There are about 100 species of abalone distributed throughout the world's oceans. Large abalones are mainly distributed in the temperate zone, while small specimens are typically found in the tropics and the cold zones. Ten species are considered of commercial value, and mainly occur in Korea, Japan, Mexico, South Africa, Southern Australia, New Zealand, United States of America, and China.
Haliotis discus hannai is an important species in Korea. They are distributed in the East and West coasts and are also found along the Liaodong and Shandong peninsula in China. In our country the measurement of isothermal line is conducted in which water temperature in 25 meters depth reaches 12 °C in February. With the isothermal line as a basis for identification, abalone species distribution can be roughly divided into two parts: northern and southern species.
H. discus hannai are found in the northern waters while the southern waters are the habitat for species such as Haliotis gigantea, Haliotis sieboldii, Haliotis discus, etc.
The world's commercial species include Haliotis ruber in Australia, Haliotis iris in New Zealand and Haliotis midae in South Africa. Ten species are distributed in the Pacific Coast of North America, 3 of which are commercially important. Haliotis rufescens (also known as the Red Abalone) reaches a maximum length of 30 cm. The optimum water depth for abalones is dependent on the species itself as well as on the environmental conditions of the site. Normally, abalones are found at depths up to 30 metres in the intertidal zone.
Abalones have their maximum population density at depths ranging between 3–10 metres where seaweeds, their natural food, grow abundantly.
From the veliger stage the bilateral symmetry of the abalone body turns into a spiral shape. Its body is divided into three parts: head, foot and saccate intestine.
The head is located on the anterior part of the body and is bilaterally symmetrical. It comprises a mouth, appendages and sensory organs. Compared to other shellfish, abalones have a developed and complex head. It has a pair of tentacles and two eyes at the tip of eye stalks which originate at the bottom of the tentacles.
The foot of the abalone is a creeping organ of muscular tissue which lies on the ventral part of the body. The well developed foot has a broad “sole” which allows the animal to strongly adhere to rocks or other hard substrates. The epipodes that occupy the lower part of foot form a broad-plate shape structure, while the interior surface has ten pairs of brown stripes on a grey background. Branch-shaped tentacles are typically found at the end of the epipodes.
The intestinal sac is located on the dorsal side of abalone and has several internal organs. On the outside, the mantle runs from the back to the ventral margin. The abalone shell is formed by the secretion released from the epidermal cells placed at the front margin and tip of the mantle.
Abalones belong to one of the most primitive gastropods and its round, elliptical or ear-shaped shell has a row of respiratory pore located along the left margin. As the animal grows, older pores are successively filled in and closed. The number of open pores varies among different abalone species. Shell exterior and interior color varies with species, but it is an unreliable form of identification since the color may not be clearly visible or the original description may not be accurate.
The shell comprises three layers. The cuticle, which is the outer layer, is an organic matter purely formed of concholin. It is very thin. The hard shell or middle layer (horny layer) is composed mainly of calcium carbonate crystals embedded in concholin whose chemical components are similar to the one in cuticle layer. The innermost layer called pearl layer has chemical components similar to the horny layer.
The pearl layer is made of secretions occurring over the surface of the entire mantle. The cuticle and horny layers are secreted by the mantle margin and the growth line appears on the edge of the shell. During the winter and spawning season, or under adverse environment conditions, no growth occurs, thus showing the growth stages and the physical condition of the abalone.
In the pharynx cavity there is a pair of calyx lobes and a radula which are located at the anterior part of digestive gland. These structures are important in terms of abalone taxonomy. The calyx lobe is made up of concholin and serves as the feeding organ. The long radula moves back and forth, chopping down the food with its small teeth (visible under a microscope) which is then further broken and absorbed during its passage through the esophagus, crop, stomach, spiral caecum, and intestine (Fig. 1).
The digestive system of abalone is placed on the left side of the adductor muscle located in the middle of the body. The digestive organ consists of mouth, gullet, stomach, intestine rectum and anus (Fig. 2).
Due to the spiral shape of the body, its digestive organs are curved, and the mouth and anus lay near to one side. The mouth is gibbous and ovate in shape and its walls composed of thick muscular tissue.
In the mouth are located the calyx lobe, radula and salivary glands. The gullet of the abalone is elongate and narrow. The abalone stomach is saccate and V-shaped and placed next to the gullet. The vermiform appendix and liver are attached to the stomach.
The liver is placed at the right side of the adductor muscle, protruding in the shape of an ox horn. The intestine and rectum start from the back side to the front margin of its body going round the left side of adductor muscle and then it curves back to the front side again. The intestine length is 3.27 times that of the shell. The digestive system of the abalone is long and complex, as it is with many herbivores.
Figure 1. External features of an abalone without its shell (1-anus; 2-tentacle; 3-liver; 4-gill; 5-epipodium; 6-eye; 7–10-mantle; 11–12-mantle tactus; 13-heart; 14-foot; 15–16-adductor muscle; 17-spiral intestine).
Figure 2. Digestive organs of an abalone (1-anus; 2-intestine; 3-liver; 4-gullet; 5–7-salivary gland; 8-radula; 9-mouth; 10-stomach).
The gills, lying immediately below the shell pores, function as the respiratory organ of the abalone. There is a pair of gills at the centre of the respiratory chamber, the left one being bigger than the right one. The left and back sides of the respiratory chamber are closed so as to allow the water into the gills from the upper and right sides of its head. The sea water undergoes gas exchange in the gills and runs out through the respiratory pores on the shell along with other excretions from the anus and ostium. The circulatory system of abalone is characterized by its patency. The heart in the pericardial cavity consists of one ventricle and two atria. The blood is colourless and contains amoebocytes. The blood from the heart runs to the blood sinus placed between organs through the circulation system. The blood in the sinus runs to gill artery and then to the gill itself. Oxygenated blood then runs to auricle through the gill vein. Abalones have a pair of gill arteries and a pair of gill veins.
The abalone nerve system is poorly developed. There are four pairs of brain ganglion located around the mouth: foot nerve chain, intestine ganglion, side nerve chain and the nerve conjunction which are connected lengthwise and crosswise (Fig. 3).
Figure 3. Nervous system of an abalone (1-brain-linked nerve; 2-brain ganglion; 3-nerve beside the brain; 4-nerve under the brain; 5-side pedal ganglion; 6-pedal nerve; 7-ventral ganglion; 8-pedal nerve chain under gullet; 9-intestine ganglion over the gullet; 10-gill ganglion).
The breeding season and its duration vary according to species and also closely related to environmental conditions of the habitat, generally being positively correlated with water temperature. The abalone spawning season in Korea (DPR) begins in July/August when the seawater temperature is around 20 °C and in some cases it lasts throughout September and/or October.
Abalone is a dieocious animal, with individuals of separate sexes. The gonad tissue can be visible when the shell reaches about 30 mm in length, usually after one year. The gonad encircles the ox horn-shaped liver on the right side or the back of the body. As the spawning season approaches the gonads are engorged with mature gametes and their colour becomes distinct. Male and female specimens are easily distinguished. The female gonad is either dark green or brownish, while the male organ is either milky-white or yellow. The gonads are clearly seen under the edge of the mantle membrane or under the foot, and can be observed without dissecting the specimen.
Abalones usually spawn in the twilight of early evening or at dawn. Males usually release sperm first, therefore stimulating females to spawn. Release of eggs or sperm by one animal usually triggers the spawning of many gravid animals nearby. The eggs are fertilized in the open water. The gametes are released into the seawater via the second respiratory pore. Male ejaculation appears like a streak of grey smoke and the female egg-laying as blue smoke.
One female measuring 7–8 cm in shell length may release 1 million eggs while a bigger one up to 10 million. Generally the number of eggs laid amounts to 80 % of the fecundity. Female egg-laying is completed within 2–4 hours but male ejaculation lasts for a much longer time, usually as long as two days.
In spawned individuals the gonads become thin and wrinkled, and lose their distinct colouration.
(a) Egg. The eggs of abalone are spherical. A matured egg cell measures 220 micrometer in diameter, while the egg yolk is 180 micrometer in diameter. The transparent cellular membrane surrounding the egg is 40–50 micrometer thick. The eggs in the ovary are compressed and shaped like irregular polygons. Spawned eggs immediately absorb seawater, become spherical in shape and sink to the bottom. In some cases, artificially stimulated spawning produces cylindrical or pearshaped eggs. Most of these eggs are immature. The eggs can be classified into three categories: a) normal, b) eggs without egg membrane and, c) eggs without colloidal membrane. The last two categories are immature. These immature eggs tend to form clumps; they are rarely fertilized, and even if successfully fertilized, the embryo has an abnormal development.
(b) Sperm. The gonad of a mature male has a yellow appearance, while that of immature specimens are milky-white. The sperm has a lanceolate head and a long tail. The length of sperm head is 8 micrometer and the tail 50 micrometer. Spermatocytes move actively after they have been released into the seawater. Their life time and fertilization potential is closely related to the water temperature and the maturity of abalone itself. Sperm fertility lasts for 3 hours at seawater temperatures of 22–23 °C. Semen ejaculated by a male with an immature gonad is an irregular mass of cells which does not disperse easily. Immature spermatocytes have an indistinct head and remarkably short tail, they move with difficulty and die within an hour.
The first and second polar bodies are formed within 15 minutes after fertilization at a seawater temperature of 20–21 °C. The pole bodies are located at the apex of animal pole (Fig. 4-1). They are lustrous when observed under a microscope. Thirty minutes after fertilization, the fertilized eggs undergo the first cleavage, the embryo is now at the two-cell stage (Fig. 4-2), each cell measuring 120 micrometer in diameter. The second cleavage takes place 80 minutes after fertilization (Fig. 4-3).
Two hours later, the third cleavage takes place, and the embryo is at eight-cell stage. Of the eight cells, four are smaller than the others, the former to be carried the side of animal pole and the latter the side of plant pole (Fig. 4-4).
In 160 minutes the embryo becomes 16-celled (Fig. 4–5) and in 195 minutes, it attains the morula stage (Fig. 4–6).
Approximately 6 hours later, the embryo becomes slightly elongated having reached the gastrula stage with the characteristic blastopore (Fig. 4–7).
Seven and-a-half hours later, the ciliary belt appears on the developing embryo. The embryo is now about 180 um long and 160 um wide. It begins to actively rotate within the egg membrane with the aid of the ciliary belt and the apical hairs (Fig. 4–8).
The larva begins moving more frequently inside the egg membrane, and 10–12 hours later the membrane becomes thinner, until it finally bursts (Fig. 4–9).
The newly hatched trocophore is 210 um long and 168 um in diameter. The free-swimming trochophore is phototactic and therefore tends to move towards the upper water layers. Approximately 15 hours later, the larva develops into the veliger larval stage; the swimming cilia are now borne by a distinctive collar of cells, known as the velum, surrounding the developing head (Fig. 4–10).
At this stage the larva is 224 um long and 196 um wide. Within 26–28 hours, the later veliger develops an eye spot, foot, operculum, and a protoconch similar to that of a snail (Fig. 4–11). The larva always keeps its shell downwards and actively swims up and down the water column with the help of the velum. When disturbed, the larva immediately retracts its foot and body inside the shell and seals the aperture with a closable operculum.
Three days later, while approaching the settling stage, the foot grows bigger and the velum degenerates and disappears. Six days later, the larva reaches 300 um in length and 220 um in width.
At this stage wrinkles appear on the peristomal shell and spread like a tuba (Fig. 4–12), and as the shell grows outward, the aperture widens. Tentacles and eye spot appear on the head, epipodium tentacles develop as well as the gills and the mantle membrane.
When the abalone shell reaches 0.7 mm in length and 0.6 mm in width, epipodes appear (Fig. 4–13) and 35–40 days later, it becomes a juvenile having the first respiratory pore (Fig. 4– 14, 15). This juvenile is 2.3–2.4 mm long and 1.8–2.1 mm wide, and the number of epipodium tentacles are about 10.
Genetic factors greatly determine growth. However there are several interrelated environmental factors affecting the growth rate. The growth is greatly influenced by the level and type of nutrients available as well as the range of environmental conditions which affect the physiological functions of the organism. Some of these factors include feed, water temperature and stocking density. Abalone grows well at a water temperature of about 20 °C. Abalones grow at about 2–3 cm a year; it takes 4 years to grow to the marketable size of more than 9 cm. In a few cases, however, it grows only about 1 cm a year, so that it takes more than 8 years to reach the commercial size. Under good feeding conditions, abalones grow 3–5 cm a year, particularly when the juveniles are artificially released and raised in the open seawater. The abalone growth largely depends on the season: at a water temperature below 6 ° C and during the spawning season, abalones tend to grow very slowly. During this slow-growing period the abalone shell becomes characterized by a thick and dense annual ring, by which the animal's age can be determined. Food preferences of abalone are in the following order: phaeophyta, chlorophyta, and rhodophyta algae species.
Figure 4. Developmental stages of an abalone (1-polar bodies appear on the fertilized egg - 15–30 min.; 2-2-cell stage; 3–4-cell stage - 80 min.; 4–8-cell stage - 120 min.; 5–16-cell stage - 160 min.; 6-morula stage - 195 min.; 7-gastrula stage - 6 hrs.; 8-trocophore in the egg membrane - 7–8 hrs.; 9-newly hatched trocophore - 10– 12 hrs.; 10-early veliger - 15 hrs.; 11-late veliger - 48 hrs.)
Figure 4. Developmental stages of an abalone - cont'd (12-peristomal stage - 6–8 days; 13-early larva with epipodes - 19 days; 14–15- juvenile abalone - 45 days).
In artificial rearing, juvenile abalones may attain over 5 mm in 80 days. Four months after fertilization, they reach more than 12 mm and can be fed with Laminaria, Undaria or other seaweeds.
Figure 5 shows the relationship between the weight of abalone and the shell length.
The DPRKorean experiences in abalone rearing show that H. discus hannai grows up to 3 cm in the first year, 5.5 cm in the second year, 7.5 cm in third year, and more than 9.5 cm in the fourth year. The weight increases faster than the growth of shell. For example, if the length of the shell doubles, the weight increases eight-fold.
The amount of oxygen consumed by abalone is an important factor in the rearing and transportation of the organism itself. It varies largely with water temperature and between day and night (Table 1).
Table 1. Amount of oxygen consumption at various seawater temperatures
|Amount of consumption|
The amount of oxygen consumption increases in direct proportion with the water temperature up to 24 °C but decreases at higher water temperature (Fig. 6). The amount of oxygen consumption is also closely related to the chloride content of the seawater (Fig. 7). The amount consumed does not vary when the chloride content is more than 14 ° /00 but below 13 ° /00 it decreases very rapidly. Oxygen consumption is also greatly affected by the concentration of NH3-N. The more ammonia-nitrogen in the seawater, the less oxygen is consumed (Fig. 8).
Figure 5. Relationship between the shell length and the weight of abalone.
Figure 6. Amount of oxygen consumed at different water temperatures (when chloride content is 17 ‰).
Figure 7. Amount of oxygen consumed at different chloride level.
Figure 8. Amount of oxygen consumed at various concentrations of ammonianitrogen.
The feeding habits of abalones depend on the growth stage, body size and season.
Up to the trochophore stage the growth is on the nutrients from the egg yolk. Veliger and settling larvae begin to feed on unicellular algae and/or organic substances. More unicellular algae cells are taken in as the mouth widens and the parts develop further.
Juvenile abalone usually feed on benthic diatoms and small benthic organisms. Juvenile intake of food increases remarkably with the appearance of the first respiratory pore; the feeding also becomes more active. Young abalones of 5 mm in length begin eating young seaweeds and grow faster. At 13 mm long they feed on a great variety of seaweeds such as Laminaria and Undaria. Young barnacles, bivalves and foraminiferans are also found in the stomach of abalones.
The feeding habits of abalones ares also affected by the seawater temperature. Abalones seldom feed at a temperature of 8 °C, while it takes in food up to 6 % of its body weight at 12 °C, and 15 % at 20 °C. Abalones are particularly active during the early hours before sunset and sunrise. They maintain their weight when the feeding quantity is 0.61 % of their body weight.
Abalone is an opportunistic feeder. In experiments where several feeds were given, the eaten rate was Laminaria 53 %, Undaria pinnatifida 38 %, Ulva 6 %, and Porphyra 2 %.
The food conversion rate of abalone depends on the kind of feed (Table 2).
Table 2. Food conversion rate
|Kind of feed||Undaria||Ulva||Carpopeltis||Laminaria||1+2||2+3||1+3|
|Feed conversion rate||5.07||2.29||1.5||16.07||4.9||1.98||5.28|
The food conversion rate also varies with the size of the abalone (Table 3).
Table 3. Food conversion rate by size
The bigger the abalone, the higher the food conversion rate (FCR). The growth rate also depends on the kind of food taken in, which also has an effect on gonad maturity. The higher the FCR, the more the weight increase. Laminaria and Undaria are good seaweed species for inducing gonad maturation while Ulva or Phodymenia are less effective.
A newly hatched trocophore swims considerable distances in the water current for 3–7 days until it enters the settling stage. From then on it becomes a benthic organism and moves with the aid of its foot. An abalone was observed to have moved 66 cm in 1 minute over a glass surface. The moving habit of an abalone differs depending on the body size, time of day, and season. As it grows bigger, it moves to deeper locations. Abalones tuck themselves under rock and stone in the daytime, and crawl around in search of food at night, returning to their shelter before sunrise. Records show that abalones can move 50–80 m during 1 night. They migrate to shallower places or gather in small groups during the spawning season.
Abalone predators include various species of marine finfish, crustaceans, echinoderms, molluscs and others (Table 4).
Studies have shown an average of 71 juvenile abalone shells and a maximum of 200 shells, found in the stomach of sea perch. Morphysa iwamushi which lives in the mud and feeds on the sole of abalone causing injury or death. Penaietis haliotis, a copepod, is a parasitic organism of the abalone digestive organ.
Table 4. Predators of abalone
|Finfish||Red skate, shark, flat bream, black porgy, perch, Sparus macrocephalus, Goniistiys quadricornis, labridae, Stephanolepis cirrhifer, Microstomus achne, Canthigaster rivulatus, and congers|
|Echinoderms||Seastar, Asterina pectinifers, Buidia maculaia|
|Molluscs||Octopus, Thais clavigera|
The number of spawners to be conditioned and used varies according to the number of the young abalone to be produced and to the rearing method and technical devices available. Up to the shell length of 5 mm, the abalone survival rate, which may range from 1– 40 %, is affected by the rearing method and technical conditions of the devices. Therefore, bearing in mind these conditions and the number of eggs to be spawned a sufficient number of spawners should be used. The ratio of female and male is 4:1, and a good spawner should be at least three years old and 7–9 cm in shell length. The bigger the spawner, the more eggs it bears (Fig. 9). About 80 % of mature eggs are usually released into the seawater. It is advisable to catch spawners in March/May and in October/November when the water temperature is comparatively low. Much care should be given during the collection of spawners and transportation to the breeding station.
Abalones with a fleshy foot are desirable as spawners. Specimens with a less developed foot and immature gonads should be replaced with mature ones.
Figure 9. Relationship between abalone shell length and fecundity.
Male and female abalones are bred separately. A proper system and devices should be installed to adequately collect spawned eggs at anytime of the year.
A 1-ton capacity plastic or concrete tank is used for abalone maturation. The optimum number of spawners per tank is 40–60 individuals. Corrugated plastic boards and V-shaped cement collectors are placed on the bottom of the tanks for the abalones to hide and feed. A temperature control device should be installed to maintain a constant temperature of 17–20 °C during the conditioning period. At this time, the amount of seawater exchanged should be adequate, according to the abalone biomass and amount of oxygen consumed. The volume of water exchanged is calculated as follows:
|V||=||the amount of water exchange (litre/day)|
|E||=||the amount of oxygen consumption (ml/kg/h)|
|N||=||the weight of abalone (kg)|
|C1||=||the amount of oxygen in fresh water (ml/l)|
|C||=||the amount of oxygen in rearing tank (ml/l)|
The amount of water exchange per hour should be 100 times the total weight of abalones in the tank. The level of dissolved oxygen in the seawater for spawners should be over 4 ml/1. It is preferable to use an aerator and a water circulation device in order to have a high level of dissolved oxygen in the conditioning water. It is better to control the light exposure time for spawners, however, the breeding tanks should be kept in the dark.
During the conditioning period it is important to provide good quality food on a regular basis.
Most phaeophyceae, especially Laminaria and Undaria are a good food for abalones. The feed is given at about 20 % of their body weight 2–3 times a week, and feeding rate should be regularly controlled. The daily feeding rate should be above 7 % until the effective accumulative temperature for maturation reaches 800–1000 °C-days, otherwise maturity is delayed or may even stop.
The daily feeding rate is calculated as follows:
|F||=||feeding rate (%)|
|F2||=||food left over|
|T||=||duration in days|
From the day when the effective accumulative temperature for maturity is over 100 °C-days, the feeding rate drops below 2–3%. When rearing spawners, the maturity of the operculum should be observed and the time of induced spawning should be expected according to the effective accumulated temperature for maturity.
At a seawater temperature of 7.6 °C, gonad growth and development begins. The temperature is called the biological zero point. The effective accumulated temperature for maturity of above 1800 °C-days ensures a spawning rate of 98 %. Therefore, in order to plan for the correct time to induce spawning, a management system should be established to control gonad conditioning and maturation.
With the rise of the effective accumulated temperature for maturity, more attention should be paid to the management of the abalones. Fully matured abalones have a tendency to release their gametes when stimulated by a number of external stimuli. Spawners should be matured for a further 80 days prior to a second spawning. Under such conditions, the number and quality of released eggs are ensured.
Abalones hold matured eggs for 3–6 months. They can be spawned and artificially collected using various external stimuli. Physicochemical and biological stimulation are widely used.
Male and female specimens are separated prior to their spawning. This method exposes gravid individuals, which are kept in the dark, to flowing and heated seawater previously irradiated with ultraviolet light. After two or three hours the male or female abalones will start to release their gametes. This method is extremely useful in commercial scale establishments as it is possible to obtain a high spawning rate, large number of eggs, and higher fertilization rate. This method is usually successful with fully matured abalone specimens. The intensity required for irradiating the seawater flowing through the UV lamp sterilizer varies with water clearness, however one twentieth (1/20) of the nominal sterilization capacity is usually sufficient.
After exposing mature individuals to air for about 30 minutes or 1 hour, they are placed into the egg collecting tanks. Filtered seawater is added into the tanks which are kept in a dark room. Spawning of eggs and sperm is usually successful if the spawners are fully mature.
This method raises the ambient seawater temperature to 27 °C for 30–60 minutes and lowers it again to the ambient temperature. Several temperature cycles are usually sufficient to induce spawning in mature individuals.
Other methods used to artificially induce spawning are: injecting 2–4 cc of either 3.7 % calcium chloride solution into the mature abalone or sterilized and filtered seawater into the head, intestine ganglion and/or foot ganglion.
This method is to induce spawning by placing adult abalones that have already been given the dry stimulus treatment into seawater which has been exposed to UV rays and with a controlled temperature.
If the eggs and sperm are released from the respiratory pore and spread out evenly, the induction is considered to be successful Following spawning the quality of the gametes should be carefully examined. Fertilization follows if the gametes are viable and of good quality.
The spermatozoan of abalone are active for 24 hours, however fertilization of eggs should be effected within one hour from spawning. Concentration of released sperm should be measured on the haemocytometer and the amount of sperm necessary for fertilization should be determined. About 1500–4000 eggs per litre and about 10,000 spermatozoan per egg should be put into filtered seawater and well mixed. Polyspermy is caused by an overabundance of spermatozoa. On the other hand, lack of spermatozoa leads to a low fertilization rate.
Egg washing is done by decantation, and the upper water layer of the tank is drained off to get rid of unfertilized gametes. This process is repeated several times. It is important to maintain a constant temperature throughout the washing process. Newly hatched larvae should be moved to a new tank.
When the larvae begin to swim, their metamorphosis and growth should be closely followed and given more care. Actively swimming larvae are transferred from the upper water layer of the spawning tank to the rearing tank.
Should the larvae be reared in the spawning tank, due to the unavailability of other tanks, the discharged egg membranes, detritus and abnormal larvae should be removed from the bottom of the tank. Sterilized and filtered seawater should also be added into the tank after hatching. At this stage, it is important to provide optimum conditions for larval growth. The optimum water temperature is about 20 °C; water temperatures below 17 °C and above 24 °C are not desirable for larval growth. It is possible to use aeration in the rearing water for evenly dispersing the larvae in the water column. The ideal larval density is around 300 specimen per litre.
The planktonic larvae search for a suitable ground for their creeping life style as a result of physiological and morphological changes: the foot begins to grow and the velum disappears within 4 days. When a suitable substratum is found they change shape several times and soon start to take food actively.
At this stage it is most important to provide the larvae with a biologically, chemically and physically suitable ground as well as other environmental requirements. When larvae enter the creeping stage, board cultches should be placed into the rearing tank. The colour of the boards should not allow light penetration and their surfaces toxic-free as well as suitable for creeping.
Normally, the boards are corrugated and made of a plastic material, measuring 30–60 cm × 30–60 cm. From 100,000 to 1,000,000 larvae per ton of rearing water are placed into the tank and 80 setting boards are fixed on the water surface. Beforehand, micro-organisms such as bacteria, yeast, protozoa measuring 5 micrometer and Navicula sp. are cultured on the setting boards.
The density of food organism below 3,000 cell mm2 is effective for the settling and survival of larvae. Larvae which set on the boards are able to feed on food organism measuring between 5–10 micrometer during the first 5–10 days. After 20 days they can take diatom of 20 um. At this stage it is important to supply the species and quantity of food organism according to the feeding habit of larvae.
The basic biological conditions for rearing larvae are as follows:
The optimum water temperature is about 20°C; above 25°C, the survival rate decreases.
A constant temperature of about 20°C should be maintained for 60 days.
The salinity should be above 30 parts per thousand and pH value between 8.0–8.3.
The degree of oxygen saturation in the rearing water should be maintained at 70–100 %. The culture water requires periodic aeration.
Much attention should be paid to the growth of food organism and other biological processes naturally occurring inside the tank.
In many cases, the larvae mortality rate is high during the first 60 days; this is due to factors such as egg quality, contamination by microorganisms, water quality, and quality and quantity of feed on the boards.
The water in the tank should be filtered and sterilized. Sometimes the mortality rate may be high due to the overgrowth of food organisms. Bearing in mind these considerations, a technical system to control the growth of diatom should be established.
At this stage diatomaceae are cultured on the surface of the board which are fed upon by the juvenile abalones until they reach 5–6 mm in shell length.
The optimum rearing conditions at this stage are as follows:
|Light intensity||>3000 Lux|
In order to supply sufficient food to the juveniles, the rearing water must contain the correct content of nutrients (5 ug-atom NO3-N/1, 5 ug-atom PO4-P/1 and 1 ug-atom Si/l).
When the juveniles reach 5–6 mm after 80 days, they are transferred from the setting boards to the intermediate rearing tanks. Separating abalones from the setting boards is not done by hand but with the use of anaesthetizing reagents such as carbonic acid gas, ether, ethanol, para-amino benzoic acid, etc.
After anaesthetizing the juveniles they are sieved and selected according to size; abalones of 5–6 mm are transferred to the intermediate rearing tank.
During the intermediate culture period the abalones are reared up to 3 cm, at high density and usually with a high survival rate. The intermediate culture is either carried out on land-based rearing tanks or in the sea. The land-based method is easier in terms of management; artificial feed can be easily provided which allows for a faster growth. However, technical facilities such as electrical pumps, blowers, etc., are required.
The sea-based method uses rearing cage hung from a floating line or raft set at sea. This method is however affected by seasonal limitation of the environment.
The density in the intermediate culture depends on the initial size of the juveniles and the amount of water exchange. In the land-based method, density is usually 3–5 kg/m2, while 2–3 kg/m2 in the sea-based method. As a rule, if the rearing weight doubles, the number of abalones should be reduced by half.
Table 5 shows the amount of water exchange according to shell length and water temperature during the intermediate culture period.
Table 5. Amount of the water exchange (%) according to abalone size and water temperature
In the intermediate stage, the quantity of feed provided varies with shell size and water temperature: the higher the water temperature the higher the feeding rate. The feed cycle is 5–10 days and the amount supplied should be 2–3 times the actual amount consumed. At a water temperature of 20°C the food easily rots, and therefore the amount of feed supplied should be carefully regulated and leftovers removed (Table 6).
Table 6. Amount of feed (g) 1,000 juveniles consume/day
In the land-based method, excrement and leftovers at the bottom of the tank should be regularly removed. Cleaning is done once a week at high temperatures, and once a fortnight at low temperatures.
In the sea-based method, fouling organisms beneath the rearing tank should be removed and a good water flow provided. In heavily fouled case the cages should be replaced.
After the intermediate culture, abalones of 3 cm may be reared in land-based facilities or released in the sea until they reach the marketable size.
Suitable rearing methods to produce abalones of marketable size are still being tested.
Optimum sites for abalone farming are coastal areas surrounded by islands, where seaweeds and algae, the natural food of abalones, grow abundantly. Farm sites should be far from fresh water inflows, have clear waters and a slow current. Stony and rocky seabeds are good for abalone culture. The site should have few predators, and be convenient for transportation.
The ecological conditions of the selected sites is important for culturing the abalones to the marketable size.
If the site has the appropriate physical and chemical conditions but lacks rocks and stones, various types of structures including stones should be sunk to meet the habitat requirements of abalones which like to cling to rocks and stones. It is also recommended to grow seaweeds and algae such as Undaria, Laminaria, etc.
The survival rate of abalones released in the sea, up to the marketable size, depends on several factors, but mainly on the size of abalones at the time of release. According to experience in Korea (DPR) the survival rate is as follows:
Survival rate of abalones of 20 mm in length was <10 %
Survival rate of abalones of 25 mm was 25–30 %
Survival rate of abalones of 30 mm was 30–60 %
Survival rate of abalones of 40 mm was 70–80 %
Survival rate is also greatly affected by the maintenance method applied after release of abalones. The average survival rate of 30 mm shell length abalones released into the sea, is about 40%. This survival rate can be increased to 80 % by improving the maintenance techniques.
When small abalones are released into the sea, their survival rate tends to be low. This is attributed to the fact that small abalones are easily preyed upon. Abalones record their highest rate of mortality within 24 hours after they have been released. This is attributable to the active search of abalones of suitable places to live in. When abalones are released into a newly built farm, the survival rate increases, because the abalones can find their places easily and without harm. In contrast, if abalones are released into a wild habitat they suffer from predation while searching for proper places to hide.
Releasing methods are of great importance is the survival rate of the released abalones. Up to now, various methods have been suggested, but the best way is to provide abalones with the opportunity to find a living place by themselves. One of the releasing methods is as follows:
Between 500–800 abalone specimens to be released are placed on a board, and a plastic setting board measuring 50×50 cm is then placed over it. The board is wrapped with net and placed in a tank with flowing water for 1–2 days. The abalones dislike the net material, and therefore cling to the plastic board.
The plastic board still wrapped in the net is placed into a netcage and sunk to the bottom of the sea. One netcage with a mesh size of 4 cm may contain 3–5 boards. Within 24 hours, 90 % of the abalones on the boards leave by themselves and the rest usually leave within the next day, in search of a living place. This method prevents high mortality that usually occurs during initial release. Throwing or releasing the abalones without using the board method results in high mortality.
The growth rate of abalones strongly depends on the water temperature and availability of feed. Water temperatures above 6 °C are acceptable for abalones to begin taking food. At 20 °C abalones grow fast but below or above 20 °C the growth is reduced. They grow fast from March to July and tend slower near the spawning season when the water temperature drops.
The growth of abalones is also related to feed quality. They normally grow well in waters that abound in Laminaria and Undaria. Shell length of abalones feeding on the above seaweed species usually grows 1.5 times and body weights increase up to 3.5 times faster than abalones cultured in sites with a different flora.
An abalone needs 6.5 mg of wet Undaria daily to maintain its body weight of 1 g, and 10 g to increase its body weight by 1 g. After release, it takes 2–3 years for abalone to reach the marketable size.
Maintenance activities should be carried out periodically, in general, 2–3 times a month except in winter. It is conducted in conformity with the physical condition of abalone and the biological features of the farms on the basis of previous surveys. In particular, the amount of feed should be controlled according to the bottom condition. Necessary measures should be taken to maintain good water quality. It is always necessary to control pests and predators.
The production system for big abalones (9 cm in length) should be established to ensure an optimum growth environment and thus high productivity.
The average body weight of a 7 cm abalone is usually below 50 g. The bigger the size of abalone in the production stage, the higher the productivity. When abalones are cultured at a stocking density of 100 m2 the productivity is high. Harvesting should be done at the time when the meat percentage is high. The meat percentage of abalones is highest, at 59–57 % of body weight, in May and November. This falls to 40 % during the spawning season. The abalones are harvested by diving using scuba equipment and hooks.