperature.
Growth rate is an important criterion in the evaluation of production efficiency. Silver carp, bighead, grass carp, black carp, and common carp, with their larger size and speedy growth, are the dominant cultured species for polyculture in integrated fish farms. Crucian carp, wuchang fish, and mud carp, however, owing to their smaller size and slower growth, are usually regarded as secondary species for polyculture. Nevertheless, these secondary species are also important in the improvement of fish yield.
Growth rates are genetically controlled, as well as being closely related to water quality, water temperature, nourishment, stocking density, and management. As a rule, growth rates (length and weight) are faster before the first sexual maturity (Table 1.1); after this, growth slows or even stops.
Table 1.1 Ages (years) at which the primary cultured fish in China experience maximum growth.
|Silver carp||Bighead carp||Grass carp||Black carp||Common carp||Crucian carp||Wuchang fish||Mud carp|
|Maximum length increase||2||2–3||1–2||1–2||1–2||1||1–2||1–2|
|Maximum weight gain||3–6||3||2–3||3–4||4–5||4–5||2||5–6|
Silver carp, bighead, grass carp, black carp, and mud carp
The natural spawning grounds of silver carp, grass carp, and black carp are vastly distributed in Pearl River, Qiantangjiang River, Changjiang River, Huaihe River, and northwards up to the Heilongjiang River systems. Those of bighead are mainly in Changjiang River, Huaihe River, and Pearl River. Mud carp, as a subtropical species, has its spawning grounds in the southern part of China: Hainan Island, Guangdong, Guangxi, Fijian, and Yunnan. Among all these river systems, the spawning grounds of Changjiang River and Pearl River are the biggest. Fry of silver carp, bighead, grass carp, and black carp proliferate in the Changjiang River and Pearl River systems. For feeding, Chinese carp usually prefer the lower reaches of rivers, river branches, or lakes, where the water flows slowly and is abundant with food. When the spawning season draws near, spawning schools begin to gather and migrate toward the middle and upper course spawning grounds. At this time, the gonads of the brood fish have, in most cases, reached stage IV and the gonads of the male fish have reached stage V. If the ecological conditions in the spawning grounds are suitable for reproduction, the fish will spawn.
Spawning occurs in the summer and varies with climatic condition. In the Changjiang River drainage, silver carp and grass carp generally start spawning in late April or early May; bighead begins to spawn in middle or late May. In the Pearl River drainage, spawning season begins in middle or late April; bighead spawns a little later. As a rule, the spawning season in the north is later than that in the south by 1 or 2 months.
During the spawning season, the mature brood fish is ready to spawn when the water level rises and the temperature is between 18 and 30°C. The optimum temperature for spawning is 22–28°C. As a subtropical species, mud carp needs a little higher temperature for spawning; its optimum spawning temperature ranged from 26 to 30°C. There are two common patterns of spawning. Spawning on the surface is termed “floating spawning” and underwater spawning is called “muffled spawning”. In floating spawning, the male chases the female excitedly, often bumping against the abdomen of the female with its head, jumping out of the water, and then splashing into the waves. Sometimes, both the male and the female would float on their backs with their pectoral fins vibrating violently. When the climax of estrus is achieved, the female and male discharge their eggs and milt, respectively. The eggs are fertilized in the water. In muffled spawning, all the chasing occurs underwater. There is usually an overwhelming majority of male fish on the spawning ground (Table 1.2). The age and weight distribution of spawning schools varies with the region. In the Pearl River drainage, fish are generally smaller and mature earlier than in the Changjiang River drainage (by 1 year). The fish in the Heilongjiang River drainage mature 1 or 2 years later than in the Changjiang River drainage.
The fecundity of silver carp, bighead, grass carp, and black carp is measured by sampling. Both the absolute and the relative fecundities can be determined. The former refers to the total number of young born to one fish in one spawning season; the latter refers to the number of young born per gram of female body weight in one spawning season. There is no relationship between the number of eggs per gram of ovary and the size of the fish.
The eggs of silver carp, bighead, grass carp, black carp, and mud carp are separated and nonadhesive. The discharged eggs expand by absorbing water through the egg membrane and become plump, transparent, and elastic. Having a greater specific gravity than water, they sink to the bottom in still water; yet, they are semi-buoyant in a current, floating until the fry hatch. The eggs must be incubated in the same temperature range as that required by brooders to spawn: 18–30°C; the optimum incubation temperature is between 22 and 28°C. The speed of embryonic development is directly related to water temperature.
Table 1.2. Sex composition of silver carp and grass carp spawning schools in three spawning grounds.
|Item||Changjiang River||Xijiang River||Songhua jiang River||Average (%)|
M = male,
F = female
Table 1.3. Absolute and relative fecundity and maturity rate of silver carp, bighead, grass carp, and black carp in the Changjiang River.
|Average absolute fecundity|
|Average relative fecundity|
|Egg (g) ovary||Averagematurity rate*|
* Maturity rate (gonad or ovary weight/body weight) × 100
Common carp and crucian carp
With their wide distribution, common carp and crucian carp do not require exact environmental conditions for gonad development and reproduction; therefore, they naturally reproduce in the still or running waters of Southern and Northern China.
Common carp and crucian carp spawn from the end of March to early April in central and central eastern part of China, from April to May in northern China, and in June in northeastern China; however, spawning begins as early as late December in the Pearl River basin. Although the spawning season begins at different times, water temperature requirements are identical, at least 18°C for common carp and at least 20°C for crucian carp. Because the eggs of common carp and crucian carp are adhesive, the fundamental requirement for spawning is the presence of substrata (e.g., aquatic plants) that the eggs can adhere to.
Spawning activity proceeds from midnight to dawn. If environmental conditions are favourable, however, these fish can spawn the whole day. In estrus, two or three males chase one female; the male repeatedly hits its head against the female's abdomen until the female is lying on its side adjacent to some aquatic plants. The female then spawns while the male discharges milt. This action occurs repeatedly so that the mature eggs stick to the aquatic plants. Age and body size at sexual maturity vary with environment and climate. In the Changjiang River and Yellow River basins, common carp and crucian carp generally reach maturity in 2 years. In northeastern China sexual maturity arrives later when the fish is larger. The fecundity is related to fish size. The average fecundity of a 3-year-old, 44–48 cm, 1-9-2.75 kg common carp is around 244,000 pieces. For a 5-year-old, 54 cm, 3.5 kg common carp, the fecundity is around 447,000 pieces. A 0.5–1 kg crucian carp has an average fecundity of 200,000–300,000 pieces. At 20°C, the fertilized eggs of common carp take 101–104 h to incubate; at 25°C, 49–53 h; at 30°C, 47–50 h.
Silver carp, bighead, grass carp, black carp, common carp, crucian carp, and mud carp live in different water layers because of their different feeding habits. Silver carp and bighead chiefly feed on plankton and dwell mostly in the upper water layer where plankton is abundant. Grass carp and wuchang fish prefer to search for food in the upper and middle water layers or near the river bank, side lake, or pond dike. Black carp, common carp, crucian carp, and mud carp prefer the bottom layer because they are benthos feeders.
Fish are poikilothermic (cold-blooded); their metabolism is dependent on temperature. Feeding diminishes when the temperature drops below 15°C and stops below 5–7 °C. As a whole, carp are very temperature resistant. They can live between 0.5 and 38°C, with an optimum range of 25–32°C; yet, the optimum temperature range for food intake and growth is 25–32°C (30–32°C for mud carp). Silver carp, bighead, grass carp, black carp, and wuchang fish begin to die when the temperature falls below 0.5°C or rises above 40°C. Mud carp, O. niloticus, and O. mossambicus, however, often freeze to death below 7.8 and 12°C, respectively.
Silver carp, bighead, mud carp, and tilapia feed on plankton and, therefore, habituate themselves in fertile water. Grass carp and wuchang fish feed on grasses. Black carp feed on molluscs and, therefore, prefer sheer water. Common carp and crucian carp, with their strong adaptability, can live in water bodies with different fertilities.
Dissolved oxygen and pH
Silver carp, bighead, grass carp, and black carp prefer a slightly alkaline environment (pH 7.5–8.5). Environments outside this pH range will retard growth.
A fish must be able to carry out normal gas exchange and demands a certain amount of dissolved oxygen. The higher the dissolved oxygen content (DOC), the greater the feeding intensity. With the DOC above 4 or 5 mg/L, feeding is intense, growth is fast, and the food-conversion factor is low. With the DOC below 2 mg/L, fish lose their appetite; below 1 mg/L, fish stop feeding and gasp for air; below 0.5 mg/L, suffocation and death normally result.
In artificial propagation, sexual maturity, ovulation, spawning, and fry incubation are all artificially controlled. The gonad of the female is the ovary; that of the male is the testis. At sexual maturity, eggs and sperm develop in the ovary and testis, respectively. The gonads exhibit a cyclical variance in its development, which is controlled by various external, ecological factors and the endocrine and nervous systems. These external and internal factors are associated and the former may restrict the latter.
Structure of the Ovary and the Ovum
Fish have one pair ovaries which are saclike and located in the body cavity symmetrically. Their walls are formed by connective tissues and smooth muscle. The inner wall of the ovary protrudes and forms the septum (ovum-producing plate) which is responsible for producing the ova. Following sexual maturity, the follicular membranes break and the ova drop into the ovarian cavity. At the end of the ovarian cavity there is a short oviduct that opens to the exterior of body. There are blood vessels and nerve branches on the ovarian tissue.
Fish ova possess all the constituents common to cells: cytoplasm, nucleus, cell membrane, etc. At the initial stage of development, the sphere-shaped egg cells only have a nucleus and cytoplasm. As they develop, the eggs accumulate yolk material. The egg yolk contains the nutritional materials essential for embryonic development (protein, fat, glycogen, vitamins, etc.). Eventually, there is much more egg yolk than egg cytoplasm. The distribution of yolk and cytoplasm in the egg is polarized. Except for the yolk, most of the egg constituents (cytoplasm, nucleus, etc.) are concentrated around the “animal pole”. Opposite the animal pole is the “vegetal pole”. It is here that the yolk, with its greater specific gravity, is concentrated. After fertilization, the first mitotic division occurs and embryonic development begins. The egg yolk neither takes part in nor hinders mitosis. The nucleus consists of a nuclear membrane, nucleoli, nuclear fluid, and chromosomes. The nucleus is sphere-shaped or leaf-shaped. Its function is to maintain the genetic material (deoxyribonucleic acid, DNA), which controls cellular metabolism and is passed from one generation to the next.
The cell membrane covers the exterior of the egg cell and can be divided into three distinct membrane systems, depending on the origin of their material. The primary cell membrane, or yolk membrane, is made of the cell's own plasm, with many radiant, ductlike pores (“radiant belt”), which are involved in the absorption of nutrients and the discharge of metabolic wastes. The secondary cell membrane, or chorion, is secreted by follicle cells in the ovary. This membrane is normally adhesive. It is common in the adhesive eggs of common carp, crucian carp, and wuchang fish. The tertiary cell membrane is composed of a secretion from glands in the oviduct. It is prevalent in the gummy cell membranes of frog and cuttlefish. Most fish do not have all three membrane systems. Silver carp, bighead, grass carp, and black carp only have the primary cell membrane and common carp, crucian carp, and wuchang fish have primary and secondary membranes.
Development of the Ovary
The stages of ovary development can be examined by visual observation or histological survey. Fish gonad development may be divided into six stages according to appearance, colour, size, weight, blood vessel distribution, and ova maturity. However, the classification of ovary developmental stages varies from country to country. Five stages are recognized in India, Japan, and the United States of America; several countries recognize seven stages; and, in China, six stages are defined. Silver carp is used as an example to describe these six stages.
Stage I ovary
By visual observation, the gonads are located at the lower part of the air bladder, closely attached to the coelomic membrane, and are lineal in shape, transparent, and flesh white in colour. It is impossible to distinguish the sexes with the naked eye.
Tissue section — Cells are tiny; diameter, 12–22 μm. The nucleus is rather large, occupying more than half of oocyte's diameter. There are few nucleoli in the centre of the nucleus (Fig. 1.14).
Fig. 1.14. Tissue section of stage I ovary of silver carp.
Stage II ovary
Ribbon-shaped, flesh white, semitransparent gonads are observed. With the naked eye, it is impossible to distinguish one ova from another; however, small eggs are visible when the tissue is examined under a magnifying glass; when fixed, the eggs are petal shaped. At this stage, it is possible to distinguish visually the sexes. The gonad index (percentage of gonad weight to body weight) is 1–2 per cent.
Tissue section — Cells are multiangular or sphere-shaped; diameter 90–300 μm. A thin layer of flat follicle cells surrounds the oocyte. The nucleoli are closely attached to the nuclear membrane (Fig. 1.15).
Fig. 1.15. Tissue section of stage II ovary of silver carp.
Stage III ovary
By visual observation, the capacity of the ovary has become conspicuously enlarged. Due to the appearance of melanotic pigment, the colour of the ovary changes to greenish grey. Eggs are visible with the naked eye but not easily separable. The distribution of blood vessels is clear. The gonad index is 3–6 per cent in this stage.
Tissue section — The follicular membrane surrounding the oocyte is a bilayer. The egg yolk begins to form. One or two layers of vacuoles appear on the edge of the cell. The cell is 250–500 μm in diameter. The nucleus in the centre is irregular or oval-shaped (Fig. 1.16.). Most of the nucleoli are distributed along the edge of the nuclear membrane; a small number is scattered in the centre of the nucleus.
The gonads of mature brooders are generally at stage III in the winter.
Stage IV ovary
The ovary is now long and saclike, occupying one-third to one-half of the coelomic cavity. Eggs are plump, greenish grey or light yellow, and can be easily separated. The ovary is fully distributed with blood vessels, and the gonad index is 12–22 per cent.
Fig. 1.16. Tissue section of stage III ovary of silver carp.
Tissue section — Egg yolk granules fill almost all the space outside the nucleus, with only a little cytoplasm spreading around the nucleus and near egg membrane (Fig. 1.17); diameter 800–1580 μm. The nucleus edge is wavy, with a few nucleoli inserted in the troughs; most of the nucleoli are moving toward the centre of the nucleus.
This stage can be further divided into three substages based on oocyte diameter and nucleus location. Early stage IV: egg diameter, 800 μm; nucleus in the centre. Middle stage IV: egg diameter, 1000 μm; nucleus in the centre or slightly toward the animal pole. Late stage IV: egg diameter, 1580 μm; nucleus at the animal pole (polarization).
Fig. 1.17. Tissue section of stage IV ovary of silver carp.
Experimental data and practical application have shown that mature eggs cannot be obtained by inducement of early stage IV oocytes from silver carp, bighead, grass carp, or black carp. Only in middle and late stage IV, when the nucleus is eccentric or polarized, can mature eggs be acquired; artificial induction of estrus will then succeed. These stages could last as long as 1, 2, or even 3 months, providing that the proper ecological conditions for spawning are not available and that no artificial propagation is performed.
Stage V ovary
In this stage, oocytes enter the ovarian cavity as follicular membranes break, and the eggs are flowing freely. The ovary and the belly of the fish are very soft. A slight pressure on the belly would cause the eggs to flow through the cloacal opening.
Tissue section — Yolk granules begin to fuse. The cytoplasm and the nucleus have moved to the animal pole. The nucleoli concentrate in the centre of the nucleus and the nuclear membrane dissolves. The nucleus looks transparent.
As the oocytes proceed to maturity, the follicle epithelial cells secrete a substance that dissolves and absorbs tissues between the follicular and egg membranes; thus, the eggs can easily be released from the follicles and flow freely in the ovary (ovulation). During spawning, the eggs are released from the body through the cloacal opening.
The oocytes proceed quickly from stage IV to maturity (stage V). In nature, the process may be complete 20–40 h after the rising of the river's water level. When estrus is artificially induced, maturity may be reached in 10–20 h or less. If the follicles discharged immature eggs, the rate of fertilization would be adversely affected. If the follicles did not release the mature eggs, the eggs would become overripe, and the rate of fertilization would certainly be adversely affected. Even if some of these eggs were fertilized, the embryos would not develop normally. In other words, the success of either natural spawning or artificial insemination depends upon knowing exactly the maturity stage and spawning time of the fish. If the mature eggs are not released, the oocytes would degenerate and be absorbed.
Fig. 1.18. Tissue section of stage V egg of silver carp.
Stage VI ovary
At this stage, most of the eggs has been laid. There are still some stage IV oocytes in the ovary. The ovary is slack and noticeably smaller, and blood vessels have become enlarged with lump-shaped extravasated blood.
Tissue section — After ovulation, there are abundant follicular membranes and some undischarged mature eggs in ovary. The undischarged eggs will soon degenerate and be absorbed, forming a semi-transparent, irregular, orange-yellow structure. Many interim oocytes can still be seen (Fig. 1.19).
Fig. 1.19. Tissue section of stage VI ovary of silver carp after spawning
Structure of the Testes and the Sperm
The testes are paired and tubular. They are situated on both sides of the air bladder, attached to the coelomic wall. The mature testes are white, and, inside, there are many irregularly arranged ampullae. The spaces between ampullae are full of connective tissues. The ampullae are composed of many spores or (seminal vesicle sacs). Spore sacs are separated by a thin layer of follicular cells. Each spore sac contains synchronously developing germ cells, and germ cells in various stages of development can be seen in different spore sacs. At the centre of the ampullae, there is a hollow cavity. After the formation of sperm cells, the spore sacs dissolve and the sperms enter this cavity. The terminal end of the testis is connected to a short seminal duct with and opening to the exterior of the body.
Chinese carp sperm cells consist of a head, a neck, and a tail (Fig. 1.20). The sperm head of silver carp is almost spherical, 2.2–2.5 μm in diameter, consisting of an apex and a nucleus. The apex is situated at the front part of the head. It is also called the “penetrator” because of its function, which is to penetrate the egg membrane. The neck is very short and situated between the head and the tail. The sperm neck of silver carp is about 1.1 μm long. The tail, narrow and long, is many times longer than the head: about 35 μm. The tail is the metabolic centre and motor organ of the sperm.
Mature sperms congregate in ampullae cavities after the disintegration of the spore sacs. Sperms mix with the fluid secreted by the interstitial cells in the testis, forming milt. This milt is exuded or pressed out of body at the climax of brooders' estrus; 1 mL of silver carp milt holds approximately 48 million sperms. The total amount of milt exuded from one male spawner could reach 30–40 mL.
Fig. 1.20. Structure of a sperm:
Development of the Testis
Like the ovary, the development of the testis may be divided into six stages.
Stage I testis
Testis are lineal in shape, transparent, and closely attached to the coelomic wall. At this stage, it is impossible to distinguish between the sexes. On the tissue slice, scattered spermatogonia, 16 μm in diameter, may be observed. The nucleus is big and round, 9 μm in diameter. Ampulli and seminal vesicles are still forming; therefore, there is no clear, fixed arrangement of sperm cells.
Stage II testis
Testis are lacelike and either translucent or opaque. Blood vessels are not clearly visible. Characteristic of this stage are the multiplication of spermatogonia and the formation of seminal vesicles, which are arrayed in bundles. At this stage, ampulli are solid and separated by connective tissue.
Stage III testis
Testis are rod-shaped, pink or yellowish, and elastic on the surface, with a clear distribution of blood vessels. On the tissue slice, a hollow cavity may appear in the middle of the solid ampullae, with one or several layers of seminal vesicles on the ampullar walls.
Stage IV testis
Testis are milky white with a clear distribution of blood vessels on the surface. It is impossible to squeeze out milt early in this stage, but becomes possible later in stage IV. On the tissue slice, some large primary spermatocytes, smaller secondary spermatocytes, and smallest spermatids can be observed; all of these cells congregate on the walls of the seminal vesicles with a small number of sperms.
Stage V testis
Testes are white and full of milt. The milt will flow out through the cloacal opening if the male's head is taken up and its belly is slightly pressed. A large number of sperms, both mature and in various stages of development, can be seen inside the ampulli on the tissue slice.
Stage VI testis
The volume of the testes has greatly decreased after milt exudation, and the testes are now yellowish white or pink. Only spermatogonia, some primary spermatocytes, and connective tissue remain in the seminal vesicles. After milt exudation, the testes revert to stage III and redevelop from there.
Sexual Cycle and Maturity Age
There is a certain sexual cycle in the development and maturity of fish gonads. When a fish reaches sexual maturity and spawns or discharges milt for the first time, its gonads begin to develop cyclically with the seasons. This is termed the sexual cycle. In pond fish culture, the sexual cycles of silver carp, bighead, grass carp, black carp, and mud carp are essentially the same. In nature, these fish spawn once a year. In southern China, the climate is warmer. Through intensive culture, pond-reared spawners, after spawning in the spring, can reach maturity in the same year and be induced to spawn two or three times a year.
Both sexual maturity and gonad development are tightly associated with environmental conditions (i.e., water temperature, food availability, DOC, etc.). Accordingly, the sexual cycle varies from region to region. Even in the same region, gonad development may vary because of environmental conditions.
Cyclic variance of ovary development
Over the winter (November-January), the ovary of silver carp in Zhejiang Province develops from stage I to stage III with a gonad index of 5–6 per cent. Lipid accounts for about 3.5 per cent of the fish's body weight. At this time, old-generation eggs are degenerating and being absorbed. The new oocytes begin to accumulate egg yolk.
In the spring (February-April), the ovary develops from stage III to stage IV, with a gonad index of 5–10 per cent. The lipid content decreases gradually as the new oocytes accumulate egg yolk. Beginning in early April, the egg cells begin to grow quickly and the fish belly becomes expanded gradually. The ovary enters stage IV.
In the summer (May-July), the ovary grows quickly with obvious increases in weight, developing from stage IV to maturity with a gonad index of 17–20 per cent. The lipid content decreases sharply and the belly is soft and expanded. If the water temperature is favourable, the fish may be induced.
In the autumn (August-October), after ovulation, the ovary is in atrophy, with a gonad index around 10 per cent. The majority of those ovaries that have not ovulated show apparent degeneration, returning quickly from stage IV to stage II. In the winter, the ovary develops again from stage II or stage III. The nutritive materials such as lipid will gradually accumulate for the next sexual cycle. Because mud carp is a subtropical fish, their gonad development requires a higher temperature. The gonad develops slowly in the winter when the ovary is at stage II. Up to next April (26°C water temperature in southern China), the gonad develops quickly to stage IV (Fig. 1.21).
Cyclic variance of testes development
Testes develop earlier than ovaries. In the winter (November-January), testes are in stage III and the maturing rate is 0.2–0.4 per cent. By the time spring (February-April) arrives, testes have developed to stage IV and the gonad index is 0.2–1.5 per cent. The testes reach maturity (stage V) in the summer (May-July), and the gonad index is 1.6 per cent. In the autumn (August-October), after milt has been released, the testes are in stage VI, the gonad index is 0.6 per cent, and the remaining sperms are absorbed. The spermatocytes of a new generation begin to grow and the testes begin a new sexual cycle (Fig. 1.22).
Fig. 1.21. Annual variation of gonad development index of silver carp cultured in pond.
-------- Water Temperature
Gonad Development Index (%)
Under different geographical and ecological conditions, the maturity age of the same species is widely different. The maturity age of male and female silver carp, bighead, and grass carp in southern China is 1–2 years earlier than that in northern China (Table 1.4). However, even in the same region, maturity age varies with ecological conditions. For example, in Jiangsu and Zhejiang provinces, the maturity age for female silver carp is 4 years; for grass carp and bighead, 5 years; and for black carp, 7 years. If ecological conditions are optimum, maturity could occur 1 year earlier. Males generally mature 1 year earlier than females.
Fig. 1.22. Annual variation of gonad development index of male silver carp cultured in pond.
---------- Water temperature (°C)
Gonad development index (%)
Table 1.4. Maturity age (years) of silver carp, bighead, grass carp, and black carp reared in ponds.
|Species||Southern China||Central China and Eastern China||Northern China||Northeastern China|
Reproductive capacity refers to the ability of fish producing their offspring. The assessment of reproduction capacity is based on maturity age, sexual cycle, fecundity or amount of the mature germ cells, effective egg production, fry survival rate, etc. Emphasis here is placed on the fecundity and spawning amount of pondreared silver carp, bighead, grass carp, and mud carp.
Chinese carp have many similar features in terms of reproduction (Table 1.5). For example, the gonad index of fully mature female spawners is between 15 and 20 per cent. The fecundity of spawners is generally high. The relative fecundity is 110–140 pieces. Fecundity does not apparently relate to region, but to culture management (mainly nutrients).
Table 1.5. Absolute and relative fecundity and maturity rate of silver carp, bighead, grass carp, black carp and mud carp.
|Average absolute fecundity|
|Average ralative fecundity|
|Average maturity rate|
Under artificial propagation, the average egg production of silver carp, big-head, grass carp, and black carp is 52 pieces/g body weight; the average maximum egg production is 95 pieces/g body weight. Mud carp shows higher egg production because its eggs are much smaller (Table 1.6).
Table 1.6. Egg production of carps under artificial propagation.
|Species||Maximum egg production|
(piece/g body weight)
|Average egg production|
(piece/g gody weight)
Relationship between the Endocrine System and Gonad Development
Just like other vertebrates, all the physiological activities of carp are regulated and controlled principally by the nervous system and the endocrine system, of which the pituitary, or hypophysis, gonad, and thyroid glands are closely associated with gonad development.
The hypophysis of fish is located below and on the ventral side of the thalamencephalon and is attached to the hypothalamus. It is divided into two parts: neurohypophysis and adenohypophysis. The neurohypophysis is directly connected to the hypothalamus, with its nervous fibres and blood vessels planted deep into the adenohypophysis. The adenohypophysis could be divided into anterior (proadenohypophysis), transitional (mesoadenohypophysis), and posterior (motaadenohy-pophysis) lobes. The anterior lobe is situated nearest to the thalamencephalon where there is a little distribution of nervous branches and blood vessels. The transitional lobe is situated at the lower front of the anterior lobe and the posterior lobe is situated at the lower front of the transitional lobe (Fig. 1.23).
Fig. 1.23. Vertical section of a grass carp hypophysis:
The mesoadenohypophysis contains basophils that secrete a sexual hormone: follicle-stimulating hormone (FSH). This hormone stimulates growth, development, maturity, and ovulation of eggs. It promotes the synthesis and secretion of estrogen in the female or the formation of sperm and the secretion of androgen in the male.
The sex-stimulating hormone of the hypophysis varies with age and season. As a rule, the amount of this hormone secreted by mature fish is greater than that secreted by immature fish and is higher before than after spawning. Hypophyseal hormones are used to artificially induce estrus and are effective within species, within genus, and, usually, within family. For example, common carp and silver carp belong to different genera; but the hypophysis of common carp is effective in inducing silver carp and vice versa.
The gonads of fish produce germ cells (sperm cells and eggs) and secrete sexual hormones. The male sex hormone secreted by the testes is called androgen. The female sex hormone secreted by the ovary is called estrogen. The hormones initiate the development of subsidiary sexual organs and secondary sexual character, and are responsible for the sexual behaviour of the fish. Other endocrine glands directly or indirectly affect gonad development. For example, thyroxine, which is secreted by the thyroid gland, stimulates spawning at low temperatures. The adrenal cortex (internal tissue) secretes adrenal cortex hormone. This hormone regulates carbohydrate metabolism and is involved in controlling the salt and water balance.
Function of the Nervous System in Propagation
Successful propagation of a species depends upon the maintenance of a balance between physiological processes and ecological conditions. The realization of this balance relies upon the coordination of the nervous system and the body fluid regulatory system. Gonad development is, to a great extent, controlled by the hypophysis, and hypophysis activity is, in turn, controlled by external factors through the nervous system (Fig. 1.24). The gonad development of silver carp, bighead, grass carp, and black carp from stage IV to spawning stage V is controlled by external, ecological conditions (Fig. 1.24).
When certain ecological conditions stimulate the external sense organs, the nerves of these organs transmit impulses to the central nervous system, which induces the hypothalamus into releasing luteinizing hormone releasing hormone (LRH). This hormone, through the portal vein of the hypophysis, stimulates the basophilous cells in the hypophysis to release luteinizing hormone (LH) and FSH. These hormones reach gonads through blood circulation and promote their growth and development. Meanwhile, the gonads secrete sexual hormones, affecting the hypothalamus and the hypophysis, and initiating sexual activity: i.e., chasing, natural courtship, spawning, and releasing milt.
Influence of Ecological Conditions on Gonad Development
If ecological conditions are favourable, the growth and development of fish will be normal. If the relevant ecological conditions are unfavourable, however, growth and development will be restricted. Excessively unfavourable conditions may result in death. The principal ecological factors are food availability, water temperature, water current, and DOC. These conditions constantly effect fish growth and gonad development.
Only under rational nutritional conditions can the gonads fully develop. At the early developmental stages (II and III) of the ovary, the gonad index is generally 5–6 per cent. As the nutrient substance (egg yolk) accumulates and the fish ingests protein and fat, coverting surplus energy to fat for storage, by late spring (late April or early May), with the increased temperature, the ovary begins to grow quickly and the gonad index rapidly increases to 12–20 per cent. If the rearing conditions for spawners are good and there is sufficient food available in the autumn after spawning, the gonads will mature earlier and the fecundity will be high. If these conditions are not met, gonad development could be restricted or even inhibited.
Fig. 1.24. Role of the central nervous system in controlling the reproduction of Chinese carp.
Among the required nutrients (proteins, fats, carbohydrates, vitamins, and minerals), vitamin E is especially important to gonad development. Table 1.7 shows that supplying vitamin-rich feeds (wheat sprouts, rice sprouts, soybean; and peanut dregs; also, lettuce leaves and Ixeris denticulata) in the early spring improves egg production. An abundant supply of nutritional feeds, however, is not sufficient to ensure proper gonad development. Proper nutrition must be combined with other favourable ecological conditions for the gonads to reach maturity.
Table 1.7. Relationship between feed composition and the fecundity of grass carp.
|No. of fish||Egg production|
(No./kg body weight)
|Corn, rice bran||125||577||51||38,214|
|Wheat sprouts, rice sprouts, soybean, peanut dregs||45||800||22||57,500|
|Rice, rice minced, barnyard grass seed, rice bran||105||240||65||27,236|
Water temperature is a significant factor affecting metabolic rate, maturing age, and the developmental rate of the gonads (Table 1.8). Because of the differences in water temperature and growth period between southern and northern China, silver carp show different maturity ages; nevertheless, the accumulated temperature required for maturity is basically identical: 18,000–20,000 degree days. This demonstrates a positive relationship between gonad developmental rate and water temperature. In northern China, raising the water temperature is an effective technique to ensure gonad maturity and an early induction of estrus during the brood fish culture period.
Table 1.8. Effect of water temperature on maturity age and accumulated temperature.
|Growth perioda (months)||12||11||8||5.5|
|Accumulated temperature during growth period (degree x days)b||9,792||8,250||5,780||3,333|
|Maturity age (years)||2||2–3||3–4||5–6|
|Accumulated temperature during maturity period (degree × days)c||19,584||16,500–24,750||17,340–23,120||16,660–20,000|
|Average accumulated temperature during maturity period (degree × days)||19,584||20,525||20,230||18,315|
Note: a The growth period is counted when the monthly average water temperature is above 15°C;
b Accumulated temperature during growth period (average water temperature during growth period) × (number of days);
c Accumulated temperature during maturity period = (accumulated temperature during growth period) × (maturity age).
Letting fresh water into brood fish rearing ponds at a definite or indefinite time keeps water quality good, which is beneficial to growth. At the same time, it can regulate the composition of the natural food and raise the nutritional level of the brood fish. Running water stimulates gonad development, especially when the germ cells develop to stage IV. It accelerates metabolism and the transfer of stored nutrients to the gonads. Furthermore, experimental data indicate that running water may stimulate the hypothalamus to synthesize and release LRH, which further stimulates the hypophysis to release sex-stimulating hormone. Spawners are then induced into estrus (Table 1.9).
Table 1.9. Effect of running water on spawning and egg fertilization.
|Running water treatment||Body weight of spawners (kg)||Relative fecundity|
(eggs/kg of spawner)
|Fertilization rate (%)|
|Slight, all year||215.0||130,000||100||82|
|Slight, all year||101.5||101,000||100||89|
Slight running water year round maximizes the spawning rate of silver carp. Slight running water year round and fresh running water before spawning have the same positive effects on the spawning rate of grass carp. If there is no running water, however, the fertilization rate drops to 50 per cent or lower.
Oxygen is essential for survival. When the DOC is 2 mg/L, normal physiological activities are drastically reduced and fish gasp for air. Also at this level, the excessive energy consumption by the fish negatively affects gonad development. In such a case, most of the induced brood fish fail to spawn properly. As spring approaches, the demand for oxygen becomes urgent, usually above 4–5 mg/L. If the water is clear with a high DOC, the brooders spawn normally. In brood fish culture, attention must be given to manuring, feeding amount, stocking density, supply of fresh water, and continuously improving the living conditions of the brood fish.
Besides food availability, water temperature, water current, and DOC, other ecological factors that affect gonad development and maturity include light intensity, salinity, and presence of the opposite sex.