2.1. Seaweed
2.2. Molluscs
2.3 Crustaceans
2.4 Echinoderms: Sea Cucumber Biology and Culture
2.5 Marine Fish: Biology and Culture of the Left-Eyed Flounder
In this chapter, the taxonomy, life history and reproductive biology of eight economically important species are presented. A review of the biology of these species provides also the opportunity to present additional details about technologies developed in China for breeding and farming these species. The various cultural systems applied for sea farming and sea ranching are presented in Chapter 3.
These commercially important species belong to five taxonomic groups, as follows:
2.1.1 Japanese kelp biology and culture
2.1.2. Laver biology and culture
Seaweed farming plays a very important role in China’s fishery industry for several reasons:
- Seaweed being autotrophic plants, no feeding is necessary for their growth and physical development. They can directly absorb and utilize nutrients present in their aquatic environment, such as nitrogen, phosphorus and other minerals.This is why, since the early 1950s, China’s government paid more and more attention to the seaweed industry. Seaweed culture became the foundation of sea farming/ranching sustainable development.- Seaweed farming is not only a clean industry, it also has a purification role in its environment, unlike shrimp and fish farming. Decades of field observations have revealed that a red tide never occurs in areas where seaweed are farmed, especially if it is kelp. On the contrary, in some areas where there is no seaweed farming, red tides are frequent.
- Because of the seaweed autotrophy, their farming requires relatively small investments.
- Seaweed have multiple uses, for example as human food, or to produce alginates and iodine, or to feed abalone and sea urchins.
As mentioned earlier (see Section 1.3.2), research on and farming of Japanese kelp (Laminaria japonica) were initiated in the 1940s. Following the development of modern breeding techniques in the early 1950s, great increases in production were gradually possible from 1958.
(a) Taxonomy and life cycle
Kelp (Figure 1) is a brown macroalga belonging to the phylum Phaeophyta. It is the only species of the genus Laminaria occurring in China, although more than 50 species have been reported world-wide and about 20 species are present in the Asia-Pacific region. Seaweed of the genus Laminaria have been given various common names, such as “kelp” in Europe and North America, “kombo” (large cloth) in Japan and “haidai” (sea ribbon) in China.
Kelp grows in temperate cold water zones. Although the species has been known in China for almost 80 years, under natural conditions it occurs only north of the 36°N latitude. In more southern latitudes, high sea water temperatures damage parent breeding stocks during the summer. This natural range was successfully extended southwards to the Fujian Province by using artificial rearing techniques, where young sporelings were grown indoors in refrigerated sea water before being transplanted to outdoor rafts for grow-out. Even with such techniques, commercial kelp farming was successful as far south as the 25°N latitude only. As a result, in China, Laminaria is present either naturally or artificially along the eastern coast of five provinces, from the Liaoning Province in the north to the Fujian Province in the south.
Male gametophyte plants produce male gametes called spermatozoids or antherozoids. Female gametophyte plants produce female gametes (eggs). At fertilization, male and female gametes fuse to form zygotes (2N), which subsequently develop into young sporelings at the beginning of the sporophyte generation (Figure 2).
Figure 2. Life cycle of Laminaria japonica
|
Life history of Laminaria Laminaria exhibits alternation of generations, the sporophyte generation alternating with the gametophyte generation. It also exhibits heterothallism. The sporophyte plant is a large multicelled macroalga whereas the microscopic female and male gametophytes are only one cell or a few cells in size. The asexual sporophyte generation (2N) produces motile zoospores (N), which develop into male and female gametophytes. The sexual gametophyte generation (N) produces male and female
gametes (N). |
1. Zoospore 2. Embryospore 3. Germination 4. Newly formed gametophyte 5a Female gametophyte 5b. Male gametophyte 6a. Mature oogonium 6b. Spermatozoid discharged from antheridia 7. Discharged egg attached to oogonium 8. Motile biflagellate spermatozoid 9. Fertilization 10. Zygote 11. Sporeling 12-13. Young sporophyte 14. Robust sporophyte 15. Mature sporophyteZoospores are produced on the fronds of mature sporophyte plants in sporangial sori or spore sacs. These sori are cup-like structures where cells divide through meiosis to produce haploid (N) male and female zoospores. The pelagic zoospores are motile, having two flagella. When released from the sporangial sori they drift and swim around for 5 to 10 minutes at 15-20°C and up to 48 hours at 5°C. Then they settle on and adhere to a substratum where they develop into male and female gametophytes. These are morphologically dissimilar, the male gametophyte having smaller cells and being more branched than the female one.
After a number of cell divisions, the microscopic male gametophyte plant develops several spermatangia (or antheridia), each one producing a single motile biflagellate spermatozoid, which is released into sea water.
The female gametophyte develops a single large oogonium, which produces an egg. The egg is extruded during ovulation but remains attached to the apical lip of the oogonium. Here the egg is fertilized by a motile spermatozoid, the fusion of male and female gametes producing the zygote (2N). This zygote germinates and develops into a young sporeling, or young seedling, which subsequently develops into a young sporophyte plant.
(b) Artificial sporeling production and farming techniques
Collecting zoospores
The collection of zoospores is made from selected parent kelp plants which are first dried in the shade for a few hours. Then, they are submerged in sterilized and cooled sea water to stimulate the release of zoospores. These attached themselves to a substrate such as bamboo rods or ropes. They are reared in a greenhouse, in cooled and sterilized sea water, where they develop into gametophytes and later, into sporelings.
Collecting season
If zoospores are collected in mid-October, the resulting young sporophyte plants are called “autumn seedlings”, whereas zoospores collected in early July produce young plants called “summer seedlings”. Because of the zoospore collection practice, summer seedlings are produced three months earlier than autumn seedlings, thus gaining three months for additional growth. Such change in the breeding season has resulted in a great increase in production and in improved conditions for the farmers. So now, in China, only summer seedlings are used.
Nursing of young sporelings
As young sporelings 3 to 5 cm long become overcrowded in their breeding station, they are moved to grow-out sites when sea water temperature drops below 20°C, e.g. around mid-October in northern China. The purpose of such move is to stimulate their growth to a length of 10 to 25 cm before their transplantation. During this nursing period, young sporelings grow very rapidly.
Transplantation of young sporophytes
At the end of the nursing period, young seedlings are transplanted to kelp culture ropes for final grow-out on floating rafts. The procedure is similar to the transplantation of young rice plants in paddy culture.
Raft culture
Three types of floating raft are used for farming kelp, depending on farm site conditions, such as water depth, currents and nutrients content:
- Raft with vertical or hanging ropesRafts with vertically hanging ropes (Figure 3A) make good use of the water column. It is a simple, easily managed procedure. The direction in which the rafts are anchored is important. Kelp will remain well illuminated if the direction of the floating rafts is at right angle to the water current. If the raft is placed parallel to the direction of the current or at a wrong angle, kelp will have a tendency to intertwine or twist around the rafts ropes resulting in production losses.
- Raft with horizontal ropes
- Dragon line raft.
Rafts with horizontal ropes (Figure 3B) are widely adopted because they can be used under various sea conditions. Usually 10 to 40 floating rafts are anchored parallel to each other and spaced 3 to 5 m apart. The culture ropes are held in the horizontal position by tying both ends to other ropes. This method is suitable for shallow and deep sites. Its main advantage is that it can be very easily adjusted in different sites in response to changing conditions, such as turbidity and light intensity, as well as according to the different growth phases.
Dragon line rafts (Figure 3C) are well suited for either turbid inshore water or for open deep water with strong sea currents. Anchored at both ends, the floating raft is 50 to 60 m long and made of a series of vertical ropes attached at regular intervals of about 1.5 m. These ropes support a long horizontal culture rope. This farming method is used by most of the southern kelp farms.
Figure 3. Types of floating rafts for kelp culture
A. Raft with vertically hanging ropes
B. Raft with horizontal ropes
C. Dragon line raft
1. Horizontal rope 2. Float 3. Anchoring rope 4. Anchor 5. Vertical or transversal rope 6. Kelp 7. Weight 8. Culture ropeHarvesting
Harvesting usually takes about 40 days and it often needs additional temporary manpower (Figure 4). Timing of the harvest should be well planned in advance to prevent biomass losses as summer water temperature rises. If kelp is harvested too early, yield and quality will be reduced because kelp fronds will have a higher moisture content. On the other hand, if the harvesting takes place too late, the fronds will be deteriorated and invaded by parasites. Typhoons become more frequent also.
The underwater forest
Since kelp farming has developed, the biomass of natural Laminaria stands has greatly increased. As there is no interest in harvesting them, they form an underwater forest which has become the habitat and the spawning ground for many aquatic animals and in particular for species with a high commercial value, such as sea urchins, abalone and sea cucumbers.
(a) Taxonomy
Laver (Porphyra spp.) is a red seaweed belonging to the Order Bangiales, Family Bangiaceae. There are about 70 species, distributed throughout the world from frigid to subtropical zones. In China, there are over 10 species with a commercial value. They are classified into three sections (Figure 5A) as follows:
Porphyra yezoensis, P. katadai, P. tenera, P. pseudolinearis and P. kuniedai
Porphyra haitanensis, P. dentata, P. erispata, P. suborbiculata, P. kwangtun and P. monosporangia
(b) Sexual characteristics of the thallusPorphyra marginata
In the Genus Porphyra, there are four types of sexual characteristics (Figure 5B):
Dioecism (a, b) ------------------------------------------------- P. dentata and P. pseudolinearisMonoecism (c,d,e) --------------------------------------------- P. yezoensis and P. suborbiculata
Majority of dioecism + minority of monoecism (a,b,c) P. haitanensis
Monoecism + minority of male thallus (c,b)------------ - P. katadai Miura var. hemiphylla
Figure 5. Characteristics of the thallus and distribution of generative cells
A. Sections 1, 2 and 3
B. a,b: dioecism and c,d,e,: monoecism
(c) Life cycle
Porphyra species have a complex biological cycle (Figure 6), which is temperature dependent and seasonal in nature. Until 1949, it was still an enigma. British botanist K. M. Drew then discovered that the genus Porphyra existed during the warm season as a filamentous shell-boring stage, which he had previously described as a separate species, Conchocelis rosu.
During winter months, Porphyra thalli differentiate and produce spores of different sizes, by successive divisions of mother cells: the largest carpospores and the smaller spermatia. Carpospores are released and collected in April-May on suitable substrates, such as old oyster shells. These carpospores germinate when water temperature rises to 25°C, thus producing the conchocelis stage. This stage can recycle asexually through monospore production, conchospores being produced from September to October when water temperature decreases. Conchospores attach themselves to give sexually differentiated thalli, which are used to produce laver.
Figure 6. Life cycle of Porphyra species
(d) Breeding and farming techniques
In China, laver culture has a long history (Section 1.3.2), but the collection of carpospores as “seed” to produce laver thalli dates from the 1950s, following research carried out by Dr C. K. Tseng and his colleagues. This enabled the farming of two species of Porphyra, P. yezoensis in the northern regions (from northern Jiangsu Province to Shandong Province) and P. haitanensis in the southern regions (Fujian and Zhejiang Provinces).
Collection of carpospores
The production period of carpospores depends on species: for P. yezoensis, it is from December to May and for P. haitanensis, it is from November to March. In order to shorten the rearing period of conchocelis filaments in greenhouses and to select the best season for the germination of carpospores, their collection should take place in mid-May for P. yezoensis (optimum water temperature, 15-20°C) and in February-March for P. haitanensis.
During the maturation season, parent plants are collected and dried, removing 60 to 70 percent of their moisture content. The algae are then preserved in a freezer at -15°C to -20°C where they can be kept for several months. When the season for collecting the carpospores arrives, desiccated parent algae are washed two or three times with sterilized sea water. As soon as carpospores are released in the water, the latter is sprayed on substrates, such as clam shells. Carpospore density on these substrates varies from 200 to 400 cells/cm2.
Rearing of conchocelis filaments
Most of the attached carpospores will become conchocelis filaments two weeks later. These filaments can be seen with a magnifying glass and, 15 days later, with the naked eye.
From May to June, favourable development conditions are as follows:
- Light intensity: 2000 to 3000 luxFrom July to September, as air temperature in the breeding room rises to 26-30°C which is too warm, light intensity should be decreased to 1000-1500 lux. As temperature decreases (17 to 22°C for P. yezoensis and 20 to 25°C for P. haitanensis), conchospores are released from the swollen filaments into the water. They attach themselves to seedling nets for further culture. Optimum conchospore density on these nets is 5 to 10 cells / cm net line.
- Water nitrate nitrogen content: 10 ppm
- Water phosphate phosphorus content: 1 ppm
Laver farming
There are three methods (Figure 7) for farming laver:
- Stake method. It is generally used in sites where there is no strong wind and no high waves. Routine operations and management are rather easy, but laver plants are exposed in the air for longer periods during low tides. Therefore, growth rate is lower than in the other two methods.Laver farming is well developed where the nutrient content of sea water is high and water depth is small. In the Jiangsu Province, laver farming is often combined with shellfish farming, which results in good harvests for both cultures.- Semi-floating method. Rearing facilities are installed in the intertidal zone. The netting frames float up during high tides and lie on the bottom, in the air, during low tides. This method is commonly used because it is much easier for routine management while producing good harvests. In general, production on a dry weight basis reaches 2400 kg/ha (P. yezoensis) or 5 000 kg/ha (P. haitanensis).
- Floating raft method. It is similar to the method used for kelp farming (Section 2.1.1b). Netting frames always float at the water surface. Laver growth is increased, nutrient absorption from sea water by the thalli happening for a longer period of time than in the other two methods. But some fouling algae invade the nets, influencing laver quality subsequently. It then becomes important to control this fouling regularly.
Figure 7. Three methods for farming laver
a Stake method b Semi-floating method c Floating raft
2.2.1 Scallop biology and culture
2.2.2 Abalone biology and culture
Scallop is an economically important bivalve. As early as 30 years ago, world fishery production reached over 170 000 mt. Later, following overfishing, environment deterioration and increased consumption, captured scallop could not meet market demand any more. Scallop farming became a profitable activity in Asia and Europe. In the 1980s, it also became a prosperous aquaculture sector in China. By 1988, farmed scallop production was 122 000 mt. Recently, it reached one million metric tons.
Figure 8. Chlamys farreri (a) and Argopecten irradians (b)
In order to meet the soaring demand for scallop spat, research on reproduction started in the 1960s. By 1974, it became possible to produce spat on a commercial scale for Chlamys farreri (Figure 8a). Then, in successive years, the technology was developed for Chlamys nobilis (1978), Patinopecten yessoensis (1981) and Argopecten irradians (1983 - Figure 8b). These four species are now successfully farmed in both northern and southern China.
P. yessoensis and A. irradians are exotic species introduced from Japan and USA in 1981 and 1982 respectively. The rearing cycle of the Japanese scallop being longer, its production through sea ranching is well developed in the Shandong and Liaoning Provinces.
(a) Breeding techniques
The technology used for the production of scallop spat is almost the same as the one used for oyster. It may be schematized as follows:
In the following sections, scallop hatchery procedures are described taking as a concrete example those applied for the production of Chlamys farreri spat.
Selection and conditioning of broodstock
In late winter or early spring, broodstock is selected either from farmed stocks or from wild individuals captured on fishing grounds. They are then transferred to conditioning tanks where, on the first day, water temperature is kept 2 to 3°C higher than natural water temperature. On the next days, water temperature is increased daily by 1 or 2°C until it reaches 12-13°C for Chlamys farreri or 15-18°C for Argopecten irradians. This temperature is kept stable for three to five days to synchronize gonad development. Then, it is increased daily again until reaching the optimum temperature, about 18°C for C. farreri or 22-23°C for A. irradians.
During conditioning, broodstock is fed either an artificial diet or unicellular algae, mostly Phaeodactylum tricornutum, Chaetoceros muelleri, Monochrysis simplex, Isochrysis galbana, Tetraselmis tetrathele and Nannochloropsis oculata.
Broodstock spawning and reconditioning
Conditioned spawners are desiccated for two hours and then placed into tanks filled with running filtered sea water for one hour. To prevent multi-sperm fertilization of the eggs, female and male scallops are stocked in separate tanks until spawning.
Spawning takes place in tanks where water temperature is kept 2 to 4°C higher. This thermal shock stimulates spawners to release gametes within 10 to 15 minutes, for about 50 minutes.
After spawning, the spawned broodstock can be reconditioned during seven to ten days before inducing a second spawning. But, in practice, such reconditioning is used only when there is a shortage of broodstock.
Eggs fertilization and incubation
Chlamys farreri is a dioecious bivalve mollusc. During its reproductive season, eggs are released from the gonads into the water and fertilized. Fecundity of a mature female scallop may be as high as 3 to 6 million eggs. Diameter of the eggs averages 65 to 72 m. The fertilizing capacity of scallop sperm lasts longer than that of abalone: at 16 to 19°C, sperm retains its fertilizing capacity for 6 hours. After fertilization, egg density is kept at 20 to 30 eggs per millilitre for incubation.
Development from fertilized egg to spat is described in Table 15 and illustrated in Figure 9.
As larvae become trochophores or veligers, the selection of healthy larvae is initiated on the basis of their activity and attraction to light. Such larvae move to the upper water layer while the unhealthy larvae usually sink to the tank bottom. This is a simple but very important procedure which ensures production of a majority of high quality spat.
Table 15. Development of Chlamys farreri from egg to spat
Sea water temperature 18.2 - 25°C and salinity 25 - 30 ppt
|
Developmental stage |
Cumulative duration |
Size (m) |
|
Fertilized egg |
0 |
65-72 |
|
Two cell |
1:20 h |
|
|
Morula |
5:30 h |
|
|
Blastula |
10:00 h |
|
|
Gastrula |
16:00 h |
|
|
Trochophore |
21:00 h |
|
|
Veliger |
26:00 h |
100 × 83.5 |
|
Umbo larvae |
6 days |
125 × 135 |
|
Crawling larvae |
13 days |
180 × 200 |
|
Juvenile I |
about 25 days |
H = 250-280 |
|
Juvenile II |
38-40 days |
H = 500-700 |
|
Juvenile III |
48-50 days |
H = 1400-2000 |
|
Spat |
80 days |
shell about 1 cm high |
1. Sperm 2. Fertilized egg 3. Showing first polar body 4. Showing second polar body 5. Showing first polar lobe 6. First cleavage 7. Two-cell 8. Four-cell 9. Eight-cell 10. Blastula 11. Gastrula 12. Trochophore 13-14. D-form larva 15. Umbo larva 16. Crawling larva 17. Young spatCulture of larvae
Development and growth of scallop larvae depend on environmental conditions such as water temperature, salinity and food. For Chlamys farreri, suitable conditions are as follows:
- Optimum temperature: 17 to 20°C- Water exchange rate: first 7 days from 1/3 to 1/2; afterwards, increasing up to 2 volumes per day
- Food and feeding regime: the following microalgae are commonly used for feeding scallop larvae: Phaeodactylum tricornutum, Chaetoceros muelleri, Monochrysis simplex, Isochrysis galbana, Tetraselmis tetrathele and Nannochloropsis oculata. Substitutes like yeast and microencapsulated feeds are used also, but live algae are better for the production of good spat. Recommended algal density varies as given in Table 16.
Table 16. Feeding microalgae to Chlamys farreri larvae
(larval density: 10 / ml)
|
Larval stage |
Shell length |
Algal density |
|
D-form larvae |
100-110 |
5-10 |
|
Early umbo |
125-135 |
15-20 |
|
Mid-umbo |
150-160 |
25-30 |
|
Post-umbo |
170-190 |
40-50 |
|
Crawling larvae |
190-220 |
50-60 |
|
Juveniles |
300-500 |
60-120 |
After 18 to 20 days at 17-19°C, larvae reach 165 to 180 m in length. During this period, appearance of the eye-spot indicates that larvae are about to become benthic organisms and to attach themselves to a substrate. Such substrates for larval attachment should be installed, just prior to this change of life style. Materials made of palm threads, nylon fibres or polyethylene fibres can be used for this purpose. Experience has shown that larvae prefer brown, red or yellow substrates.
Spat nursing
Juveniles over 500 m long, also called young spat, become easily detached from their substrate, sinking to the bottom of the rearing tanks. In order to avoid this, they should be transplanted to a good nursery site on time.
During this nursing period, routine management is very important to increase survival rate. Every five to seven days, net bags should be cleaned to guarantee an adequate sea water exchange.
As young spat reach 2 mm in length, they should be transferred to other net bags with a larger mesh size, where they will grow up to their commercial size of 10 to 20 mm.
At present, some hatcheries have land-based nursing ponds built in greenhouses. This has the advantage of accelerating the growth of young spat in early spring because of the higher temperature of nursing pond water. Management is also easier.
(b) Sea farming and ranching techniques
The technology used for farming Chlamys farreri is schematized in Figure 10.
Sea ranching is widely practised in areas with a sandy or rocky bottom, 15 to 20 m deep. Before stocking young scallop, sea stars, crabs and other predators should be eliminated. Juvenile scallop are then experimentally stocked in a selected area. If survival rate is at least 60 percent one month later, the area can be selected for sea ranching. Stocking density ranges from 10 to 20 scallop juveniles per square metre. Commercial size may be reached 20 months later.
Abalone are large herbivorous marine gastropods. There are about 100 different species, all belonging to the same genus, Haliotis. They are found in both hemispheres, the largest species in temperate regions and the smaller ones in tropical regions. The greatest number of species is present in the central and south Pacific regions and in parts of the Indian Ocean, but none of them has a large size.
In China, two indigenous species are cultured, Haliotis discus hannai (Figure 11a) and H. diversicolor (Figure 11b) and its varieties. The former is mainly found in the northern part of China, such as the Shandong and Liaoning Provinces, the latter preferring the southern regions, the Fujian, Guangdong and Hainan Provinces. Young abalone of both species are produced in commercial hatcheries, most of which belong to private and collective entrepreneurs.
Figure 10. Scallop farming procedures (Chlamys farreri)
Figure 11. Haliotis discus hannai (a) and Haliotis diversicolor (b)
Haliotis discus hannai (a)
Haliotis diversicolor (b)
(a) Breeding techniques
The technology used for the production of young abalone can be schematized as follows:
Development of H. discus hannai from egg to juvenile is described in Table 17 and illustrated in Figure 12.
Table 17. Development of H. discus hannai from
egg to juvenile
(at water temperature 22.5 to 24°C)
|
No. |
Phase |
Cumulative duration |
Size |
|
1 |
Fertilized egg |
0 |
100-270 m |
|
2 |
2 cells |
40-50 min. |
|
|
3 |
4 cells |
80 min. |
|
|
4 |
8 cells |
2 h |
|
|
5 |
16 cells |
2 h 15 min. |
|
|
6 |
Morula |
3 h 15 min. |
|
|
7 |
Gastrula |
6 h |
|
|
8 |
Early trochophore |
7-8 h |
|
|
9 |
Early veliger |
15 h |
|
|
10 |
Post veliger |
26 h |
Shell length: 0.27 mm |
|
11 |
Peristomial larva |
6-8 days |
0.3 mm |
|
12 |
Epipodium larva |
19 days |
0.7 mm |
|
13-14 |
Juvenile |
45 days |
2.3-2.4 mm |
1. Fertilized egg 2-6. Mutiple-cell stages 7. Gastrula 8. Early trochophore 9. Early veliger 10. Post veliger 11. Peristomial larva 12. Epipodium larva 13-14. Juvenile.Free-swimming larvae
After hatching, actively swimming larvae are transferred from the upper water layer of spawning tanks to rearing tanks. Best stocking density is about 1 000 ind. per litre. Optimum water temperature is 20°C for H. discus hannai and 26-28°C for H. diversicolor.
Crawling larvae and juveniles
Following physiological and morphological changes, planktonic larvae search for a suitable substrate for their crawling life. After about one or two days, they develop into crawling larvae and settle on corrugated plastic sheets placed in the rearing tanks. To ensure that settlement of the larvae is evenly distributed, tank water should not be changed and light intensity should be kept below 100 lux for the first few days.
It is most important to control settlement density. Optimum survival rate and growth are obtained at densities of 100 to 200 individuals per 100 cm2, because at higher densities, benthic diatoms can not meet the food demand of growing larvae.
Water salinity has also a very significant effect on growth and survival as young abalone are typically stenohaline. Optimum salinity for larval development and grow-out ranges from 32 to 35 ppt. Both larvae and young individuals will die if the salinity falls below 24 ppt. Juveniles will die after 60 days in 25 ppt or 30 days in 20 ppt. All hatcheries and farming facilities should therefore be located away from estuarine areas.
Nursing young abalone
As young abalone reach 5 mm shell length, they have to be dislodged from their settlement substrates and transferred to nursing substrates placed into indoor rearing tanks. In order to do this safely, anaesthetic chemicals are commonly used, such as alcohol, ethylcarbamate (NH2COOC2H5) or ethylaminopionate (NH2C2H4COOC2H5) (Table 18).
Table 18. Use of anaesthetic chemicals
|
Chemical |
Concentration |
Treatment time |
Recovery time |
|
Alcohol |
2 |
3-10 |
8-25 |
|
Ethyl carbamate |
1 |
1-2 |
10 |
(b) Sea farming/ranching and stock enhancement
In Japan and China, initial development of the abalone industry dates from the late 1950s and early 1960s. Following successful research on mass production of seed (Table 19), abalone farming rapidly developed in the 1990s. By 1999, total world production reached about 13 000 mt, of which over 8 000 mt were farmed abalone. According to existing statistics, China is the world largest producer and in 1999, its farmed production was over 5 000 mt (Table 19 and Figure 13). Almost all of it is sold on domestic markets.
In China, there are two popular methods to farm abalone: the intensive method indoors and cage culture on floating rafts. Most of the farmers use seaweed as food. In northern China, brown seaweed such as Laminaria and Undaria are preferred, but in the southern regions, red seaweed (e.g. Gracilaria) and formulated feeds are commonly adopted.
As production costs of the intensive system are higher, its use is often restricted to the production of small abalone (2.5 to 3 cm shell length) for the restocking of selected sites where wild seaweed are available. Within a couple of years, these abalone grow up to 6-8 cm length and they are then harvested by divers. Such ranching method giving a higher profit margin than farming, it has been widely adopted in northern China. Recently, the price per kilogram of commercial size abalone has reached US$50 to 60, depending on size.
The technology to produce hybrid and triploid abalone has been successfully developed by researchers. It is ready to be introduced to the commercial production sector.
Table 19. Abalone: production of seed and commercial size, 1991-1999
|
Year |
Number of seed |
Production (mt) |
||
|
Farming |
Fishing |
Total |
||
|
1991 |
26 460 |
175 |
178 |
353 |
|
1992 |
72 860 |
314 |
157 |
471 |
|
1993 |
216 680 |
544 |
248 |
792 |
|
1994 |
228 120 |
569 |
377 |
946 |
|
1995 |
190 440 |
879 |
579 |
1 458 |
|
1996 |
239 990 |
1 218 |
672 |
1 890 |
|
1997 |
322 090 |
2 045 |
592 |
2 637 |
|
1998 |
544 690 |
3 766 |
794 |
4 560 |
|
1999 |
776 700 |
5 178 |
639 |
5 817 |
2.3.1 Shrimp biology and culture
2.3.2 Mud crab biology and culture
China has a great diversity and abundance of marine shrimp. There are about 100 species, among which 40 have a high commercial value (Liu, R.Y.1955; Liu and Zhong, 1986). Due to overfishing, production of Penaeus chinensis in the Bohai Sea and the Yellow Sea declined from 32 896 mt in 1980 to 7 324 mt in 1982. In order to meet the increasing demand and to protect natural resources, shrimp farming and enhancement became priority subjects from the late 1970s to the early 1980s.
(a) Farmed shrimp species
Nine species of shrimp belonging to three different genus are farmed in China:
|
Penaeus chinensis |
Metapenaeus ensis |
Litopenaeus vannamei |
|
P. penicillatus |
M. affinis |
L. stylirostris |
|
P. merguiensis |
|
|
|
P. japonicus |
|
|
|
P. monodon |
|
|
But in China, research has concentrated much longer on the farming of Chinese shrimp (Penaeus chinensis syn. P. orientalis) and it is this species which will be considered in the next sections.
(b) Obtaining broodstock
Shrimp broodstock may be obtained from three sources:
- Soon after the spawning migration, it may be captured on spawning grounds. Presently, this is the main source of broodstock in northern China. Shrimp can be spawned soon after capture and they are the least expensive. But there is concern that continued use of such broodstock is threatening wild stocks and that it impacts on ocean fishery production negatively.(c) Breeding techniques- Broodstock may be captured during its reproductive migration, before being sexually mature. It is then held in ponds until it becomes ready to spawn.
- Broodstock may be collected either from farm ponds at the end of summer or from the sea during the wintering migration. Shrimp are then overwintered in tanks or ponds. They are spawned the following spring when they become sexually mature. At present, this method is widely used. It has the least impact on natural resources.
Spawning
P. chinensis is a typical multiple spawner. Under hatchery conditions, a female can spawn four to five times, at the average interval of 15 days (range: 5 to 20 days). Egg production per spawning varies from 400 000 to 500 000. One female can produce about 1.7 million eggs during its spawning season.
Since spawning usually occurs at night, gravid females are selected and placed in spawning tanks in the afternoon or at dusk. Fertilized eggs are collected on the next morning.
Hatching and larval development
Water quality and ambient conditions are especially important for ensuring a high hatching rate, normal embryonic development and production of healthy nauplii. In China, good hatching conditions are defined as follows:
- Salinity: 25 to 35 pptAfter hatching, shrimp go through four developmental stages and 26 moults during their larval and postlarval development, before becoming a juvenile similar in shape to the adult shrimp. Environmental requirements also change as the larvae develop.
- Temperature: 18 to 20°C
- Dissolved oxygen content: above 4 mg/l
- Total ammonia nitrogen content: below 0.6 mg/l
Transport of postlarvae
Depending on water quality and temperature, postlarvae (PL) are ready for pond stocking 20 days after hatching. They are then typically 7 mm long. Their transfer from the hatchery to the farm is done by truck, in canvas barrels. A barrel of 1 m diameter containing 20°C aerated water can hold 300 000 to 400 000 PL 7 to 10 mm long, for six to eight hours, without excessive mortality. PVC plastic bags filled with water and oxygen are also used. A 10-litre bag can hold 10 000 to 20 000 PL for 10 hours or more.
(d) Farming techniques
Successful pond farming of P. chinensis depends on a series of procedures which should all be optimized, such as site selection, pond construction, PL transport and stocking, predator control, water quality management, feeding, disease control and harvesting.
In China, several cultural systems are used to farm shrimp, including: fish pond polyculture, shrimp monoculture without feeding, shrimp monoculture with feeding and pen culture in open waters (see Chapter 3).
The first two systems are extensive, with low yields and limited economic efficiency. But since 1993 (epidemic viral disease), a disaster year for the shrimp industry in China, these extensive systems have been widely preferred because both risks and production costs are lower.
For pond culture, site selection is most important, especially with respect to elevation and topography, soil conditions and water quality. For best results with P. chinensis, water pH should be in the range of 7.8 to 8.6 and salinities from 5 to 35 ppt. Metal ions should not exceed the following values: Hg2+ 0.0002 mg/l, Cu2+ 0.017 mg/l, Zn2+ 0.03 mg/l and Pb2+ 0.16 mg/l. Sandy clay makes the best pond bottom: it is firm and impermeable, and it does not crack excessively when dry. Acid sulphate soils are not suitable because their oxidation forms sulphuric acid, decreasing pH and solubilizing iron and aluminium. These last two ions are not only toxic, but they also bind phosphorus and thus lower pond productivity. Other factors to consider when siting a farm include availability of labourers, reliable source of postlarvae for stocking, vehicle access to the ponds, local climate (precipitation, temperature) and post-harvest processing.
Precautions should be taken to minimize shrimp losses due to diseases and predation. Thorough drying of the pond bottom between crops will help reduce pathogens and solubilize nutrients in non-acid soils. Since 1993, great efforts have been made in the prevention of viral diseases, such as using Special Pathogen Free (SPF) broodstock and applying chemical treatments to reduce the risk of epidemics and great financial losses. But for curing these diseases, existing treatments are still unsatisfactory.
In extensive farming systems, increasing natural food production in ponds improves shrimp growth and lowers production costs. In China, to this end, Corophium spp. are stocked in ponds, giving good results. Other food organisms such as Unciola spp, Gammarus spp. and polychaete worms are also widely used.
(e) Shrimp stock enhancement
Experiments on the enhancement of shrimp stocks through stocking of postlarvae were carried out from the 1980s to the early 1990s (Section 3.8). Even if recapture rate was reported to be as high as 8 percent, the project was stopped because such method was judged to be too unreliable. Our full understanding of all processes involved in such enhancement is still far from being complete.
The mud crab, Scylla serrata (Figure 14a), is an economically important crustacean occurring in tropical regions of the Indian and Pacific Oceans. It has been widely cultured in China, especially in the southern regions such as the Fujian, Guangdong and Hainan Provinces.
(a) Life habits and reproduction behaviour
Mud crab is a euryhaline animal which can tolerate water salinities ranging from 5 to 33.2 ppt. Optimum salinity ranges from 13.7 to 26.9 ppt. When salinity decreases below 7 ppt, they often dig holes to survive adverse environmental conditions.
Optimum temperature ranges from 180°C to 32°C. Feeding rate decreases when water temperature drops below 18°C. Crabs will survive in holes when water temperature drops as low as 12°C. As water temperature continues to drop to 7°C, they stop feeding and become dormant. During the hot season, as water temperature rises up to 35°C, they feel obviously unadapted, erecting their body and keeping their abdomen away from the earth when crawling on the sea beach. A grey-red spot appears on their tergum and, as water temperature increases above 39°C, they gradually emaciate until death.
Figure 14. Mud crab (a) and a berried female (b)
Crabs moult 13 times during their life span: six times during the larval stage, six times during the grow-out stage and once during reproduction. Moulting occurs only when water temperature is at least 15°C, but preferably when it is above 18°C. When crabs moult, they breathe rapidly, their oxygen consumption being higher. For two to three hours, newly moulted crabs cannot swim and lay on the bottom.
Scylla serrata is a carnivorous animal, its preferred food consisting of small molluscs, trash fish and other crustaceans.
Usually, the mud crab reaches the reproductive stage when its shell width is greater than 7.8 cm and its body weight over 100 g, females being normally a little bigger than males (shell width over 8.5 cm and body weight over 130 g).
Reproduction season varies according to local water temperature. In southern China, female crabs carrying eggs are few in winter, but in tropical regions, they can be found almost throughout the year.
Mating occurs about one hour after moulting of the female. The male turns the female on its back and climbs upon its abdomen. Then, it grasps the female with its walking legs, the female opening its abdominal plate. The male inserts his copulating apparatus into the female’s aperture genitalis and ejaculates sperm into the spermatheca.
Generally, mating lasts for one or two days, a period which may vary from nine hours to three days. After mating, the aperture genitalis is blocked by ovarian secretions. Mating occurs mostly at night, especially at the beginning of the high tide. For mating to be successful, water temperature should preferably be higher than 18°C.
Both female and male crabs do not feed during the mating period. After mating, females consume a large amount of food to support rapid ovarian development. Under suitable conditions, ovulation may take place 30 to 40 days later.
Normally, spawning occurs early in the morning, between 0500 h and 0800 h. Eggs are released from the aperture genitalis to meet the sperm released from the spermatheca. After fertilization, two-membrane layers are formed to protect the fertilized eggs. The inner one is a yolk membrane, the outer one being a secondary ovarian membrane. Fertilized eggs stick to the bristles of the abdominal legs of the female which is then called a “berried” crab (Figure 14b). One female crab can produce about 2 million eggs.
Development of fertilized eggs is accelerated by an increase of water temperature. At 18°C to 28°C, hatching occurs after 25 to 15 days, while at 32°C, it takes 11 days only.
(b) Larval development
A newly hatched embryo of Scylla serrata is called a zoea larva (Figure 15A). This zoea stage is made of five sub-stages, the zoea larvae developing into megalopa larvae (Figure 15 B) through five moults. Then, through another moult, these megalopa larvae metamorphose into juvenile crabs (Figure15 C). Normally, it takes 23 to 24 days at 26-29°C for the zoea larvae to develop into young crabs: 4 to 5 days from sub-stage I to sub-stage V and 6 to 7 days from megalopa to juvenile.
The megalopa larvae gradually adapt themselves to a benthic life. Because of their phototaxic behaviour, larvae are often attracted by light at night.
The moulting process depends on body size and environmental factors. For example, moulting of big crabs takes longer than for small ones. Newly moulted mud crabs lose their swimming ability and sink to the bottom of the pond. It takes two to three hours for soft-shell individuals to regain this swimming ability. Hardening of the shell lasts six to seven hours, three to four days being needed to complete this process. With each moult, shell width, shell length and body weight generally increase by about 28.4 percent, 30 percent and 41 percent respectively.
Strong stimuli or mechanical damages often result in the loss of appendages, a process called self-cutting. New appendages can be regenerated several times.
Figure 15. Development of mud crab from zoea larva to juvenile
(c) Breeding techniques
Selection and rearing of broodstock
Crab broodstock can be collected either from the wild or from farm ponds. The selected individuals should be healthy, with a body weight of over 300 g and ovaries having reached development stage V. Berried females are not selected as broodstock due to a lower fertilization rate and their contamination by parasites, such as ciliates, and by disease pathogens.
Broodstock rearing consists in ensuring maturation, reaching the berried stage and bringing the eggs close to hatching. Concrete tanks and earthen ponds can be used for this purpose. The facilities and equipment used for shrimp rearing in ponds are particularly well adapted for rearing crab broodstock, except that shelters made of bricks or stones should be added on the bottom of the ponds.
In concrete ponds, stocking density is usually less than two crabs per square metre. A higher density could result in fights and injuries. A rich and diversified food is required, which includes small mussels, fish, crab, shrimp, etc. Crabs are fed in the evening and feeding rate is determined by the amount of food left over on the next morning.
Optimum salinity ranges from 26 to 31 ppt. If salinity is lower than 22 ppt, ovarian development slows down. Similarly, development is slower at water temperatures below 20°C and crab will die at temperatures higher than 32°C.
Artificial aeration is needed in concrete ponds and there should be one complete water exchange daily.
Spawning and hatching
Under good management, mature female crab should be reared about 10 days before spawning, even if well selected. After spawning and during the whole incubation period, it is very important to feed berried females a high quality food, to control water quality and to keep a rational stocking density.
Observing the colour changes of carried eggs is an important routine work. With the development of the embryos, this colour varies from bright orange, to grey and to dark-grey. When this last egg colour is reached, it indicates that the larvae will hatch soon. The duration of the incubation period depends on water temperature as shown in Table 20.
Table 20. Duration of incubation period according to water temperature
|
Water temperature |
Duration of incubation period |
|
16 |
60 - 65 |
|
18 |
40 - 45 |
|
20 |
30 - 35 |
|
22 |
25 - 30 |
|
24 |
18 - 20 |
|
25 |
15 - 18 |
|
30 |
10 - 15 |
The phototaxic behaviour of the zoea larvae is used to collect them, discarding those which are not able to reach the water surface. They are transferred to rearing tanks where stocking density varies from 20 000 to 50 000 per cubic metre.
Zoea I larvae start feeding mostly on unicellular algae, rotifers, eggs and trochophores of bivalve molluscs. During later stages, larvae are fed brine shrimp nauplii and copepods. Towards the end of the larval rearing period, natural food is gradually replaced by artificial feed.
It has been reported that rotifers are good food for zoea-I and zoea-II, at the density of 60 rotifers /ml, but from the zoea-III stage, this diet should be complemented with brine shrimp nauplii.
When zoea larvae become megalopa larvae, a substrate such as netting should be placed in the tanks to prevent cannibalism.
Nursing of young crabs
Megalopa larvae moult to become juvenile crabs living on the bottom. Appearance and feeding habits of the latter are the same as those of adult crabs and they can feed on meat of shrimp, crab and fish. Gradually reducing water salinity to 15-20 ppt can accelerate moulting and growth.
When young crabs reach development stages II and III (shell width: 6-7 mm; body weight: 18-32 mg), they can either be transferred to other nursing tanks or grown in farm tanks. But to be stocked in earthen ponds they should be reared for about another 10 days, when they should be 13 mm in width and 5 g in body weight.
(d) Farming techniques
Site selection for pond culture
Estuarine and flat tidal areas can be selected for farming mud crab. Brackish water with a salinity ranging from 13 to 25 ppt is pumped into the ponds. A sandy clay bottom is good for crab, giving them a healthy look for the seafood market. The necessary supplies of juveniles and feeds should be reliable, as well as transportation means.
Feeds and feeding
Scylla serrata are carnivorous, but sometimes they also eat rotten plants. At present, pelleted feeds are commonly used, although many farmers still prefer to use cheap meat of molluscs, trash fish, shrimp and crab to feed farmed mud crab.
Feeding rate depends on water temperature and water quality. Optimum temperature for growth being 25°C, feeding rate should be increased as the water temperature reaches 18°C and above. On the contrary, it should be sharply reduced when temperature is higher than 30°C or lower than 13°C. Normally, for example if trash fish is used, daily feeding rate should equal 5 to 7 percent of the mud crab biomass present in the pond.
Since Scylla serrata feeds mostly at night, 60 to 80 percent of the daily food ration should be distributed in late evenings and the rest in early mornings. Mud crab should never be fed at noon when water temperature is high. Food should be evenly distributed along the edges of the ponds, so as to reduce competition and fighting.
Routine management of ponds
In subtropical regions, regulation of water levels in rearing ponds is routinely done in order to keep a suitable water temperature. In summer, the presence of deep water helps avoiding too high temperatures. In winter, on sunny days, it is advisable to lower the water level to increase the temperature of pond water.
Pond water should be changed daily or at least every two to three days. It depends on water quality, in particular on dissolved oxygen and ammonia nitrogen contents.
Feeding behaviour should be regularly checked and the leftovers should be removed to ensure a high growth rate and a good survival rate, and to prevent diseases. An adequate diet helps to reduce cannibalism and to maintain good water quality.
Harvesting
Farmed Scylla serrata grow rapidly as shown in Table 21. Depending on the size of the juveniles initially stocked, it usually takes four to five months to produce 250 to 400 g individuals. Female crabs whose ovaries are fully developed (called yolk crabs) fetch the highest price in seafood markets. Commercial value of male crabs mainly depends on individual body weight and size.
Table 21. Growth in weight of farmed mud crab
|
Group |
Mean body weight (g) |
||||
|
10 days |
60 days |
75 days |
96 days |
180 days |
|
|
1 |
5 |
- |
- |
340 |
- |
|
2 |
- |
120 |
177 |
- |
574 |
(a) Biology
Taxonomically, sea cucumbers belong to Families Holothuridae and Stichopodidae. Processed to produce bêche-de-mer, they have a world-wide distribution. They are found in large numbers in the Indo-West Pacific region including the islands of the Western Indian Ocean, Mascarene Islands, East Africa and Madagascar, the Red Sea, Southeast Arabia, the Persian Gulf, the west coasts of India and Pakistan, the Maldives and the Lakshadweep, Sri Lanka, the Bay of Bengal including the Andaman and Nicobar Islands, the East Indies, North Australia, the Philippines, China and southern Japan, the South Pacific islands and the Hawaiian islands. In China, it is traditionally considered as a tonic food and a natural medicine.
The species Stichopus japonicus (Figure 16a), present in northern China, Korea, Japan and other regions of the North Pacific, is considered to be the best among the group. Another species, Holothuria scabra (Figure16b), is widely distributed throughout the tropics. Both species are farmed and stocked for sea ranching. In the 1950s, total production (dry weight) was 130 to 140 mt in the Shandong Province. In the 1970s, production declined to less than 40 mt. In the 1980s, increased market demand stimulated their culture again. Sea cucumber farming is regarded as a highly profitable business. Decades of practices have also proved that sea cucumbers are good marine animals for sea ranching in northern China.
Figure 16. Stichopus japonicus (a) and Holothuria scabra (b)
Stichopus japonicus (a)
Ecological conditions
Stichopus japonicus is a cold water species. Its optimum temperature ranges from 5 to 15°C. Feeding activities start to slow down as water temperature drops below 3°C or increases above 17°C. As it reaches 24°C, dormancy begins.
Optimum salinity for growth and development ranges from 27 to 35 ppt. Their tolerance to higher salinities is better than to lower ones. When placed in 10 ppt brackish water, LT50 equals 8 to 17 hours and LT100 equals 9 to 24 hours. Therefore, this species is never found in estuaries.
Sea cucumbers have a very strong capacity for regeneration and thus, it is very difficult to tag them in experimental enhancement programmes. They can extrude their viscera out via their anus in order to escape from predators or harmful objects. About two months later, new viscera are present again. Normally, a half cucumber can grow up to a whole animal.
(b) Breeding techniques
Sea cucumber is a dioecious animal, but it is hard to differentiate male from female with the naked eye. Its very small genital pore is located on the posterior part of the back of the head. Most individuals only possess one genital pore with a spawning function, but a few animals may have two or three such pores,.
Two-year old individuals, weighing about 250 g, become mature. Female fecundity may be as high as 1 to 2 million eggs, even up to 10 million sometimes, fecundity depending on body weight normally. When reaching maturity, there are 220 000 to 290 000 eggs per gram of ovary. A good time for collecting broodstock is when bottom water temperature reaches 15 to 17°C.
Artificial reproduction of sea cucumber may be summarized as follows:
- Broodstock collection in late May to early July, when their maturity index is over 10.Development of sea cucumber from fertilized egg to juvenile is illustrated in Figure 17 and further described in Table 22.- Broodstock rearing in land-based tanks: 30 individuals/m3, dissolved oxygen content above 5 mg/l, daily feeding rate 5 to 10 percent of body weight.
- Induced spawning: thermal shock (water temperature increased by 3 to 5°C), desiccation, sea water jet for 10 to 15 minutes.
- Fertilization: oocyte diameter around 120 to 130 m; maximum density in spawning tanks 200 to 300 eggs per ml; one million eggs/ m3 in hatchery tanks.
- Hatching of larvae
- Nursing of juveniles
Figure 17. Development of sea cucumber from egg to juvenile
Larval rearing
On entering the pre-auricularia stage, larvae begin to feed on phytoplankton. In commercial sea cucumber hatcheries, Dunaliella salina, Phaeodactylum tricornutum and Chaetoceros simplex, as well as bakery yeast, are commonly considered to be the best food.
Feeding rate depends on developmental stage. From the pre-auricularia to the post-auricularia stage, density of algal cells should gradually increase from 10 000 cells/ml to 25 000 cells/ml, the feeding frequency increasing also from twice to four times a day. Research has clearly shown that survival rate of juveniles is related to food quality and quantity (Tables 23 and 24).
As larvae develop to the doliolaria and the pentactula stages, their bodies start to constrict and to shrink to half the size. About one or two days later, they metamorphose into early juveniles (about 400 m long). Behaviour changes from swimming to settling. Substrates should be placed in the rearing tanks on time. Normally, the density of attached juveniles should be limited to 20-50 individuals per 100 cm2.
Table 22. Sea cucumber development from fertilized egg to juvenile
(Water temperature: 20-21°C)
|
Stage |
Size (m) |
Density (ind/l) |
Cumulative duration |
|
First polar body |
|
|
20-30 min |
|
Second polar body |
|
|
30-35 min |
|
First cleavage |
|
|
43-48 min |
|
Blastula |
|
|
3h 40 min-5h 40 min |
|
Hatching of larvae |
|
1000-1500 |
12-15 h |
|
Pre-gastrula |
|
500-600 |
14-18 h |
|
Gastrula |
|
|
18-25 h |
|
Pre-auricularia |
360-430 |
|
25-30 h |
|
Mid-auricularia |
600-700 |
|
5-6 days |
|
Post-auricularia |
800-1000 |
|
8-9 days |
|
Doliolaria |
400-500 |
|
About 10 days |
|
Pentactula |
|
|
About 11-12 days |
|
Juvenile |
300-400 |
50-100 |
About 12-13 days |
(Density of pre-auricularia: 1000/l)
|
Food |
Auricularia stage |
Juvenile stage |
||
|
Density |
Survival rate |
Density |
Survival rate |
|
|
Marine yeast |
190 |
19 |
55.5 |
5.55 |
|
Tetraselmis sp. |
9 |
9 |
1.85 |
0.18 |
|
Phaeodactylum sp. |
20 |
20 |
50.9 |
5.09 |
(Density of juveniles: 1300/l)
|
Food quantity |
Density |
Survival rate |
|
5 000 |
59.1 |
4.56 |
|
3 000 |
13.7 |
1.54 |
|
1 000 |
5.5 |
0.42 |
Enhancement of sea cucumber natural stocks includes at least three aspects:
- Protection of broodstock and their larvae.A simple enhancement technique was tried in the Shandong and Liaoning Provinces, consisting in throwing stones into selected sea areas. Selection criteria included the annual cycle of water temperature, water currents, nutrient levels, presence of predators and growth status of aquatic weeds. Results revealed that successful enhancement depended mainly on site selection and routine management. For example, in one site located in the Shandong Province, production increased 16 times following such practice.- Improvement of the environment of selected sea areas and their stocking with broodstock and juveniles.
- Improvement of the environment of existing natural stocks.
In China, marine fish farming entered a new era since the 1990s, following the great advances made on understanding cultured species biology and on their rearing technologies. The most important breakthrough was the improvement of seed production, the main constraint for sea farming and sea ranching development. The successful farming of the left-eyed flounder (Paralichthys olivaceus) (Figure 18) and of the large yellow croaker (Pseudosciaena crocea) are two examples. In the next sections, the breeding and farming techniques used for flounder production are presented.
Before the 1990s, because of overfishing, flounder capture fisheries produced less than 500 mt a year. Then as flounder farming developed, production increased until reaching more than 5 000 mt since 1998, the highest sea farming/ranching production achieved by one species. Experimental release of tagged fingerlings into the sea was carried out since the1970s and results showed that this was an effective way to increase natural stocks.
Figure 18. Left-eyed flounder
|
Paralichthys species belong to the Pleuronectidae Family. This genus has 20 species distributed world-wide. In China, only one species exists, mostly in the Bohai Sea, the Yellow Sea and a small area of the South China Sea. In the early 1970s, annual production was about 2 000
mt. |
Flounder does not display any external secondary sexual characteristics. It has a cyclical pattern of reproduction characterized by massive gonad development. Water temperature is an important factor for the maturation of gonads. Under natural conditions, spawning takes place between April and June, May being the peak spawning season in the Bohai Sea and Yellow Sea. Under artificial conditions, vitellogenesis occurs at temperatures ranging from 5 to 11°C. Optimum temperature for spawning is 16 to 19 C, the lowest value being 10.6 to 11°C. As water temperature increases above 21°C, gonads start to degrade.
Light intensity triggers gonad development and, under artificial conditions, light intensity and photoperiod should be controlled. Results from experimental studies showed that 400 to 600 lux and 14 to 16 hours of light a day are indispensable for maturation and development of gonads.
Spawning occurs at night only, from about midnight to early morning.
(b) Breeding techniques
The hatchery phase is generally considered as the major bottleneck in the development of flatfish farming, such as flounder, turbot, sole and halibut. Flounder eggs are 0.83 to 1.1 mm in diameter. Both eggs and yolk sac larvae are particularly difficult to rear, compared with other marine fish such as the Atlantic salmon. At the end of the yolk sac stage, larvae start feeding and then they metamorphose, the symmetrical larvae (shaped like round fish) becoming flatfish. Weaning follows.
Development of flounder from fertilized egg to end of metamorphosis is illustrated in Figure 19. It takes 70 to 80 days to produce a juvenile about 50 mm long. Survival varies from 20 to 50 percent, being best in batches fed rotifers, brine shrimps and micropellets.
Figure 19. Development of flounder from egg to juvenile
From A to B: from fertilized egg to newly hatched
larva (total length 2.21 mm)
From C to D: from newly hatched larva to end of
metamorphosis (35 days, TL 13.7 mm)
Broodstock and spawning
Up to now, farmers and researchers obtained flounder eggs from broodstock, which was either caught in the wild or selected from farmed fish at least three years old. Natural spawning season lasts from April to June, but under farming conditions it varies. For commercial purposes, control of rearing environmental factors makes it possible to have batches of fish ready to spawn at all times of the year, especially in autumn.
Flounder broodstock is held in tanks rather than cages, so that they can be handled more easily for spawning. Water temperature is usually kept below 21°C. Before complete feeds were developed, broodstock was fed raw fish, which did not always provide the required nutrients and carried with it the risk of disease transfer. But now, fish are mostly fed a formulated feed which is moistened just before feeding.
Flounder do not release all their eggs at once. Eggs may be obtained six to ten times from the same fish during its spawning cycle, at intervals of two to eight days. In natural populations, minimum spawning size is about 30 cm in two-year old fish. One spawner weighing 2.5 to 4 kg can lay a total of about 3 million eggs, through successive batches of 40 000 to 450 000 eggs each.
The use of a hormonal treatment and of stripping is not required because ripen males and females can spawn naturally in captivity. Normally, it is expected that at least 90 percent of the eggs are fertilized and that over 80 percent of these fertilized eggs produce larvae.
More and more broodstock being hatchery-raised fish, this gives the opportunity for improving flounder broodstock. In 2000, the construction of a modern fish farm was approved by the fishery authorities of the Shandong Province. Its objectives are not only the selection and genetic improvement of broodstock, but also the production of healthy fertilized eggs and fingerlings to be distributed to farmers or to be used for the enhancement of natural marine stocks.
Egg incubation
Ripe eggs can easily be collected from spawning tanks. Once fertilized, eggs are collected into screened containers (egg collectors) and transferred to incubation tanks. Hatching rate is higher in a darkened room. Water temperature is also important: hatching rate reaches 90 percent at 14 to 16°C but drops to 60 percent when water temperature rises to 22°C (Table 25).
Table 25. Relationship between water temperature and flounder eggs hatching rate
|
Water temperature |
14 - 16 |
18 |
20 |
22 |
|
Incubation period |
63 h 30 min |
59 h |
51 h |
45 h 10 min |
|
Hatching rate (percent) |
90 |
74 |
70 |
57 |
Dead eggs and debris are removed daily to prevent bacterial and fungal contamination. Live eggs are disinfected immediately before hatching. At 14 to 16°C, hatching occurs after about 63 hours (Table 24) producing fragile yolk sac larvae from 2.13 to 2.95 mm long. The yolk sac is relatively large but there is no functional eye or mouth yet (Figure 19).
Yolk sac larvae development
The yolk sac stage lasts for four to five days. During this period, larvae develop from their yolk reserves. They are sensitive to light and temperature. Larval development needs a light intensity of 400 to 600 lux, survival and growth being affected if light intensity is lower than 40 lux or higher than 1000 lux. At the water temperature of 20°C, larvae become females and to increase the proportion of males, water temperature should be maintained between 15 and 19°C. Survival during generally ranges from 50 to 70 percent. By the end of this period, larvae are about 3.8 mm long.
First feeding
About four to five days after hatching and just before the mouth of the larvae opens, rotifers should be distributed. In China, Brachionus plicatilis tipicus and B. plicatilis rotundiformis are widely used for the first four to five days. Then, brine shrimp nauplii are preferred.
Larval rearing
Larvae are fed brine shrimp (Artemia) nauplii hatched from dried eggs and/or copepods obtained by filtration of natural sea water.
The quality of these live feed was the subject of much research. Brine shrimp nauplii are an incomplete source of nutrients, responsible for low survival, incomplete metamorphosis and/or abnormal pigmentation. They have to be supplemented either by copepods or by enriched brine shrimp nauplii.
Metamorphosis
Flatfish start their life upright, like a round fish. They turn on to one side which then becomes the belly, during metamorphosis. The eye and nostril on that side move up and over the head, joining the other eye and nostril on what now becomes the back. This extraordinary biological change usually occurs 35 to 40 days after hatching, depending on larval growth rate and water temperature. Not all larvae metamorphose at the same time. As bigger larvae start eating smaller ones, size grading becomes necessary. Therefore, synchronization of metamorphosis should be considered as a priority.
Weaning period
Weaning occurs when the diet of newly metamorphosed juveniles is changed from live food to artificial feeds. Young fish are then 10 to 20 days old and they weigh between 20 and 100 mg. Both types of food are offered together, the supply of live food being gradually reduced. Feeding rate depends on average size of juvenile fish (Table 26). Feeding frequency is five to six times a day. This process usually takes 30 days to be completed, by which time juvenile fish weigh about one gram. Expected survival is about 70 percent.
Table 26. Feeding rate of moist formulated feed according to flounder size
|
Fish total length |
Fish weight |
Feeding rate |
|
15-25 |
20-100 |
10-20 |
|
25-30 |
100-200 |
35 |
|
30-35 |
200-300 |
45 |
|
35-40 |
300-500 |
25 |
|
40-50 |
500-1000 |
20 |
Nursing period
This is an ill-defined period as regards duration or targeted fish size. The purpose of nursing is to rear young flounders until they can be moved or sold to an on-growing system, but the size at which such transfer occurs can vary substantially. For example, if they are to be on-grown in land based tanks, transfer can take place rather early, when fish are 10 to 15 cm long.
Once weaned, young flounders grow quickly. If optimum water temperature is maintained, they should reach 5 to 10 cm total length about 150 days after hatching. During this period, moistened pellets are fed at the rate of about 5 percent of body weight. Up to the length of 10 to 15 cm, young fish are still very vulnerable to Vibrio disease. Vaccines can now be used to provide some protection.
To optimize growth until fish average 15 cm, water temperature should range from 18 to 23°C and light intensity from 500 to 1000 lux. Depending on location, different strategies are used in flounder hatcheries, either individually or in combination, to maintain optimum water temperatures during the nursing period. In early spring or late autumn, warmer water from a power station or a deep well is commonly used for this purpose. Recently, recirculation systems have become the method of choice.
Although salt water recirculation systems are not economically competitive in the long run with net pen farming for the production of market size fish, they might be ideally suited for rearing juveniles which, on a per unit weight basis, are more valuable. Another reason for this, is that recirculation systems can use extremely shallow raceways (water 50 to 60 cm deep) where flounder species have been shown to do well.
On-growing period
Flounder are naturally docile and not easily agitated. As a result, they subject themselves to little stress under farming conditions and, therefore, do better than more excitable species. They also like crowding together, though, as flatfish, they do not fully use the water column as do round fish such as large yellow croakers. In fact, stocking densities for flounder are usually expressed in terms of kilograms per square metre, rather than kilograms per cubic metre as they are for round fish. Optimum stocking density for flounder varies from 20 to 30 kg per square metre. This does not appear to stress them and, in this respect, they are similar to other farmed flatfish such as the European turbot and Atlantic halibut.
Flounders accept dry formulated feeds well and convert them efficiently, the feed conversion ratio (the weight of distributed feed per unit weight gain) being equal to 1:1 or a little more. This might be due to an intrinsic virtue of flounder metabolism and/or to a sedentary life style. If such excellent feeding efficiency could be achieved in large scale commercial systems, it would provide flounder farmers with a significant advantage from the economic point of view.
