There are more than 300 species of scallop that have been identified worldwide. Among the 40 species distributed along the coast of China, Chlamys farreri, Argopecten irradians, Pactinopecten yessoensis and Chlamys nobilis are the most important economic scallops being cultivated in the country.
Scallops are distributed world-wide. However, various species require specific environmental conditions for survival and reproduction and are therefore found only in coasts of certain geographical regions.
Chlamys farreri is a species native to the coasts of Northern China and western Korea. The suitable habitat for this scallop is where the water depth is 10–30 m, tidal current is swift, water temperature is relatively low, the bottom is rocky reef and the salinity and transparency are relatively high.
Chlamys nobilis is mainly distributed in Japan, Southern China and Indonesia. The environmental conditions of its habitat are quite similar to those of Chlamys farreri, except for water temperature, the suitable level of which for the former being higher.
Pactinopecten yessoensis occurs only in Japan. Through transplantation, it has been popularly cultivated in China for many years. Being a cold water species, P. yessoensis can only be cultivated in the temperate sea regions of Northern China.
Argopecten irradians, the bay scallop, has a wide geographical distribution along the Atlantic and Gulf coasts of the United States. After being transplanted into China in 1982, the bay scallop gradually became one of the dominant species of scallops cultivated in China.
The shell of this species is medium in size and fan-shaped. Both shells are convex, but the right one is flatter than the left. The height is slightly greater than the length, while the width is ⅓ of the height. The straight hinge is composed of umbo and an anterior ear and a posterior ear in each shell. The anterior ear is bigger than the posterior one. The shape of anterior ears in both left and right shells is different. The sunken ventral portion of the anterior ear of the right shell makes it possible to leave an aperture when the two shells are closed. The aperture is the only exit for byssus, which keeps the scallop firmly attached to a hard substratum.
The color of outer side of shell is variable, it being dark brown, light yellow, apricot pink or grayish white. There are about 10 (8–13) well developed ribs on the left shell and about 30 relatively thin ribs on the right shell, with many sharp brambles extending from the ribs on both shells.
The inner side is generally white with brown spots. However, the colour is pink in some individuals.
Though slightly smaller in size, both shells are more convex than those of Chlamys farreri. The outer side of the shell is yellowish-brown, with about 20 high brambles and rather wide ribs. The anterior ear is bigger than the posterior.
This is bigger than other species, its shell being more than 20 cm long. The left shell is flatter than the right. The right shell is yellowish-white in colour, while the left shell purplish-brown. The anterior and posterior ears are same in size. There are about 23 (21–24) ribs on each shell.
The height of the shell is almost equal to its length. The colour of the outer side of shell is variable, it being light purplish-brown, yellowish-brown or pink. There are about 23 ribs on each shell.
The description of digestive and reproductive systems given below is based on Chlamys farreri (Fig. 1).
The digestive system is mainly composed of mouth, labellum, stomach, descending and ascending intestine and anus.
| a. | Labellum. | It is situated at the end of gill and is composed of two pairs of labial palps, with one pair on each side. Each pair of labial palps can be divided into inner and outer labial palps. There are a lot of cilia on the inner side of each labial palp. The food is sent toward the mouth by means of movements of the cilia. Another function of the labial palps is to prevent the food from going back. |
| b. | Mouth. | Looking like a split in the centre of two labial palps, the mouth is connected to the oesophagus, which is about 1 cm long and 3–4 mm in diameter. |
| c. | Stomach. | Surrounded by the digestive gland, the flat oval-shaped stomach is connected to the oesophagus at its upper end and to the intestine at its lower end. |
| d. | Intestine. | The intestine consists of the descending intestine, ascending intestine and rectum. |
The descending intestine is located on the left side of the body at the ventral end. From the ventral position of the stomach, the descending intestine runs downward through the gonad and then turns upward along the lower end of the gonad to form the ascending intestine. There is a cream-colored crystalline style located in the cavity of the descending intestine. Its real function has not been clearly proved, but there are several conjectures, such as that it is an appendage of the reproductive organ, a part of the reduced tongue, helps in digestion, adjusts the digestion rate and stores food.
The ascending intestine initially goes upward along the back inner edge of the gonad, continues upward along the inner side of gonad until it enters into the digestive gland (digestive diverticula). The ascending intestine is connected to the rectum at the back of the soft part.
The rectum passes through the ventricle and goes downward along the back of the adductor muscle until it makes a ‘U’ turn to form the anus.
Among the four species, only the bay scallop is hermaphrodite. The gonad is usually located between the posterio-ventral part of the foot and the front side of adductor muscle and is crescent shaped. When the gonads mature, the cream coloured testis and bright orange ovary can be easily distinguished by naked eye.
The sperms and eggs are discharged out of the body through the breeding holes of kidney gland, which is situated in the region near the gonad.
Figure 1. Internal structure of C. farreri. (1. ligament; 2. oesophagus; 3. stomach; 4. precordial cavity; 5. ventricle; 6. heart-ear; 7– 8–10. abductor muscle; 9. rectum; 10. anus; 12. eye-spot of mantle; 13–14. right shell; 15. gill; 16. mantle; 17–18. kidney; 19. mantle cavity; 20. gonad; 21. intestine; 22. foot; 23. crystalline style; 24. labial palps; 25. mouth; 26. labellum).
As in the case of other bivalves, the feeding style is passive. The food is filtered by gills and then sent to mouth by the movement of labial palps.
The food ingested by scallops is composed of phytoplankton, zooplankton, bacteria and detritus. Among the phytoplankton, diatoms constitute the major component.
The filtration rate of scallops varies with the alternation of day and night. Generally speaking, the filtration rate is faster at night than during daytime.
Under normal living conditions, the two shells of a feeding scallop are slightly open and the tentacles on the edge of mantle are extra-extended. Looking into the open shells, the black eye spots can be seen clearly on the mantle.
If the environmental conditions become unsuitable, the scallop is capable of cutting off its byssus and swimming to the ideal location by means of a water-jet generated by closing and opening of both shells. The scallop can swim faster than any other species of bivalves. When it finds an ideal place, it secretes a new byssus and attaches onto the substratum again. Since the right shell is somewhat bigger than the left one, the scallop usually positions its body in such way that the right shell is lower than the left.
The ability to secrete the byssus is determined by the size of the scallop and the water temperature. A 1983 study showed that in the size range 1.0–3.0 cm, the bigger scallops themselves faster than the smaller ones at a water temperature of 13.5 °C. The scallops cultivated in water of 19.5 °C attached themselves more quickly than those in 13.5 °C. After it has grown up, the bay scallop discard its byssus, while C. farreri retains it throughout its lifetime.
In the bay scallop, the gonad condition of bay scallop is classified into six stages. Stages I–III are immature, IV is mature, and V–VI are partially spent and spent conditions respectively. Stage I gonad is small and transparent, and the only reproductive tissues seen are narrow tubules with primary germ cells. In stage II, the gonad has increased in size and is translucent. In gross examination, testicular or spermary and ovarian regions cannot be distinguished. However, microscopic examination would reveal that a few follicles have developed spermatogonia and oogonia and the gonad became enlarged. In stage III, the bisexual nature of the gonad can be seen from the proximal white testis area and the distal pale orange ovarian portion. Spermatogonia increase in number and can be seen in clumps. A few spermatozoa are also seen, as also many half-grown oocytes with stalk and a large germinal vesicle.
In stage IV, the gonad increases considerably in volume and assumes a round form. It contains thickly packed follicles. The testicular and ovarian portions are cream and bright orange respectively. Microscopic examination shows free, active spermatozoa and mature pear-shaped oocytes. In the partially spent gonad condition (stage V), testis and ovary are differentiated by the pale white and orange colors of their respective regions. The gonads retain some residual mature germinal products. Empty spaces in the follicles of stage V gonad distinguish it from stage III gonad. The completely spent (stage VI) gonads are light brown in color, with no differentiation between testicular and ovarian regions. The gonads are shrunken and flaccid with empty follicles.
In Jiaozhou Bay, the bay scallop has two spawning peaks in a year. One occurs from late May to June and the other around September. The maturity of bay scallop can be advanced to March in early spring by conditioning with suitable water temperature and adequate food supply. This enables the raising of several batches of spats in a year in Northern China.
The bay scallop, which is hermaphrodite, usually discharges its sperms before releasing the eggs, with a time lag of about 15–20 minutes. Simultaneous discharge of both types of gametes has not been observed so far in the laboratory or in commercial spat rearing establishment. Spawning usually occurs from 7 to 9 o'clock in the evening. A few individuals may spawn during daytime.
The average fecundity of bay scallop in a single spawning is about 0.5–0.6 million, although a mature gonad contains as much as 2–3 millions eggs.
The eggs of the bay scallop are enclosed in a thin membrane, are pale orange in colour and range in diametre from 53–56 μ. Being of slightly higher density than the seawater, the eggs sink to the bottom of the container a little after spawning (Fig. 2).
Much smaller in size, the mature sperm swims actively in the seawater. The sperm of the bay scallop retains its fertilizing ability for 6 hours after discharge in waters of 16–19 °C.
Fertilization in case of bay scallop, as in bivalves, takes place in seawater. About 20 minutes after fertilization, the first polar body appears on the animal pole of the fertilized egg in water of 20–22 °C. The appearance of the second polar body follows after another 5 minutes. The appearance of polar body is usually used as the sign of fertilization in commercial spat rearing establishments.
About 40–50 minutes after fertilization, the fertilized egg begins to undergo cleavage in water of 20–23 °C. After a series of cleavages, the embryo gradually develops into a swimming ciliated larva, which eventually reaches the trochophore stage about 10 hrs after fertilization, depending on the original condition of the egg, culture method and water temperature.
During the later phase of the trochophore stage, the shell gland begins to excrete the shell. When the shell completely encloses the soft body, the larva has reached the early straight-hinge or D-shaped larva stage. From fertilization to this stage, a time of about 22 hrs is required in seawater of 20–23 °C. The larva in this stage is also known as veliger.
When the umbo begins to appear at the hinge region, the larva reaches the umbo stage. During this stage, the development of some special internal organs take place. To start with, a pair of translucent round balance organs are formed at the back of the digestive gland in the central region of the shell. Some very small particles are seen moving inside both the balance organs.
An “eye” spot appears at the lower part of the balance organ when the larvae are approximately 180 μ long. In some individuals, this spot is inconspicuous and irregular at first, but after 1–3 days it becomes regular in shape (round) and conspicuous with brown color. The appearance of eye spot signals the end of the swimming stage and the approach of metamorphosis. This is an important for indicator for the culturists to introduce spat collectors should into the tanks for the settlement of larvae.
The appearance of foot and round-shaped eye spot and thickening of shell margin indicate the end of umbo stage of veliger.
Metamorphosis, which is a gradual process, is preceded by a stage in which advanced larvae have both a functional velum and a foot, and alternately swim about and crawl on the bottom or some clean hard surfaced substratum. This stage, termed pediveliger, may last for several hours or even days depending on the conditions provided.
Initially, metamorphosis is characterized by the disappearance of the velum and retention of the functional foot. A definite narrow band then appears on the shell margin, which indicates the ending of prodissoconch and the beginning of dissoconch or post-larval shell.
The larvae in this stage are also called crawlers. During the early period of this stage, the larvae can stop crawling and swim again if the conditions are not suitable for their settlement. After a few days of crawling, the swimming ability of larvae is gradually reduced until the velum is completely reabsorbed and the settling is completed.
Once settled, organs such as foot, velum and eye spot degenerate, while the gill and adductor muscles develops quickly. The appearance of dissoconch indicates that larvae have already metamorphosed into juvenile scallops.
Temperature is a very important factor for the growth of larvae, since the enzyme in the larvae requires a relatively stable temperature to maintain its activity. The various metabolic activities of the larvae, such as digestion, respiration, growth, etc., are influenced by temperature.
Experiments and practice show that the optimal temperature for rearing of bay scallop larvae ranges from 20–23 °C. If water temperature is below the optimum, the growth rate of larvae decreases and the rearing period gets prolonged. If the temperature is higher than optimum, most of the larvae sink to the bottom of tanks and die during the early veliger stage and a only few individuals grow up to metamorphosis.
Under normal conditions, the osmotic pressures inside and outside the larva's body are balanced or equal. If there is any variation in salinity, the larvae must consume a certain amount of energy to adjust their inner osmotic pressure to reach a new equilibrium with sea water. Thus salinity can directly affect the growth and survival of larvae. If salinity goes beyond the normal tolerance range of the larvae, they may die due to non-equivalent osmosis. The optimal salinity for the larvae of bay scallop is 25±1 ppt.
Figure 2. Developmental stages of the bay scallop, Argopecten irradians. (1. sperm; 2. egg; 3. fertilized egg; 4. extrusion of first polar body; 5. extrusion of second polar body; 6. first cleavage; 7. second cleavage; 8–9. morula stage; 10–11. trocophore stage; 12. veliger stage; 13. umbo larvae; 14. spat).
The optimal pH value for the larvae of bay scallop ranges from 7.8–8.2, while dissolved oxygen (DO) must be maintained at a concentration of not less than 5 ppm. The heavy metal content in seawater should be kept at the following concentrations:
| Hg | ≤ | 0.004 | ppm | Cu | ≤ | 0.01 | ppm |
| Cd | ≤ | 0.03 | ppm | Zn | ≤ | 0.1 | ppm |
| Pb | ≤ | 0.1 | ppm | A1 | ≤ | 0.1 | ppm |
The purpose of larval rearing is to produce as many larvae as possible in a limited water volume at minimum cost. Although the larvae can certainly grow quickly at a low larval density, the total number of juvenile scallops would be insufficient to meet the requirement of culture. On the other hand, over-crowding is undesirable, since it may reduce the growth rate of larvae and increase their susceptibility to diseases. Therefore, the determination of maximum concentration at which larvae can survive and the optimal concentration for their growth is an important consideration in the rearing of bay scallop larvae.
Under normal conditions, the optimum larval density in the case of bay scallop is 8–10 individuals per millilitre. If the conditions, such as food supply, water exchange, etc., are kept at the optimal level, the density may be increased to 20 individuals per millilitre. The rearing concentration should be adjusted according to the conditions provided in the hatchery.
The effect of illumination on veliger larvae is significant. When they are exposed to intense illumination, the swimming larvae gradually sink to the bottom of the container due to their negative phototropism. If the illumination is too weak, the phytoplankton cultivated as food sinks to the bottom, which is harmful for the growth of larvae. Furthermore, a dark room is inconvenient for the hatchery operator. Based on observation and experiments, an illumination intensity ranging from 400–700 lux with diffused light has been found to be suitable for the growth of larvae.
As soon as the digestive system is completely formed on the second day of D-shaped stage, the larvae can readily feed on unicellular phytoplankton. The naked flagellates, such as Monochrysis lutheri and Isochrysis galbana, are ideal food organisms, which can induce better growth than any organism having cell wall. Platymonas sp. and Chlorella sp. are also popularly used as feed for bay scallop larvae. Experiments have shown that mixed food composed of several species of phytoplankton promote more rapid growth than one-species food.
Artificial diet can also be used as larval feed, but it should be incorporated with unicellular phytoplankton. The optimal concentration of food cells varies with the size and developmental stage of larvae. The suitable concentration of food cells for the larvae of bay scallop is described in subsequent sections.
Even under the best of conditions, there have been occasionally heavy mortalities of larvae and juveniles that could not be accounted for by mistakes in technique. In some cases, the heavy population of bacteria may cause epizootics and kill almost all the young bivalves in a short period of time.
Apart from treating the water as much as possible to keep it pure, antibiotics, such as terramycin, chloromycetin, penicillin, etc., are usually used to eliminate fungal infections in the larval culture tanks. The concentration of antibiotic depends on the degree of infection. In addition, if the concentration of heavy metals is higher than the permitted level, 3–6 ppm of EDTA should be kept in rearing tanks throughout the cultivation period.
In the temperate zone, the shell height of bay scallop increases at the rate of 1 cm every month during summer. When water temperature goes below 10 °C, the growth rate of shell gradually slows down, but does not stop unless the temperature drops below 5 °C. During early winter, the weight of the soft part begins to increase rapidly. In Northern China, the growth of shell of bay scallop stops during January - March.
The enemy organisms of the bay scallop are usually divided into three groups, according to their living behavior.
| a. | Predators. | These include sea star, flounder, drilled conchs, etc. Among them, the sea star is the most harmful. |
| b. | Parasites. | These usually live in the gill cavity of the scallop and absorb nourishment from its soft part. Their action also directly affects the filtration rate and thus impairs the growth of scallop due to lack of food. Pinnotheres is typical among such parasites. |
| c. | Foulers. | There are many species of sessile fouling organisms, such as mussels, sea squirts, sponge, diatoms, seaweeds, hydroids, oysters, flat worm, fungi, annelids, etc. Most attach themselves to the on-growing facilities such as raft, rope and net cage and some attach directly onto the shell of scallop. These fouling organisms not only compete for space and food with scallop, but also block the mesh of net cages, affecting the growth and survival of scallops due to lack of food and poor water exchange. |
Mortality in the older culture sites is generally higher than in new seafarming areas, and occurs more frequently during summer and autumn than in winter and spring. Furthermore, heavier mortality occurs with higher culture density and bigger sized individuals.
An unsuitably high culture density may be the main cause of mortality. The reasons are: (1) the scallops cannot obtain sufficient food; (2) a greater amount of metabolic waste is produced, the decomposition of which will consume great quantity of dissolved oxygen; (3) the products of decomposition, especially sulphide, are toxic to scallops; some experiments have shown that when the sulphide concentration reaches 0.3 ppm, the filtration rate of scallop is reduced by about 30 %, and at 0.7 ppm, feeding and respiration stop; and (4) the weak individuals in a crowded and adverse environment are susceptible to disease.
Other important factors that cause mortality are high temperature and high density of fouling organisms.
The density of scallop culture should be determined based on the carrying capacity of the sea region and the technique of culture.
The equipments required for the culture of bay scallop larvae include a water supply system, rearing tank, unicellular algal culture tank, heating system, aeration system, power system, observation room, laboratory, spat collector, etc. The design and construction of the facilities should depend on local conditions. The essential facilities and main equipments are described bellow.
The hatchery must be situated in a pollution-free area with good quality seawater and mild waves. It should also be suitable for nursery culture. The hatchery should not be situated in an area with muddy bottom that contains high organic matter and water of low transparency.
Other factors that should be taken into consideration are availability of electricity, freshwater supply, good transportation facility and other supplies and services.
The installation should be compact and rational, with the different facilities linked to each other. The observation room and the unicellular algae culture tanks should be near the hatchery workshop. The heating system should be concentrated, with the pipeline as short as possible to reduce the surface area for thermal diffusion. The foundation must be laid firmly to prevent land depression and tank leakage.
The water supply system comprises the pump, pipeline, filter tank and purification tank.
Pumps are made of various materials, viz. iron, stainless steel, glass-fibre reinforced plastic, ceramic and plastic. The pumps that contain heavy metals or other harmful materials must be avoided. The pump house should be situated near the seashore to reduce the drawing distance. The drawing distance of pump should be greater than the drop between the pump house and the sea level and its lift should be greater than the drop between the top of the filter tank and the pump. In addition, the pump should be able to deliver the total water requirement of the hatchery in three hours at least. The inlet for the water supply should be placed where the water is deep and clear, and protected by cage. Two pumps should be available to ensure proper maintenance.
Wave-resistant wire-pipe or iron-pipe should be used as inlet pipe, since it extends into the sea. When iron-pipe is used, a drainage valve must be set to discharge the rusty water before transporting seawater to the filter tank. Polyvinyl chloride pipe is generally used for the indoor pipeline.
The precipitation tank should be built on an elevated area or platform to save on electric energy. If it is built at floor level, a secondary lift facility should be used.
This tank should be divided into 2–4 chambers and its holding capacity should be 3–4 times that of the rearing tanks. Generally, the tank is 2 m deep, with a cover and an inlet pipe at the top. The bottom should have some slope and a sewer pipe with a valve. The tank's outlet pipe should be 30 cm above the bottom. The tank should be cleaned at least once a week when it is in operation.
Sand filter is a mechanical filter device, which uses electrostatic force to absorb the suspended solids and separate them from the water.
The sand filter is placed in a concrete structure. Its top is almost at the same level as that of the bottom of the precipitation tank, but higher than the rearing tanks and unicellular algae culture tanks. If the precipitation tank, the sand filter and the rearing tank are on the same level, a secondary pump should be installed for pressurized filtration. The total filter area (m2) should be about 1/30 – 1/40 to the total volume of rearing tank (m3). For the same filtration area, two filters will be better than a single filter for performance and maintenance.
To the top of the filter tank is fixed an inlet pipe, which comes from the precipitation tank. At the bottom, there is an outlet pipe to the rearing tanks in the hatchery workshop and a water storage space up to 15–20 cm in height, above which is placed a plastic or cement sieve plate of 1 cm mesh to support the pebble and sand filtering layers above. Sand and pebbles are filled in according to size in different layers, which are separated from one another by sieve plates. The composition of these filtering layers is given below:
| Layer (from the top) | Granular Diameter (mm) | Thickness of Layer (cm) |
| 1 | 0.1–0.2 | 15–30 |
| 2 | 0.2–0.3 | 20 |
| 3 | 0.4–0.5 | 10 |
| 4 | 1 – 2 | 5 |
| 5 | 5 – 10 | 5 |
| 6 | 20 – 30 | 5 |
When a filter is used for the first time, it should be filled in reverse with water entering from the base, to ensure its normal working. Then it can work normally. Filling water from the top will destroy the sand layer. The layer should not be allowed to dry up when the filtering stops, in order to avoid the rising air bubbles that would stir up the sand. A reverse washing should be resorted to every 3–4 days.
The rearing system includes a parent scallop-conditioning tank, spawning-cum-hatching tank, larval rearing tank all three of same size, preheating tank, aeration and inlet pipeline and heat exchangers. All of these are built in the hatchery. There is need for heat conservation and a light control device. An observation room should be provided at the side. Detailed specifications of these facilities are given below.
Rearing tanks. The rearing tanks used for commercial production have different sizes. The small, medium and large rearing tanks have volumes of usually less than 10 m3, 10–30 m3 and 30–100 m3 respectively, with respective depths of 1.0–1.2 m, 1.3–1.5 m and 1.5–2 m. The tanks are generally rectangular with rounded corners. Tto facilitate easy handling, the tanks are sunk into the ground with only about 80 cm of their height above the. The inside surface of the tank should be smooth. The slope of the bottom is 0.5–1 %. At the bottom it has 1–2 outlets, each with a 5–10 cm long plastic pipe extending to the sewer channel. The discharge capacity should be such that all the water from the tank can be drained in half an hour.
The newly built concrete tank must be soaked with water for one month to remove the alkalinity, and the water should be changed every 3–5 days to keep the pH below 8.4.
Inlet and outlet pipelines. Each tank should have at least one inlet valve. The main inlet pipe and its branches must have enough flow and should be able to fill all the tanks in about 8 hours. Below the outlet pipe, there is a sewer channel with a slope of 0.5–1.0 % to avoid water accumulation. The channel must be able to drain off the water without overflowing in case all the tanks are being drained simultaneously. It should be 80 cm in width and 60 cm in depth beneath the tank bottom and can be used for larva transfer from tank to tank. There are cover plates on the channel for easy routine work and walking.
Large scale larval rearing stations use boiler for heating. Filtered water is poured into the preheating tank and then steam produced by the boiler is let in to heat the water. A boiler with an evaporating capacity of 1 t/hr could meet the demands of a 400 m3 rearing volume.
The preheating tank should be near the boiler and its total carrying capacity should be about ¼ – ⅓ of the capacity of rearing tanks. The steam intake pipe, which extends vertically down to the lower part of the tank, has a control valve just before it enters the tank. A little before this valve, there is a branch pipe for draining purpose.
Roots blower, which has large airflow, steady pressure and cannot be easily polluted by oil, is suitable for large scale hatcheries. A blower with pressure of 0.2 kg/cm2 will be suitable for a water depth of 1 m.
Polyvinyl chloride (PVC) pipe is used for aeration pipe, while a plastic flexible pipe with an airstone at the end is used for branch pipes. Airstones of 100 and 80 meshes can supply sufficient air to 4–5 m3 and 2–3 m3 water masses respectively.
Unicellular algae comprise the main food of cultured scallop. Their culture is, therefore, important for use in hatcheries.
Details of various components of the unit and their operation are furnished below.
a. Equipment and Materials
i. The algal culture room
The algal culture room should face South or North and there must be no high building around to block the sunlight and keep out the wind. The light intensity should reach 10,000 lux on sunny days, which is achieved by providing for light incursion from more than half of the total area of the roof and walls. For this purpose, corrugated tiles of glass fibre reinforced plastic are used in the roof, while doors and windows are provided on the walls. Curtains are used to regulate light intensity.
An aeration system should be installed, failing which aeration is done by regular manual agitation.
The water supply system is similar to that used for the larval rearing pond. But the water must be filtered by ceramic filter or disinfected with UV light or by a suitable chemical.
The capacity of the algal culture tank should be the same as that of the larval rearing tank.
The algal culture tank should be at a higher level than the larval rearing tank or it can be built upstairs, to facilitate easy transfer of algae to rearing tanks. However, the drainage pipeline systems of the two tanks must be separate, in order to prevent chemically polluted water of the algal culture tank from entering the larval rearing tank.
ii. Seed algae preservation room
Besides having the same equipments and requirements of the culture room, the preservation room requires both heating and cooling. The temperature should not be lower than 15 °C in winter, or higher than 25 °C in summer.
iii. The algal culture tank
The tank should be rectangular with rounded corners and laid out from South to North. The area of one pond is about 10 m2, with a depth of 70 cm. There should also be some small ponds and deeper ponds. The smaller ponds can be used for seed algal culture and the deep ponds can be used for large scale algal culture to feed the broodstock. The deep ponds can also be used as larva rearing ponds.
The bottom and the walls of the tank should be covered with white ceramic tiles. The bottom should have a certain slope, so that the water can drain away completely by gravity flow. The inlet pipe and the algal transport pipe should be easy to clean.
If a new cement tank that has not been water-soaked needs to be urgently used, it must be painted with Rt waterproof coating.
b. Equipment Disinfection
The flasks and other glass containers should be washed, their mouths covered with a piece of paper and then sterilized in an oven for about 2 hours at 120°. The spoons are sterilized in alcohol and the emulsion pipe and PVC pipe with boiling water. Tweezers and other metal tools can be heated over a fire for disinfection.
The tanks are disinfected by one of the following methods:
i. Bleaching powder disinfection
The tank is covered with bleaching powder paste and then washed with water.
ii. Potassium permanganate disinfection
The tank is first washed with 25 ppm potassium permanganate solution, which is then diluted to 5 ppm and allowed to soak for 15 minutes. It is then drained and the tank washed with water.
iii. NaClO disinfection
The tank is cleaned and filled with water, to which is added the requiste quantity of sodium hypochloride (NaClO) to provide 3–5 ppm effective chlorine. After 4 hours, it is neutralized by the addition of sodium thiosulphate, which renders the water suitable for inocculation.
iv Phenol disinfection
The tank and the tools should be washed with 3–5 % phenol solution and soaked for an hour before washing with water.
Cupric sulphate and some other chemicals are also used for disinfection.
c. Culture of Stock Algae
i. Solid culture media
In the preparation of any solid medium, at first the required quantity of nutrient salts are added to a certain quantity of disinfected seawater. The nutrient salt composition differs for different algae. For example, in the case of Platymonas spp., the requiremen is N:P:Fe = 10:0.1:0.1 ppm. Agar is then added at the rate of 1.5–2.0 % of the medium solution, which is heated till the introduced agar dissolves completely. The culture tubes should be blocked with cotton stopper and the culture dishes covered. All the tubes and dishes should be wrapped in paper, after which they are placed in an autoclave for 40 minutes disinfection. The tubes must be kept in a standing position. After the culture tubes and dishes have cooled down, they are inocculated with the stock algae and cultured under optimum light (100–500 lux) and temperature (10 °C) conditions. When the algal colony is established, the tubes and dishes are moved to the stock algae preservation room, where they can be stored for 6–12 months at 5–8 °C.
ii. Liquid culture media
For preparing liquid culture medium, flasks of 250–500 ml capacity are filled with filtered seawater and heated to boiling (90–100 °C). After the water cools down, requisite quantity of nutrition salts are added, followed by inocculation with the stock algae to be cultured. Culturing is done in suitable conditions of light and temperature. Change of bottles and addition of nutrition salts enhances the growth of the algae.
d. First-level Culture
10,000 ml capacity flasks are used for first-level culture. The flasks should be cleaned by water and disinfected in the autoclave. Requisite quantity of seawater is poured into the flasks and boiled water and nutrient salts are added after it has cooled down. It is then inocculated with the concerned alga taken from the pure culture. The flasks are covered with disinfected paper. Different kinds of algae should be laid separately. The flasks are kept under optimum illumination from an indirect light. The flasks should be shaken at least 3 times a day. For getting luxuriant growth, some extra quantity of culture medium is added to the extent of 1/5; – ¼ of the total volume of culture medium in the flask once every 2–3 days or even daily. If one algae is contaminated by another kind, it should not be used as stock algae any more, but can be used to feed the larvae. If the algal culture is contaminated by protozoans, it must be dicarded outright.
e. Second-level Culture
Flasks and jars of 20,000 ml and small tanks of about 1 m3 capacity can all be used. The water used for flasks should be boiled, while the water used for jars and small tanks is treated similarly as in the case of third-level culture.
f. Third-level Culture
The third-level culture is the production culture. The tanks and the tools (such as mixer, air pipe, airstone) should be disinfected. The depth of culture medium (seawater + nutient salts) in the tanks is about 10–20 cm. The inoculation volume is about 1/5; – ⅓ of the total volume, depending on the amount of the stock algae.
The tank should be aerated or stirred 4 times a day and extra culture medium added every 2–3 days. The light intensity must be regulated. Artificial light may be considered on cloudy days.
Within about 5 days of culture, the density of the alga can get to 2 million cells/ml. It can be readily used as larval food. Algae slightly contaminated with protozoans can be used, but cultures heavily infested with protozoans must be discarded.
Feeding the larvae with several kinds of algae has been seen to be better than feeding with only one species.
g. Culture Conditions of Selected Unicellular Algae
The algae which are commonly used for feeding the larvae of the bay scallop and sea cucumber include Phaeodactylum tricornutum, Isochrysis galbana, Dicrateria zhenjiangensis, Platymonas spp. Their culture requirements are detailed below.
Phaeodactylum tricornutum Bohlin
Light: 3,000–5,000 lux
Isochrysis galbana Parke
Light: 3,000–8,000 lux
Dicrateria zhanjiangensis Hu var. sp.
Light: 3,000–8,000 lux
Platymonas spp.
Light: 5,000–10,000 lux
h. Algal Count
There are many counting methods, but the haemocytometer method is most commonly used.
The haemocytometer is a glass plate, which is thinner than the commonly used glass slides. The central depressed part of the naemocytometer is divided into 9 squares, each measuring 1 mm2 with a depth of 0.1 mm. When covered by a cover slip, each square has aspace of 0.1 m3.
For counting, a small amount of water containing the algae is put on the counter beneath the cover slip and the number of algae in 2 of the squares are counted under a microscope and their average taken. Two sub-samples are taken from every sample for counting.
The number of algal cells per ml of algal culture water is calculated as folloes: average number per haemocytometer square × 10,000 × dilution factor.
i. Stock Solution of Nutrient Salts
Usually, NaNO3 is used as nitrogenous fertilizer, KH2PO4 as phosphate fertilizer, FeC6H5O7.xHO as ferric fertilizer and Na2SiO3 as silicon fertilizer.
It is more convenient to use stock solutions in algal culture. The concentration of nutrients in the stock solution should be several folds that of the same nutrients in the seawater that is going to be used for algal culture.
The account given below refers to the production of spat of the bay scallop, Argopecten irradians. The various steps include (1) cultivation of parent scallops, (2) induction of spawning, (3) fertilization, (4) rearing of larvae and (5) collection of spats.
Being a hermaphrodite each individual bay scallop has both testis and ovary, as proximal and distal parts of the germinal gland. The ovarian and testicular parts are located along the outer margin and inner side respectively of the ventral region of soft parts. An immature gonad is usually covered by a black film. When the black film disappears, during the maturation period, the bisexual nature of the gonad can be made out from the cream coloured testis region and the orange coloured ovarian portion. In Northern China, the bay scallop has two spawning peaks in a year, the first in May and the second around September. In Southern China, the bay scallop spawns several times in the course of a year.
By culturing the scallops with adequate food in water of temperature maintained at optimum level, it is possible to advance their maturity from May to March in Northern China.
In the normal course, the bay scallop breeds in May and the resultant spats can be cultured only for about 6 months before the temperature drops down sharply after December. Since the size attained by then is less than the more preferred marketable size of 6 cm shell height, it is necessary to prolong the culture period. For this purpose, the breeders are cultivated in Northern China in indoor tanks, where the temperature can be maintained at the optimum level, which makes it possible to advance their maturity from may to March. The advancement of spawning by two months, by which time the scallops easily attain the marketable size. However, several factors are required to be taken care of in order to be able to bring the breeders to good conditions.
a. Selection of Breeders
Cultured scallops with 5–6 cm in shell height can be selected as breeders. Fouling organisms attached to the shells must be removed and cleaned by brushing as soon as the scallops are taken from their culture sites. Care should be taken while brushing so as not to damage the ligament. If the ligament is damaged, both shells of the scallop would never close as usual and the scallop soon dies. Well-selected parent scallops are usually put into lantern net cages and cultivated in the breeder tanks. The lantern net cages, tanks, etc., which could get in touch with scallops must be sterilized with potassium permanganate or any other disinfectant.
b. Breeders Culture Density
A density of 80–100 scallops per cubic meter has proved to be suitable. Each chamber of a lantern net cage can provide adequate space for 15 scallops. At this density of stocking, adequate fertilized eggs can be obtained for larval rearing in each tank.
The water in the breeder tanks should be totally renewed every day with pre-heated water. When the old water has been drained out completely, the sediment on the bottom of tanks must be removed and cleaned every day before the tanks are filled again. Scallops in the lantern net cages should be checked at least once a day for dead individuals. Dead scallops must be removed as soon as possible in order to prevent the rest from being infected with disease.
c. Water Quality Requirement
In order to keep the temperature as stable as possible, two or more tanks are used as the pre-heating tanks. With the constant availability of pre-heated water in the pre-heating tanks, the water in the broodstock tanks can be renewed any time so as not to cause temperature fluctuation. Two pre-heating tanks are usually used alternately.
The concentration of dissolved oxygen in the water of the broodstock tanks must be kept at more than 4 ppm and at pH 7.7 to 8.2.
d. Food
Unicellular algae, such as Phaeodactylum tricornutum, Isochysis galbana and Thalassiosira pseudonana, are effective food for parent scallops. In the early stage of broodstock rearing, P. tricornutum is the main food for parent scallops as it can be easily cultivated in low temperature. The other algae become the dominant food during the later stages of rearing.
The daily ration of food organisms for parent scallops is 20–80 l per m3 of breeder tank water, with the exact quantity depending on the size of cells and the feeding ability of the scallops. It is generally given in six equal installments. Experiments have also demonstrated that mixed feeding with several species of unicellular algae promote more rapid gonad development than feeding the same quantity of any one species alone.
Once a film appears on the water surface, the ration must be reduced as quickly as possible. The appearance of a film means that the food ration has gone far beyond the feeding ability of the scallops; pseudofecaes are produced to form a film on the water surface. Overfeeding is harmful and can cause heavy mortality. By adopting the steps described above, the parent scallops can be maintained at a survival rate of 70–80 %.
The artificial diet that has been formulated by the Yellow Sea Fisheries Research Institute has proved to be an effective food for parent scallops, even better than unicellular algae to a certain extent. Fed with this diet, the scallops can attain a gonadal index, which is 25 % higher than that obtained with unicellular algae, and have a better survival rate up to even 98 %. With such advantages as easy processing, low cost, rich nutrient content, this diet can completely replace the unicellular algae as food for parent scallops during breeder rearing.
Feeding the artificial diet at night and the unicellular algae in daytime is an effective and practical method for broodstock rearing.
e. Aeration
Among the shellfishes, bay scallop is one species that consumes a lot of oxygen for survival and growth. If DO concentration is lower than 4 ppm, scallops open their two shells much wider than normal and their behavior, such as feeding and moving, becomes sluggish. Thus aeration is a critical measure. Aeration can supplement enough DO for scallops.
Furthermore, aeration can help to check if spawning has occurred. In an aerated tank, a lot of bubbles would appear on the water surface as soon as the scallops begin spawning.
As the date of spawning approaches, intensive checking and observation of gonads are required. Gonadal index is usually used to determine the development of gonad.
The formula for calculating gonadal index is : G=(gw/sw) 100 %, where G is the gonadal index (%); gw is fresh weight of total gonad (g); and sw is fresh weight of total soft parts including the gonad (g).
When the average gonadal index reaches about 16 % and the black film on the surface of the gonad region disappears, it is to be understood that the scallops are about to spawn.
As mentioned above, spawning can be easily noted according to the appearance of a number of bubbles on the water surface. If there is no aeration system in the tanks, a water sample should be taken from the bottom of the tank every 2 hours before the water is replaced during the later part of breeder rearing and the sample examined under a microscope for the presence of gametes. When the scallops are about to spawn, their food should be given at least two hours after the renewal of water. Once spawning occurs, feeding must be stopped and aeration increased. When the density of eggs reaches about 30 eggs per ml, the aeration must be stopped and the parent scallops taken away from the tanks.
Proper counting is a very important measure to estimate correctly the number of fertilized eggs and larvae. The counting helps in regulating the density of fertilized eggs in the tanks, in determining the quantity of food organisms required for the larvae and in determining the number of spat collectors required and the timing of their introduction into the tanks.
There are several methods for counting. The simplest but very effective counting method is described below.
A plastic or glass tube, with a diameter of 0.5 – 1.0 cm, is usually used as the sampling tube. The length of tube is about 20 cm more than the depth of the tank. A 1,000 ml beaker is kept ready for collecting the water sample.
Before sampling, the water in the sampling tank should be stirred by a swing board, so as to evenly distribute the eggs or larvae in the water column. As soon as stirring is stopped, the sampling tube is stuck vertically from the surface of water to the bottom of the tank. When the tube reaches the bottom, the hole on the upper end must be closed by the thumb and the tube lifted up as quickly as possible. Allow the sampled water in the tube to flow into the collecting beaker by loosening your thumb. Several samples should be taken from different points in one tank and all samples taken from one tank should be collected into one beaker. The beaker must be aerated to evenly distribute the eggs or larvae in the sample, after which at least 3 samples are taken by quickly sucking out certain volumes of water from the beaker. The number of eggs or larvae in the samples are then counted in a haemocytometer under low power (10×4) microscope.
Bay scallops being hermaphrodite, the ration of sperms and eggs is impossible to control. Therefore, the suitable density of fertilized eggs and normal development of larvae should be ensured by an adequate food supply and optimum environmental conditions. Experiments have shown that the fertilized eggs could successfully develop into the stage of D-shaped larvae if the density does not exceed 30 eggs per ml.
After the spawning is over, the water in the hatching tanks should be stirred with the swing boards for 30 minutes to prevent the eggs from sinking to the bottom.
i. Effect of salinity on hatching
The fertilized eggs can be normally hatched in a salinity range of 17–35 ppt, the suitable range being 22–33 ppt, with 27 ppt as the optimum salinity (Fig. 3). The salinity can be adjusted by addition of fresh water or sodium chloride, as necessary.
ii. Collection of D-shaped larvae
The fertilized eggs of bay scallop can develop into D-shaped (straight hinge-larvae) 22 hours after fertilization in water of 23 °C. Just like the larvae of other bivalves, the well-developed D-shaped larvae are usually found swimming in the upper layer of the water column. The D-shaped larvae can be collected by a small trawl net made of JP-120 sieve cloth with a mesh of 41 μ and transferred from the hatching tanks to larval rearing tanks. By towing several times, most of the larvae swimming in the upper water layer can be collected and transferred to the rearing tanks. Fewer larvae swimming in the middle layer and bottom of the tanks can be collected by siphoning or draining the water out of the tanks.
The rearing density depends on the rearing technique, food supply, capacity of tanks and quality of larvae. It ranges from 5–15 larvae per ml.
Experiments have shown that at higher temperatures within the desirable range a higher growth rate can be obtained. When grown in water of 16–21 °C, the first metamorphosed individual was noticed 12 days after fertilization. When grown in water of 22–23 °C and 27–25 °C, larvae reached settling stage in 10 days and 8 days respectively.
In Northern China, it has been shown that rearing larvae of bay scallop in 22–23 °C water temperature could shorten the rearing period and reduce costs and mortality. During the rearing period, the fluctuation of water temperature should not exceed ± 2 °C.
The larvae of bay scallop can develop well into the juvenile stage under an illumination range of 300–800 lux, but they may hide in the corners or swim in the middle layer of tanks if exposed to more intense illumination. Aggregation of larvae may cause heavy mortality and therefore the illumination is usually controlled in the range of 300–500 lux. The larvae and food cells are all well-distributed in the rearing tanks under suitable illumination.
Aeration is not imperative for larval rearing, especially at low density culture level. When larvae are reared in higher density, aeration should be provided to prevent the larvae from aggregating.
The larvae of bay scallop can survive and grow in salinity range of 18–36 ppt; 23 ppt being most suitable (Fig. 4, 5, 6).
The sediment containing dead bodies of both larvae and food organisms, metabolites and faeces of larvae, must be removed and cleaned from the bottom of larval rearing tanks regularly. Change of tanks, namely, transferring the larvae from the dirty tanks to the clean tanks has been observed to be a very effective and important technical measure for the larval rearing. The procedures and equipments are similar to those described above, except for the different mesh size of sieve.
From D-shaped larval stage to metamorphosis, two or three changes of tanks are enough to keep the water in good quality under the normal rearing density. If the density of larvae is higher than normal, change of tanks should be carried out every two to three days.
During the early period of rearing, water in the rearing tanks is replaced two times a day and about one-third or two-thirds of the volume is drained out and refilled each time. In the later period, especially when the spat collectors have been put into the tanks, the amount of renewed water should be increased based on water quality and growth rate of larvae.
Isochrysis galbana, Dicrateria sp., Phaeodactylum tricornutum, Platymonas sp. and Chlorella sp. are the main food organisms for the larvae of bay scallop. Once replenishment of water is completed, the food organisms should be fed as soon as possible. Besides this initial feeding the larvae should be fed five more time over 24 hours.
The larvae may sink to the bottom of tanks in the case of overfeeding, while under-feeding may reduce the growth rate of larvae. Therefore, calculating the suitable concentration of algal cells is a key measure for the rearing of larvae.
The following formula is usually used to calculate the daily feeding rate for the larvae of bay scallop.
| Where: | V1: | volume of water containing unicellular algae |
| V2: | total volume of water in the rearing tank | |
| C1: | concentration of unicellular algae | |
| C2: | desired concentration of unicellular algae in the rearing tank |
i. Daily observation of larval growth and development.
During the period of larval rearing, the feeding behavior and growth of larvae are observed every morning before the water is renewed. In general, the normal larvae are usually swimming in the upper and middle layers of the water. The feeding ration can be adjusted according to the food content in larvae's stomach. The number and shell length of larvae should be counted and measured every two or three days.
The growth and development of larvae are closely related to salinity, temperature, rearing density, water quality and food supply. Under suitable rearing conditions as mentioned above, the eye spot, which is located in the stomach region, will appear 10 days after fertilization, with the shell height at 180 to 190 μm. In poorly fed, crowded or generally neglected culture, the beginning of spat stage is usually delayed and the time difference between beginning and end of settling is considerably extended. Sometimes, on account of culture under unsatisfactory conditions, the larvae are unable to metamorphose and eventually die.
The appearance of eye spot is an indication that the larvae are approaching the settling stage. Once the eye spot appears, the larvae should be sieved and transferred into another well-cleaned tank. The spat collectors are then put into the tank for larvae to settle on as soon as the transfer is completed. Two kinds of collectors are commonly used in China. One is made of palm fibre rope and the other of pieces of polyethylene net.
With a high tensile strength, good resistance to rotting, large surface area per unit, amenability to easy handling and lack of toxic substances, the collector made of palm fibre ropes is the ideal one for collecting spats of bay scallop. Before using, the palm rope should be thoroughly cleaned to remove harmful organic compounds, especially tannic acid. The procedure for processing and cleaning palm fibre ropes is as follows:
First, the newly twisted coir rope must be made pliable. This is done by “dry hammering”, in which the rope is pounded with a special hammering machine. This process removes all remaining palm bark and other undesired fragments and makes the palm rope very flexible and thus easy to handle.
After soaking, the palm rope should be pounded again in a procedure called “wet hammering”, which uses the same hammering machine as in dry hammering. During hammering, the rope should be sprayed with freshwater to wash away any exuded substances.
After wet hammering, the rope should be boiled in large vats for three to five hours and left soaking in the water overnight. Finally the rope is washed in clean freshwater and dried in the sun.
A collector is composed of a cluster of curtains. Each curtain is made of palm rope of 3 mm diameter and unit weight of 3 grams per metre. Each cubic metre of water needs a number of collectors equal to 1,500 metres in length of palm rope under normal conditions.
Although efficient in collecting spats, the palm rope collector is unsuitable in turbid sea regions, since it is easily silted by mud. The heavy silt on the collectors may cause heavy spat mortality during the intermediate culture period in natural sea region.
Besides being easily processed, the collectors made of pieces of worn out polyethylene net can keep the water quality better than palm rope. Furthermore, it is especially suitable in turbid sea area regions, since it is not easily silted by mud.
The procedure for processing and cleaning the polyethylene net is as follow:
First, it is soaked in a solution of 0.5–1.0 % NaOH (sodium hydroxide) for at least one hour and then pounded and cleaned. After that, the pieces of net are washed in clean fresh water and dried in the sun.
About 2.5 kg worn out pieces of polyethylene per cubic metre (2.5 kg/m3) is considered to be suitable for spat collection.
Collectors must be distributed evenly in the tank. The tank bottom is usually covered by a layer of collectors. A number of horizontal and parallel polyethylene ropes are placed side by side over the tank. Collectors are vertically hung from the horizontal ropes and weighed down with small stones at their lower ends. This way, the collectors can move up and down freely with the fluctuations in water depth, especially during the period when water is renewed. Once all larvae get settled, food ration and illumination should be increased to the recommended levels.
Before the juvenile scallops are transferred from the rearing tanks to the sea for their intermediate (nursery) culture, the temperature of water should be decreased by 1– 2 °C each day until it is close to that of the natural sea.
Figure 3. Effects of salinity on eggs of bay scallop Argopecten irradians.
from fertilized eggs to D-shaped larvae.
from trochophore to D-shaped larvae.
Figure 4. Effects of salinity on larvae of bay scallop Argopecten
irradians.
survival rate in 72 hours from fertilization.
survival rate in 144 hours from fertilization.
Figure 5. Effects of salinity on growth of larvae Argopecten irriadians.
mean shell length in 72 hours from initial size of 120μ
mean shell length in 140 hours from initial size of 135μ
mean shell length in 164 hours from initial size of 109μ
Figure 6. Growth of Argopecten irriadians larvae throughout the whole rearing period (1985).
Nursery or intermediate culture involves transfer of the spats to the open sea and rearing them until they attain 5 mm in shell height.
Besides establishing more hatcheries, increasing production per unit area and development of multi-crops larval rearing, it is very important to increase the survival rate in nursery culture for producing enough spats to meet the demands of culture.
Because of the differences in the size of spats, condition of the sea area, culture materials and management, high mortality occurs after the spats are transferred to the open sea. At present, the survival rate in nursery culture ranges from as low as 10 % to 30–40 %.
The size of the spat depends on the materials used, net mesh, water temperature, etc. In about 20 days after fertilization, when the shell height reaches 400–600 μm, the spats can be transferred to the sea.
When all the spats have settled, the rearing water temperature should be lowered by 1–2 °C every day to approximate the temperature of the sea. This is important for increasing their survival. Each transfer leads to better survival and better growth rate. The water temperature of the sea area must be higher than 10 °C when the spats are transferred.
Two kinds of materials are used at present. One is a plastic pipe, 60 cm long and 25 cm in diameter, covered with plastic net (mesh smaller than the shell height of the spats) at both ends. The other is made of polythene bags. The size of the bag depends on the size of the spat collectors and is generally 30×40 cm. Every 8–10 bags are strung together on a rope. The bags should be separated to prevent them from rubbing each other in the sea.
The following factors should be considered in choosing a site for nursery culture.
Floating long-line rafts that are set transverse to the current and 60–70 m in length are used for nursery culture. The distance between the rafts is 8 m, while that between two culture lines is 0.5–1 m.
The culture density significantly influences survival. The spats grow fast and have high survival in low density. Generally, a suitable density per plastic pipe is 100,000 spats, while for a 30×40 cm polypropylene net bag of 40 or 50 mesh it is 10,000–30,000 spats, depending on the sea area condition.
The timing and the thinning out the growing spat also influences the growth and survival of the spats. The juvenile scallops should be distributed to more bags of certain mesh on time according to their growth.
The nursery culture in the sea may be divided into two phases. The first is from spat transfer to the first thinning when the shell height reaches 1–3 mm. The second is from the first thinning till the spats reach the marketable size of 5 mm.
First phase of nursery culture. There are two methods in this phase. One is to use 40×40×70 cm netcage. The other uses 30×50 cm polypropylene net bag. The former contains one palm screen spat collector and three spat-free collectors of the same material. The collectors are hung horizontally in the cage. The latter contains half a spat collector and 15–20 g polypropylene net piece of 1–2 cm mesh to extend the bag. Survival in the first phase nursery culture is about 30 %.
Second phase nursery culture. The second phase nursery culture uses 30×50 cm net bag of 1 mm mesh, extended by net piece. About 2,500 spats are put into each bag and every ten bags are linked on a string. When the shell height reaches 5 mm, the spats may be sold.
The management practices directly affect the survival of spats during their culture in the sea. The bags should be cleaned every 5–7 days, depending on the turbidity of the water. The operation should be done carefully to prevent the scallops from falling off. The mud outside should be washed off gently. The cage should not be taken out of the water and the cleaning should be done from bottom to top. Cleaning time and frequency depend on the amount of silt. Cleaning improves water exchange, which enables the scallop to maintain a good filtration rate to obtain adequate food organisms.
Transfer of the spat should be done in the morning or evening. The spat bags should be laid over canvas or straw bags soaked with seawater and covered with a plastic or canvas sheet, a layer of straw bags soaked in seawater and a layer of canvas. Exposure of the spats to wind, sun and rain should be avoided.
The various transport links should be well coordinated to ensure a quick and smooth transportation to the culture ground.
Grow-out culture, during which the juvenile scallops (shell height 5 mm) are reared to marketable size, takes about six months (June-November).
The bay scallop has a fast rate of growth. The juvenile scallops produced in May can reach marketable size at the end of the year. If the reproduction time is advanced to March the shell height at the end of the year would be more than 6 cm, which is the more preferred marketable size. In a warmer sea area, the scallop grows faster with a growth rate of about 1 cm per month. It grows slowly when the temperature is lower than 10 °C and would stop growing below 5 °C. However, even if water temperature drops and growth slows down from October, the soft body weight of the scallop still increases. Around August, the high growth period, the shell grows at an average rate of 0.4–0.6 mm per day.
Epiphytes and other epicommensal animals are harmful to the growth of the scallop. Silt is deadly to juveniles with shell height of less than 1 cm. The scallop grows faster in slow current. Experiments have shown that the scallop grows fastest when the current velocity is 1–5 cm per second. Although scallop can secrete byssus throughout its lifetime, the older individuals secrete only rarely. The scallop can swim at all sizes. The bay scallop has a high filtration rate, with an average of 24.4 l/hr. Its average life span is about 12–16 months and a few can live up to 18 months, but rarely more than 24 months. A large number of scallops will die after their first reproduction. Their biological minimum size is 2.2 cm. If the harvest time has to be delayed to the next year, it must be made before April, or the yield and quality would be lower.
The visceral mass condition index is the ratio of the dry soft body weight to shell weight. The visceral mass condition index of the scallop cultured in Jiaozhou bay shows an upward trend from late April, followed by a downward trend at the middle of May and upward trend again in May. At the end of May, it reaches the maximum peak of the year, followed by a rapid drop in June. It goes up again at the middle of July and reaches the autumn peak in September. In late September, it drops gradually and becomes stable. The visceral mass condition index of the bay scallop is closely related to the development of the gonad and its reproductive cycle.
The percentage of fresh weight of the adductor to fresh soft body weight is defined as the adductor rate or adductor gain. Experiments have shown that the adductor rate of the bay scallop in the Jiaozhou Bay is more than 11 % in October and reaches above 13 % in November. It is lower than 8 % from June to August and drops to its minimum of about 5 % in June. The taste is also poor at this time.
As can be made out from the above account, the seasonal variation of the adductor rate is slightly ahead of the condition index. This is probably because the development of the gonad necessitates the transfer of nutrients from the adductor.
The extent of mortality during grow-out culture would depend on the technical expertise of the handling personnel. The mortality during nursery culture and early grow-out culture, when the shell height increases from 5 mm to 1.5 cm, ranges from 20–30 %. After that the mortality rate comes down to 5–10 % towards the end of the year, if fouling organisms are removed regularly. However, inadequate management, typhoon and tremendous variations in environmental conditions as well as pollution could cause mortality of 30–50 %.
Laboratory experiments have shown that the bay scallops can tolerate temperature as low as -1 °C and as high as 31 °C. They will stop growing at a temperature lower than 5 °C and grow slowly at temperatures less than 10 °C. They will grow faster at the temperature range of 18–28 °C.
The tolerable salinity range of the bay scallop is 16–43 ppt, with 21–33 ppt as the optimum range. It cannot adapt to sharp changes in salinity.
The bay scallop grows well at the middle layer of the shallow sea; fairly well at the surface layer and poorly at the bottom layer. The preferred layer is 2–3 m below the sea surface.
The organic matter in seawater includes particulates, detritus and plankton. When the organic content is 150–398 μgc/1, the production of scallop can reach 45 to 52.5 t/ha.
During the culture period, the cage may be attacked by many kinds of fouling organisms, such as Tubularia marina, sponges, bryozoans, Polysiphonia, Enteromorpha, tunicates (ascidians), mussels, oysters and barnacles. These organisms plug the cage and block the water exchange. The epicommensal animals also compete for food and even cause diseases. Culture experiments have shows that fouling by oysters and barnacles is much less in areas far from the coast. The quantity of foulers attaching to the cage in water layer deeper than 3 m is only 10–20 % of that in the surface. At the sea bottom, fouling is practically nonexistent.
The predators include starfish, sea urchin, crab, octopus, etc., which must be removed immediately when discovered.
The areas used for Laminaria, Mytilus and Chlamys farreri culture are suitable for bay scallop culture. The general requirement are given below:
The area should have fertile water and adequate tidal exchange.
The water depth should be more than 10 m, with flat bottom of mud-cum-sand.
It should be pollution-free.
The lantern cage has been found to be the most economical, durable and easy to handle. The cage is a net tube woven with 6–12 ply polyethylene thread, separated into 7–8 chambers by plastic discs of 30 cm diameter with some round holes on them. There is a space of 15 cm between every two discs. The number of chambers depends on the depth of the water. The cage is generally about 1.4–1.5 m in height.
An important step in grow-out culture is the need to prevent overcrowding of the growing juveniles to ensure proper growth. When the juveniles have grown to 1.5 cm in the nursery bags, they are taken out and distributed on to a number of grow-out cages. About 25–30 juveniles are usually stocked in each chamber of the cage. This works out yo an average stocking density of 1.5 million juveniles per hectare. The cage has two layers of netting, the inner of 2–3 cm mesh and the outer of 1–3 cm mesh. When the scallops grow to 2–5 cm in shell length, the outer layer is remouved. This method serves to reduce labour and overall production cost, which otherwise would be more if the scallops are to be transferred to new cages. The remouval of the outer layer also helps in getting rid of the fouling organisms.
The juvenile scallops are generally placed in grow-out cages around July and are harvested in November or December, after about 5–6 months. During this period, fouling organisms should be removed. After September, with the growth of the scallops, the flotation capability of the main raft lines must be increased. The hang links should be checked regularly to prevent the cages from touching the sea bottom and getting damaged by wear and tear.
