Marine Fisheries Research Institute, Liaoning Province, China
This report summarizes the latest progress in the artificial breeding and farming of sea cucumbers, Apostichopus japonicus Liao, along the coast of Dalian, China. The development of specialized techniques will be discussed, emerging problems analysed and future prospects outlined. For the artificial breeding, the broodstock sea cucumbers are maintained under low temperatures (15-16 °C) to maximize the quality, quantity and maturity of the gametes. The density of larvae is kept under 1.0 individuals/ml. The algae, Dunaliella euchlaia, Chaetoceros gracilis, Chaetoceros muelleri, Nizschia closterium and Phaeodactylum tricornutum are used as food for the larvae, whereas Sargassum sp. is used to feed the juveniles. High quality seawater is a basic requirement for successful larval and juvenile production.
The growout of sea cucumbers is mainly carried out in former shrimp ponds and newly built ponds in the inshore regions of Dalian. It has become an important industry after more than ten years of development. The area used for farming now exceeds 7 000 hectares. A recently developed model of culture in the open sea will be presented in this paper.
Keywords: Cultivation, technology, larvae, juvenile, diseases
Major progress on artificial breeding of the sea cucumber Apostichopus japonicus was made in the mid 1980s. Since then, and particularly in the 1990s, these techniques have been improved considerably and spread widely throughout the Liaoning Province in northern China (Zhang & Liu, 1998).
In recent years, a considerable number of hatcheries have been established that produce sea cucumber juveniles for commercial purposes. The total water volume used for breeding has exceeded 2×105m3. The current production capacity of young sea cucumbers (body length >2 cm) is between 100-200 tonnes which is equivalent to 0.8 and 1.6 billion sea cucumbers (approx. 4 000 individuals/kg). It is estimated that the ongrowing area, including former shrimp ponds and newly built ponds, currently exceeds 7 000 hectares.
During the last two decades, the living standards of Chinese people have increased considerably, which has stimulated the consumption of sea cucumber despite sharp increases in price. On the other hand, the increased demand for this product in the Chinese market has encouraged the development of sea cucumber aquaculture.
The present report introduces the latest technology on sea cucumber breeding.
Improvement of artificial breeding techniques
The condition and gonad maturity of the sea cucumber broodstock have a direct impact on the quality of the gametes. In the past, the broodstock was collected from the wild and almost immediately induced to spawn. In recent years, however, the spawners are maintained in the hatchery at a low temperature (15-16 °C) in order to allow the gonads to attain full maturity. The seawater temperature of the conditioning tanks is gradually raised to favour the maturation of the broodstock allowing the hatchery operators to produce relatively large sea cucumber juveniles early in the season. The temperature is increased daily by 0.5-1.0 °C while the tanks are kept in total darkness. Feeding rate is in the range of 5-7 % of the body weight, but it should be adjusted under different temperatures conditions. The quality and composition of the food has a direct impact on the gonad development. The ETA (effective temperature accumulation) is about 800 °C/day. Under these conditions, high numbers of spawning individuals are ensured as well as gaining an extension of the spawning season. This allows the production of sea cucumber juveniles over a longer period of time.
Gamete collection and fertilization
The natural spawning periodicity of A. japonicus remains poorly understood. However, technological improvements have enabled hatchery operators to induce spawning using a variety of methods that include drying, water jetting and thermal shock. Artificial spawning allows the hatchery operators to better control the concentration of spermatozoa and the stocking density of eggs. The hatching rate may exceed 90 %.
Rearing the pelagic stages of A. japonicus requires considerable attention and constant monitoring of the culture medium.
Larval density: In order to ensure a fast growth and high metamorphosis rate, larval density should be maintained at between 3 to 4x105 individuals/m3. At higher concentrations the growth rate of the larvae can dramatically decrease and the incidence of malformations is considerably higher (Sui, 1990).
Feeding: The use of different species of microalgae is crucial for the development of the larvae. Dunaliella euchlaia, Chaetoceros gracilis, C. muelleri, Nizschia closterium and Phaeodactylum tricornutum can be used to feed the larvae. The most important species are D. euchlaia, C. gracilis and C. muelleri, whereas Dicrateria zhanjiangensis, Isochrysis galbana and Chlorella sp. are usually added as a supplement and are never used alone. For a balanced diet, a mixture of 2 to 3 species is highly preferable. The microalgae are fed at a concentration ranging between 10 000 and 40 000 cells/ml. Food levels are increased gradually as the larvae develop. In order to enhance larval growth and decrease the rate of malformation of the young sea cucumbers, marine yeast and/or photosynthetic bacteria (PSB) are often supplied.
Water quality: High quality seawater in a sea cucumber hatchery is an important prerequisite. Research findings indicate that numerous physical and chemical factors (e.g. temperature, pH, salinity, ammonia, dissolved oxygen, heavy-metal concentration, turbidity, etc.) will influence the success of a culture. As these parameters tend to vary significantly from one region to another, careful monitoring of the seawater quality is essential.
Selection and use of settling bases: The traditional substrates used for the settlement of the sea cucumber juveniles are frames fitted with fine polyethylene (PE) or polypropylene (PP) cloth. In recent years, some hatcheries have started to use PE corrugated plates measuring 50x50 cm fixed together in stacks of 8-10 pieces. This latter method has been used with some success. In the traditional method, benthic diatoms need to be cultivated on the settling bases before they can be used. Currently, some hatcheries no longer cultivate benthic diatoms, but rather provide a food supply soon after the juvenile sea cucumbers have settled. This method has two advantages: no equipment is needed to rear benthic diatoms and the settling plates are easier to clean during routine hatchery operations. The settled sea cucumbers are fed with benthic diatoms that have been using mesh bags or other materials placed in the rearing tanks and with a powdered macroalgae soup typically prepared using Sargassum spp. such as S. thunbergii. The correct feeding rate is essential to ensure a high survival rate of the juveniles.
Rearing juvenile sea cucumbers may take several months, but may require as long as 6 months if the rearing conditions are not favourable.
Food: The food must be free of contamination, of the right particle size and contain all the essential nutrients. A balanced diet not only accelerates the juvenile sea cucumbers growth rate, but also increases their survival rate.
Transfer of the juveniles: Juvenile sea cucumbers are particularly vulnerable during the early rearing stages. High mortality rates are caused by high density, overfeeding, faeces on the settling plates and competition for space amongst themselves and other opportunistic organisms such as Ciona intestinalis. This species of tunicate can also secrete a toxin that can kill juvenile sea cucumbers. Therefore, juveniles should be regularly transferred to new settling plates, sorted by size and injured individuals transferred to separate tanks. Light anaesthesia is usually used to reduce stress and facilitate handling of the sea cucumbers. Microsetella sp. (Ectinosomatidae) is commonly found in rearing tanks and can form large colonies in a short time killing all the sea cucumber juveniles in 1-2 days when the situation gets out of control. Trichlorfon, a biocide, was formerly used to kill Microsetella sp., but the copepods have developed a strong resistance to the biocide and therefore have become difficult to eradicate. In 2003, a new and effective pesticide known as Mei Zao Ling was developed by the author. This product has little side effect on the sea cucumbers.
Nursing of juvenile sea cucumbers: As the juveniles grow, the water quality and dissolved oxygen must be maintained at the optimal level. Increasing aeration and water exchange rates becomes necessary. The oxygen level has to be maintained above 5 mg/l. It is also important to use formulated feed that can be digested and absorbed easily. Experimental results have shown that the growth rate of juveniles fed on the formulated feed is at least two times higher than that of individuals fed on traditional feed during the 20 to 30 day period. In recent years, studies on a series of formulated diets revealed that diet is a key factor for improving the survival and growth rates of juveniles in the nursing stage.
As the accumulation of excess food and faeces increase, harmful germs tend to multiply rapidly and can cause very serious disease outbreaks among the juvenile sea cucumbers, including what is known as the stomach ulcer. Another disease is white muscle syndrome which causes muscle tissues to turn white and rigid. More applied research is urgently required to find effective remedies to these problems.
Advances in farming technologies
The cultivation of sea cucumbers is mainly carried out in former shrimp ponds and newly built ponds along the coast of Dalian. It has become an important industry after more than ten years of development. The culture area currently exceeds 7 000 hectares.
Selection and construction of the culture ponds: The farm sites should provide suitable conditions for the growth of sea cucumbers. A supply of clean and unpolluted seawater should be easily accessible and the ponds should be constructed in such a way to ensure that the banks are leak-proof and that complete drainage can be achieved. Salinity levels should range between 25 and 35, with optimal values around 27-32. Ponds with muddy and sandy bottoms and of 2-3 hectares in size are preferred; however some operators use ponds as large as 7 hectares. If necessary, stones or other artificial materials are placed in the pond to provide an adequate substrate for sea cucumbers to aestivate and live through cold winters. Hard substrates should cover 50-70 % of the bottom. The depth of water should be between 1.5 to 2 m and the seawater temperature maintained between 0-30 °C.
Transfer of young sea cucumbers to the pond: It has been demonstrated that the release of young sea cucumbers measuring 2-3 cm in body length produce the best farming results. The survival rate can reach up to 50-70 % when the individuals are introduced at a size equivalent to 2 000-4 000 individuals/kg. These will attain commercial size after 1.5 years. For an optimal growth the culture density should not exceed 10 individuals/m2.
Management: Sea cucumbers can be farmed with shrimp and certain species of finfish, although they are commonly reared alone. Prior to stocking the ponds with the hatchery-reared sea cucumber juveniles, it is necessary to clean and sterilize the ponds as well as inoculate the seawater with benthic diatoms. These measures will provide an appropriate culture environment and ensure high survival rates. The addition of formulated feed will also enhance growth particularly during spring and autumn. Some field tests have shown that the growth rate of sea cucumbers fed on formulated feed is as high as two times that of non-fed individuals.
Constant monitoring of several environmental and biological parameters such as the body length and weight of the sea cucumbers as well as the feeding and excretion rates is essential in order to properly adjust the food supply in the culture ponds. Furthermore, particular attention should be paid to changes in the colouration and temperature of the seawater, presence of fouling algae and predators, as well as to fluctuations in salinity, dissolved oxygen, etc. Finally, the depth of the water should be monitored closely as the appropriate levels and exchange rates must be maintained to prevent thermal stress.
Problems and outlook
A number of problems have emerged following the relatively fast development of sea cucumber aquaculture in China. The most notable of these is the shortage of good quality broodstock and, consequently, the production of poor quality fertilized oocytes. Furthermore some hatcheries are not able to provide proper feed due to the lack of appropriate equipment and techniques. Other hatcheries fail to ensure good water quality due to poor design and inappropriate selection of the farming site. The emergence of numerous diseases, such as stomach atrophy, stomach ulcer and body ulcer, are also major constraints affecting the industry. Because sea cucumber farming is a relatively new aquaculture practice, pathogens and their treatment are still poorly understood. Further research is needed.
In view of the problems mentioned above, the author deems that improving the overall design and layout of the sea cucumber hatcheries and farming facilities, effective transfer of the culture techniques to the farmers and further research on the prevention and treatment of diseases are priorities for the development of the sector. Indoor and land-based cultivation should be considered so as to shorten the cultivation period and reduce production costs. The author believes that the artificial breeding and cultivation of sea cucumbers in the inshore regions of Dalian will expand and achieve technological improvements in the future.
Zhang, Q. & Liu, Y.H. 1998. Cultivation techniques on sea cucumber and sea urchin. Oceanology, University of Qingdao Press, p.22-72.
Sui, X. 1990. Seed production and cultivation of sea cucumber. Agriculture Press of China, p. 107-153.