Hainan Institute of Fisheries, Haikou, China
Dongsha Islands, Nansha Islands, Xisha Islands and Zhongsha Islands in the South China Sea, located in the subtropics, are rich in sea cucumber resources. There are eighteen species of sea cucumbers in the area: Actinopyga echinites, A. lecanora, A. mauritiana, A. miliaris, Bohadschia argus, B. marmorata, Holothuria arenicola, H. atra, H. cinerascens, H. edulis, H. impatiens, H. leucospilota, H. nobilis, H. pervicax, H. scabra, Stichopus variegatus, S. chloronotus and Thelenota ananas. These species mainly inhabit the sea bed of coral reefs and are found as deep as 70 m, feeding on organic matter and microorganisms in the sand. For more than 400 years, fishers in eastern Hainan Island have visited Xisha and Nansha Islands to collect sea cucumbers using a specially designed tool known as the sea cucumber fork. Processing of sea cucumbers includes three steps: removal of the viscera, cooking and drying. During the initial sea cucumber processing considerable care is required during the boiling phase and the intensity of the fire is regulated based on the colour changes of the specimens being cooked. Sea cucumbers are rich in protein and are used in China as a traditional medicine. They are also a highly sought after food item. Excessive fishing has caused the resources of sea cucumbers in these four island groups to gradually decline. To promote the sustainable utilization of these resources, a plan to protect some areas from fishing should be considered. At the same time, the fishing season and minimum legal size for capturing sea cucumbers should be restricted to preserve adequate breeding populations. Moreover, studies are needed on the artificial reproduction of economically important species of sea cucumber to maintain an ecologically stable resource by transferring emphasis from capture fisheries to aquaculture.
Keywords: South China Sea, sea cucumber, resource, fishery, management, holothurians
Located in the tropics and subtropics, Dongsha Islands, Nansha Islands, Xisha Islands and Zhongsha Islands of the South China Sea are famous fishing grounds. Characterized by their special environment and extensive habitats, these fishing grounds are valuable for the growth and reproduction of many species of marine taxa, including echinoderms, molluscs, crustaceans, seaweeds, some reptiles and especially sea cucumber.
Status of the resources
There are a total of 18 species of sea cucumbers that live in these four island groups according to previous studies (Liao, 1997). Among these 18 species of sea cucumbers, Thelenota ananas, Stichopus variegatus, and S. chloronotus belong to the family of Stichopodidae, while all others (Holothuria nobilis, H. atra, H. leucospilota, H. scabra, H. edulis, H. cinerascens, H. pervicax, H. arenicola, H. impatiens, Actinopyga lecanora, A. echinites, A. mauritiana, A. miliaris, Bohadschia marmorata and B. argus) belong to the Holothuriidae family. It has been estimated that there are about 2 000 tonnes (dry weight) of exploitable of sea cucumbers in these islands off Hainan (Li, 1990). On the Qionghai coast, Hainan, the available resource was estimated at 250 tonnes (dry weight). Among these species, T. ananas, H. atra, H. leucospilota, A. mauritiana and B. argus are considered high yielding species, whereas T. ananas, H. nobilis, B. marmorata, S. variegatus and A. mauritiana are the commercially important species.
Compared with the Dongsha, Xisha and Zhongsha islands, the sea cucumber resources are most abundant in the Nansha Islands. The marine areas where sea cucumber resources are most abundant are: the southern and northern areas around the Xisha Islands; the Yuya Reef, Chang Reef, Guangxingzai Reef, South Shallow Bank, Nankang Reef, Meiji Reef and Hua Reef in the Nansha Islands; North Reef, Huaguang Reef, Yutuen Reef, Langhua Reef, Yinyupan and Zhongjian Islands in the Xisha islands group; and Paibo Reef, Meixi Reef and Huaxia Reef around the Zhongsha Islands.
Sea cucumber habits
Sea cucumbers in the four island groups are found on the coral reef flats at up to 70 m depth. Some species, such as T. ananas, H. nobilis and B. argus, live among coral reefs while others seek shelter in sandy bottoms and among the seaweed. Some species live in shallow waters near the sublittoral zone, such as A. mauritiana and H. cinerascens; some live in areas with an abundance of sand and seaweed, such as A. echinites, H. atra, H. edulis and S. hermanni; some prefer areas affected by a strong wave action, such as A. miliaris and H. scabra; while others prefer calm waters, such as S. chloronotus, A. lecanora, H. nobilis and B. marmorata and they are mostly found in deep waters (40-60 m). T. ananas, A. lecanora, B. argus and A. mauritiana are distributed in waters between 10 to 50 m depth. H. atra, H. leucospilota and S. hermanni all live in shallower waters, up to 15 m in depth.
Generally, sea cucumbers stop feeding and burrow in the sand at midnight and burrow when the water temperature is low (Li, 1990). Many species emerge from the sand at dawn and feed from noon to dusk. Sea cucumbers feed on organic matter and microorganisms, such as diatoms, foraminifera, radiolarian, small crustaceans and gastropods typically found in the sand (Li, 1990). Some species of sea cucumber, for example T. ananas and B. argus, have symbiotic fish in their cloacae. The internal organs of sea cucumber can generally regenerate if body is cut in two.
Tools and fishing methods
Fishing for sea cucumber is a traditional activity among the local fishers in Qionghai, Hainan. According to local historical records, sea cucumber fishing in Xisha, Zhongsha and Nansha islands started in 1681 and has continued from generation to generation without interruption.
Collections relied on free diving and capture of sea cucumbers by hand in the early periods, so the fishing grounds were restricted to waters with a maximum depth of 20 m. At some point, however, fishermen became unhappy with the collection efficiency as it was impossible to capture the larger specimens living in deeper waters. As a result, a tool known as the sea cucumber fork was designed and used in the 1920s to capture sea cucumbers. The fork was subsequently improved and nowadays it consists of a fork, a weight, a rope and a buoy (Figure 1).
The specifications of these components are as follows:
1) Fork: the fork is made of steel with sharp agnail ends. The pointed elements of the fork are arranged in a triangular fashion and secured at one end of a cylindrical weight. The pointed elements are 11 cm in length and 1 cm in diameter and fixed 6 cm apart from one another. The sharp ends are 3.7 cm in length and 0.8 cm in width.
2) Weight: The cylindrical element is a 9 kg lead weight with a diameter of 9.5 cm.
3) Rope: the rope is usually 80 m in length.
4) Buoy: the buoy is made of plastic with a buoyancy of 20 kg.
The boats used in sea cucumber fishing are generally 50-80 tonnes, in gross tonnage, fitted with a 120-250 hp engine and generally have 12-16 people onboard. Each support vessel usually carries 3 to 4 smaller boats (15-25 hp, 3-5 gross tonnage each) to enable the divers to easily move around the reefs. At the fishing grounds the collection of sea cucumbers is carried out using the smaller boats which carry a crew of 3; one in charge of steering the boat while the other two engage themselves in fishing with the use of masks or view buckets. Upon detection of the sea cucumbers on the sea bottom the divers enter the water and swim directly above the animal. Fishing is carried out by dropping the fork on the specimen seen by the diver. One end of the fork rope is attached to the boat in order to facilitate the retrieval of the captured sea cucumber. Because of the relative seawater turbidity around the islands the fishing activity is generally restricted in waters not deeper than 60 m. Commonly 20 kg of sea cucumbers, such as T. ananas, are collected by each boat (or 2 000 kg/year). Furthermore, fishermen engaged in the collection of sea cucumber very often fish for other species such as sharks, molluscs, as well as collect a variety of seaweed species.
Figure 1. The traditional tool used for fishing tropical sea cucumbers (Drawings of Chinese Fishing Tools published in 1989).
The processing of sea cucumbers involves three steps: removal of the viscera, cooking and drying.
Prior to the evisceration process the sea cucumbers are sorted by species. In the case of T. ananas the incision is made on the ventral side, while it is done on the dorsal side for all other large species. In the smaller species a small incision in done beside the mouth. Following the removal of the internal organs the sea cucumbers are rinsed with seawater and then placed in a suitable boiler. Small individuals are cooked for 30 minutes, while larger individuals may require an additional 10-15 minutes. The optimal water temperature when cooking species such as H. nobilis and B. marmorata is around 90 °C. During this phase the sea cucumbers become stiff and lose 50-70 % of their body fluids, assuming a yellowish colouration in species like T. ananas and S. hermanni, and blackish colouration in most other species. The sea cucumbers are then rinsed again once they are removed from the hot water. Large species, such as T. ananas, H. nobilis and B. marmorata, are further baked over hot coals for an additional 30 minutes. The cooked product is then sun dried (small bamboo sections are used to keep the incision wide open in large individuals) while ensuring that each sea cucumber is regularly turned over every few hours. Up to 3-5 days may be required to dry the products completely.
Proper cooking and drying of sea cucumbers is essential. If not cooked completely the sea cucumber will soon start to rot and acquire an undesirable smell. Overcooking may also damage the product as a very soft sea cucumber may not be processed into a high quality product. Fishers in Qionghai have acquired considerable processing experience over the years and tend to produce a higher quality product compared to that produced in some neighbouring countries.
The ratio of dry weight to wet weight of the processed sea cucumber is 27:1 in H. nobilis and H. leucospilota, 25:1 in T. ananas, 20:1 in S. hermanni and S. chloronotus, 17:1 in A. lecanora, B. argus and A. mauritiana and 10:1 in H. nobilis and B. marmorata.
Nutritional value and cooking method
Sea cucumber is a nutritious seafood with a high protein and low lipid content and is rich in gluten, nitrogen, iodine and other nutritional elements (Wang, 1997). The protein content of a dried sea cucumber may be as high as 68.7 %. Considered as one of the most popular Chinese seafood dishes, sea cucumbers are also used as a traditional medicine. It is believed that the consumption of this marine organism may have beneficial effects on the kidneys and stomach as well as being a cure for some cancers. Some research findings report that sea cucumbers have helped impotent individuals as well as patients affected by other sexual conditions (Ran, 1993). In the culinary tradition, sea cucumbers are prepared and consumed in a variety of ways. Some of the most popular dishes are: sea cucumber stew, braised sea cucumber, boiled sea cucumber, lotus seed-sea cucumber and chicken-sea cucumber soups.
Management of sea cucumber resources
Sea cucumbers have been classified as a protected group according to a number of regulations issued by the Chinese Ministry of Agriculture (MoA). Furthermore the authorities are promoting the development of aquaculture activities and ensuring that adequate conservation measures are established to suit local conditions in the different geographical areas (i.e. temperate and subtropical regions).
Nevertheless, the development of a sea cucumber management plan for the Dongsha, Nansha, Xisha and Zhongsha islands has been relatively slow and the plan inadequately implemented. This has caused excessive fishing and a gradual decline of the natural resources. It is suggested that a suitable number of protected areas are established around some of the islands, a minimum catch size for the different species is set, along with the establishment of authorized fishing seasons. Moreover, applied research on the artificial reproduction of economically important species of sea cucumbers may be necessary for the long term sustainability of the sector in the islands of Hainan.
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Ran, X.D. 1993. Chinese Medicine Encyclopedia (Zhonghua Yaohai). Ha-E-bing Publisher, Beijing. p.1681-1682.
Wang, F.G. 1997. Nutrient analysis of frozen sea cucumber (Acaudina molpadioides). East China Sea Marine Science, 15(4):65-67.