Key Techniques for Artificial Breeding and Seed Rearing of Abalone

Abalone is a valuable marine delicacy. In recent years, artificial abalone farming in China has developed rapidly, which makes abalone seed rearing crucial. So how exactly should abalone be artificially bred, and what is the specific procedure? Below we introduce it in detail.

(1) Raising temperature to promote maturation

In the Bohai and Yellow Sea region, the natural spawning period of Haliotis discus hannai begins after late July. If naturally matured brood abalone are used for seed collection, by mid-January of that year the juvenile abalone are on average rather small, and the overwintering survival rate at sea is low. In addition, when seed is collected in July and August, because the rearing water temperature already exceeds 20°C, the benthic diatom feed reproduces slowly and easily detaches, making it difficult to guarantee feed for the early rearing of juvenile abalone. Maturation-accelerating culture of brood abalone and earlier seed collection help solve these shortcomings and improve the egg-collection results.

The equipment needed for warming culture includes: a rearing room with heating equipment, cement tanks or fiberglass-reinforced plastic water tanks as culture tanks, a preheating water tank with electric heating equipment or a boiler, and aeration equipment.

Based on the estimated egg output, 0.5 female abalone with a shell length of 7.5—8.5 cm per square meter of seed-collection water surface is sufficient to meet the need, and the number of males kept is one-fifth that of females. Warming culture of brood abalone to promote maturation should begin two and a half months before seed collection, so that the effective accumulated temperature can reach the requirement.

Culture method: about 80 brood abalone per cubic meter of water, which should be placed in net cages and suspended for culture. Black corrugated attachment plates are laid on the bottom of the net cage. The natural water temperature is gradually raised to 20°C at a rate of 2°C per day, after which a constant temperature is maintained. Fresh wakame or kelp is fed once every 2—3 days, with a daily feeding amount of about 20% of body weight. Fresh seawater is fully exchanged once a day; the new seawater must be preheated first, and the temperature difference should not exceed ±1.5°C. Water exchange can be done simply by moving the net cage. During the culture process, the aeration rate must be kept at no less than 100 liters/hour·cubic meter. Precautions: the water temperature should not exceed 20°C, and residual feed should be removed frequently.

(2) Egg collection

1. Selection of brood abalone

Good brood abalone are easy to spawn, have a high hatching rate, and their embryos develop well. The conditions for selecting brood abalone are: healthy individuals of Haliotis diversicolor with a shell length of more than 6 cm, or Haliotis discus hannai with a body length of more than 8 cm, with gonads that appear extremely plump, enveloping most of the stomach and liver, with the base protruding beyond the shell edge and the end becoming blunt and rounded because it is filled with reproductive glands.

2. Induced spawning

It is also possible to use temporary holding for natural spawning, but generally various means are used to stimulate the brood abalone to spawn. Common methods include:

(1) Ultraviolet-irradiated seawater method

The principle of this method is not yet clear, but the induced spawning rate can reach 100%. It is more convenient to use a flow-through ultraviolet sterilizing lamp. If an ordinary quartz ultraviolet sterilizing radiation tube (30 watts) is used, insulation measures must be taken: add insulating leads to the terminals and seal them with epoxy resin. Place the prepared sterilizing lamp in a water tank, inject 23°C fresh filtered seawater into the tank, and when the water level exceeds the lamp tube by a certain height, the lamp can be turned on for irradiation. The irradiation dose is generally 300—500 mg/hour.liter, which is sufficient. The irradiation time can be determined according to the power of the lamp and the volume of seawater irradiated.

Egg collection is best carried out in the first half of the night. The best effect is obtained if the brood abalone are first stimulated by drying in the shade before entering the ultraviolet-irradiated seawater. The method is to place the abalone foot upward, cover with a layer of clean moist gauze, and dry in the shade at room temperature for about 1 hour.

For convenience of operation, the egg-collection tank should not be too large. In the absence of a dedicated product, a commercially available 30-liter polyethylene basin will do. During egg collection, males and females are separated, and one or several are placed in the egg-collection tank, into which ultraviolet-irradiated seawater is injected. Within 1 hour, except for a few, most individuals will not spawn. At this point the ultraviolet-irradiated water should be replaced once, and before long the brood abalone can release large amounts of sperm and eggs. Male abalone release sperm quickly, so their induction time can be 0.5 to 1 hour later than that of the females, to ensure that the sperm have sufficient vitality during artificial insemination.

(2) Hydrogen peroxide method

This method is relatively simple and convenient, and requires no special equipment.

Either commercially available hydrogen peroxide (H2O2 content 30%) reagent or medical disinfectant hydrogen peroxide (H2O2 content 30%) can be used. Prepare a concentration of 4 mol, that is, add 0.3 ml of 30% hydrogen peroxide to one liter of seawater.

During egg collection, use net bags made of plastic window screen to separately hold the selected male and female abalone, immerse them in the prepared hydrogen peroxide seawater for half to 1 hour, take them out and rinse clean with seawater, then place them separately in egg-collection tanks with about 23°C fresh seawater added, and within 1 hour they can release sperm and eggs.

3. Artificial insemination

At water temperatures above 20°C, artificial insemination must be completed within 1 hour after the eggs are released. During insemination, it is best to first quantify the number of eggs and the sperm density. Tests show that the fertilization rate is highest when more than 1500 sperm per egg or more than 300,000 sperm per milliliter are added. If the concentration is too high, the egg membrane easily ruptures; if too low, the fertilization rate is unsatisfactory. However, in production there is often no time to measure, so the method of adding sperm while eggs are being released is used. If fertilization is timely, the eggs are also vigorous, and as long as the amount is roughly appropriate, the effect is also very good.

(3) Hatching of fertilized eggs and larval rearing

1. Egg washing

This mainly uses the characteristic that the relative density of eggs is greater than that of seawater, soaking and washing with seawater to remove excess sperm in the water and around the fertilized eggs. The method is: fill the egg-collection tank with fresh filtered seawater, preferably at a water temperature not lower than 20°C, let it stand for 20—30 minutes, and after the eggs have fully settled, tilt the egg-collection tank to pour off the upper clear water, then refill it with water. Generally repeat more than 5 times. After egg washing, change the water once per hour until the larvae float up.

2. Hatching and selection

Usually, direct hatching in the egg-collection tank is sufficient. Under normal conditions, it takes 13 hours from egg fertilization to the hatching of trochophore larvae that float to the water surface. The larvae float because they are phototactic; by using this characteristic, the surviving larvae can be selected out. Pour the larvae together with the seawater into another water tank, and the remaining bottom liquid can still continue to be used for selection. During the trochophore larval stage, cloth net cages must not be used to concentrate them, as this easily causes mechanical damage. After they enter the veliger larval stage, cloth net cages can be used for water exchange.

Precautions: during the planktonic larval stage, there is no need to feed, or some single-celled algae may be given. The larvae need frequent water changes, and the new seawater must be filtered. The water tank also needs to be scrubbed to keep it clean. During water changes, mechanical damage to the larvae should be avoided as much as possible. If conditions permit, frequently measure the oxygen content, pH value, dissolved oxygen, soluble organic matter, and hydrogen sulfide content of the seawater in daily work, to ensure the normal life of the larvae.

(4) Attachment and juvenile abalone rearing

1. Preparation of feed plates

Corrugated plates made of polyethylene plastic film, transparent plastic, or organic glass can all be used (cut into strips and wound in corrugated form around a support). Before use, soak in 0.5% sodium hydroxide for two days, then scrub and wash off the chemical solution before use.

Benthic diatoms can be collected from rock surfaces or from the walls of indoor culture ponds. After microscopic examination, select small species such as Navicula, Chaetoceros, and Cocconeis. When collecting algal species, care should be taken not to mix in large amounts of green algae.

Inoculation should begin one and a half to 2 months before seed collection. Set up suitable racks so that the feed plates are placed horizontally in the pond, arranged neatly, and inject filtered seawater. The algal species, after filtration, are also poured into the inoculation pond. After sufficient stirring, let it stand to allow automatic inoculation. The next day, turn the plates over, and proceed with inoculation in this way. After inoculation, spread them out, place the feed plates vertically on the racks, and supplement nutrients in the pond. The nutrients include 1.2 g urea, 22 g monopotassium phosphate, 24 g sodium silicate, and 24 g ferric citrate, added to 500 ml of fresh water to make a stock solution. Apply 100—200 ml of stock solution per cubic meter of pond water, change the water once a week, and after each water change replenish the nutrients according to the standard.

During the culture process, attention should be paid to frequently changing the orientation of the feed plates. Light should not be too strong, and copepod pests should be captured and killed promptly.

2. Attachment of creeping larvae

The time for larvae to move from the planktonic stage to the creeping stage, calculated at 20°C, is about 52 hours after floating up, when preparations must be made, including preheating seawater and rinsing the feed plates, and removing pests and miscellaneous algae.

The feed plates are placed in the pond at a depth of 10—20 cm below the water surface. The number of larvae to be introduced is determined by the area of the plates. When 22 square meters of feed plates are used per cubic meter of water, the number of veliger larvae introduced is 40,000—50,000. If the density is too high, growth is slow and the survival rate is low. After being placed in the pond, light should be reduced and aeration carried out to ensure uniform attachment.

In the early stage of larval attachment and growth, attention should be paid to fertilizing and changing water. To maintain the proliferation of the feed diatoms, one week after attachment, fertilize daily in conjunction with water changes, applying 10—20 ml of stock solution per cubic meter of water. When changing water, it should be taken into account that the larvae have not yet fully fixed themselves after attachment, so the overflow outlet should be fitted with a filtering device to prevent the larvae from being lost. In addition, attention should be paid to promptly killing pests and monitoring water conditions and growth status.

3. Transfer to net cages for flow-through flat rearing

Previously, the Japanese aquaculture industry transferred juvenile abalone to net cages for rearing after their shell length reached 6—7 mm, because the juvenile abalone were relatively large and benthic diatom feed could hardly meet their needs. It is now believed that the formation period of the first shell pore of abalone (1—3 mm) is the key period for food conversion. Japanese culture experiments have shown that feeding frozen wakame cut into appropriate algal pieces achieved a good effect with a survival rate of 88%, and the growth rate was faster. Therefore, they can be moved into net cages for rearing when the shell length reaches 3 mm. After juvenile abalone become larger, they like to hide from light, so black attachment surfaces should be used. Currently, the more widely used are fiberglass-reinforced plastic and polyoxyethylene corrugated plates, with a certain number of holes drilled in the plates to facilitate the crawling of juvenile abalone. The corrugated plates are placed in net cages for rearing. At this time, flowing seawater should be used instead of regular water changes, with a flow rate greater than 7 times the rearing water volume, and the dissolved oxygen content not lower than 3 ml/liter. The specific measures are:

(1) Detachment of juvenile abalone

Detachment is required when juvenile abalone are transferred from feed plates to net cages, when individuals are sorted during net-cage rearing, and when they are transferred from indoors to the sea. There are two commonly used methods:

Chemical anesthesia detachment method: a 10% ethyl carbamate solution or a 20% alcohol solution prepared with seawater can both cause juvenile abalone to lose their adhesive power. Lay nylon silk cloth in a water tank containing the chemical solution to catch the detached juvenile abalone. Immerse the feed plates together with the attached juvenile abalone in the chemical solution. After 1—2 minutes, the juvenile abalone raise their shells and twist in place. At this point, vibrating the feed plates or gently wiping with an artificial sponge will make them fall off. The detached juvenile abalone should be taken out and rinsed as soon as possible, not exceeding half an hour.

Temperature stimulation detachment method: this method is used for detachment when juvenile abalone are transferred from indoors to seawater in spring and autumn. The effect is better when the culture temperature is 12°C. First immerse the corrugated plates together with the juvenile abalone in high-temperature seawater about 8 to 10°C higher than the culture water temperature for about half a minute, then immediately return them to normal-temperature seawater. The juvenile abalone will then move vigorously, and at this point they can be peeled off by hand wearing cotton gloves. Place net cages at the bottom of the normal-temperature and high-temperature water tanks to collect the juvenile abalone.

(2) Feed selection

Artificial feed can completely replace algae as feed. The supply of artificial feed is guaranteed, it is convenient to use, is not restricted by season, and its feeding effect is also better than that of seaweed feeding. Abalone fed only wakame grew to 2.5 times their body weight in 5 months, while those fed artificial feed increased their body weight by 4.1 times. Artificial feed is wakame mixed with white fish meal and vitamin agents, etc. Its protein content reaches 27.1%, while the protein content of dried wakame is only 11.4%. The feeding amount is related to individual size and water temperature. Generally, the daily feeding amount can be controlled at 5% to 7% of body weight, and the time should be in the evening.

(3) Rearing management and overwintering

The focus of management is to maintain water quality, frequently remove residual feed, clean the net cages and pool walls, and prevent water stoppage and local obstruction of water supply to the net cages. If juvenile abalone are seen crawling out of the plates during the day or climbing out of the water surface in large numbers along the net cage walls, the pond should be cleaned and the water changed immediately; otherwise large numbers of juvenile abalone will suffocate to death. At the same time, detachment and sorting should be carried out. About one month after the juvenile abalone are transferred to net cages for rearing, when the average shell length is about 6 mm, differences in individual size appear, and timely sorting should be carried out, separating large, medium, and small individuals for separate rearing.

In northern regions, if juvenile abalone seedlings are transferred to the sea for grow-out in late autumn and early winter, the water temperature is relatively low and the juvenile abalone are small, so the survival rate is low. If they are put out to sea after mid-April of the second year, when the water temperature has risen and the individuals are larger, the survival rate is significantly improved. Therefore, indoor overwintering is a necessary and effective measure.

At present, the technology for indoor overwintering of large quantities of juvenile abalone is still not very complete. There are mainly two methods:

Water purification machine method: the overwintering pond is covered with a plastic film greenhouse, and each pond is equipped with a sewage purification machine, which has the dual function of increasing oxygen and purifying water quality. The water temperature is kept above 15—16°C, fresh seaweed is fed, and generally no water change is needed.

Original culture pond overwintering method: overwintering is carried out directly on the basis of flow-through net-cage rearing. The survival rate is high, but energy consumption is large and the cost is high, and water changes are needed. To reduce energy consumption, the seawater must be recovered, filtered, and reused.