SEA-HORSE CULTURE

1.1 Taxonomy

1.2 Morphology

The body of the sea-horse, covered by septanangle bone-rings, is flattened on the sides, with a characteristically protruding abdomen and horse-like head. The bone-rings covering the tail are of the tetragon type. The mouth is very small and toothless, with pipe-like lips. The abdomen and tail fins are absent. The male sea-horse has the typical baby sac for brooding the offspring. The body color of both sexes is grey-brown or dark brown, however it may change in response to external stimuli as a protective measure or to capture food organisms.

Hippocampus kuda and H. trimacutus are the main cultured species dwelling in South China Sea. The major external differences are the following:

1.3 Habitat

1.3.1 Temperature

Sea-horses are eurythermal, however changes in the seawater temperature have a direct influence on growth and survival. The optimum water temperature varies among species. For example, the temperature limits of H. japonicus are between 5–36 °C, 9–34 °C for H. kuda, and between 10–30 °C for H. trimacutus. In general, however, the optimum temperature for most species is around 28 °C.

Figure 1.1. Four species of sea-horses found in Chinese waters: a) Hippocampus kuda, b) H. japonicus, c) H. trimacutus, and d) H. histrix.

The effect of temperature on the reproduction activity is very strong, the optimal level ranging from 20–30 °C. When the seawater temperature increases to 20 °C the sexual activity of the broodstocks tends to increase considerably. Male specimens actively search for females. After mating, the females lay their eggs into the baby sac of the males.

The hatching-out time of fertilized eggs is closely related to the seawater temperature. The process may take 10–20 days at temperatures ranging between 28–30 °C, while at 25 °C hatching will commence on the 14th day. However, at temperatures below 20 °C, the fertilized eggs will not develop into healthy larvae which frequently die in the baby sac. In the above case, the broodstock will also die.

1.3.2 Salinity

Sea-horses are euryhaline organisms. They can live in seawater with salinities ranging between 9–37 ppt, however adaptation to salinity is closely related to the stage of maturity. The lowest salinity the juveniles can survive in is 15 ppt, while adult specimens can tolerate a salinity of 6 ppt or even lower.

1.3.3 Dissolved oxygen

Although sea-horses are slow moving organisms, their oxygen requirement is relatively high, particularly during the hatching period. The DO in a culture tank should always be above 3 ml/L.

1.3.4 Light intensity

A certain light intensity is required for normal growth and development of sea-horses. The optimal light intensity ranges from 1,000–10,000 Lux. In a culture environment where the light intensity is either too strong or too dim, diseases and other abnormalities often occur. For example, sea-horses cultured in a dark pond become blind after several days. It is, therefore, very important to maintain the broodstocks under conditions of suitable light intensity.

1.4 Reproduction and growth

1.4.1 Sexual maturity

Sea-horses are dioecious animals. The age of sexual maturation varies among species, as well as being influenced by seawater temperature, nutrition and other environmental conditions. When sea-horses are cultured under optimal conditions, they attain sexual maturity in 100 days. Table 1.1 shows the age of sexual maturity of three sea-horse species.

Table 1.1. Age of sea-horses at sexual maturity.

1.4.2 Reproduction season

The reproduction season is closely related to water temperature. The suitable temperature for reproduction is 20–28 °C. As water temperature increases to 26–28 °C, they enter the peak period of reproduction.

1.4.3 Fecundity

The fecundity of sea-horse cultured under controlled conditions is higher than that of wild specimens. Over 70% of the broodstock have reproductive capabilities, and a mature specimen can reproduce 10–12 times in one year. During each spawn one female can lay several hundred eggs. As the animals grow older the fecundity gradually decreases.

1.4.4 Growth

The growth rate of sea-horses is comparatively and proportionally faster than other marine finfish. The larvae attain the size of the parents within 3–4 months. Table 1.2 shows the growth rate of three species.

Table 1.2. Growth rate of three species of sea-horses.

1.5 Culture method

1.5.1 Site selection

The culture site should be selected as near as possible to the seashore in order to facilitate seawater pumping, as well as collection of live food organisms. Sea-horses prefer to take live rather than dead food or formulated diets. In addition, salinity and clear seawater are two very important environmental factors. Experiments have shown that the salinity level should be in the range of 15–35 ppt.

1.5.2 Culture tank

Small hatchery tanks are required for intensive stocking of broodstock and larvae. The tank bottom area should be about 2 m² with a depth of 0.6–1.0 meter. The bottom and walls of the tank must be painted black. The two ends of the tank should be fitted with an inlet and an outlet, respectively.

Nursery tanks are especially used to culture 7–15 day-old juveniles. They should have an area of 2–6 m² with a depth of 0.8–1.0 meter. Tanks built outdoors must be fitted with some sort of cover in order to prevent strong sunshine from directly radiating the culture water.

The grow-out tanks are usually quite large with an area ranging between 5–20 m² and a depth of 0.8–1.0 meter.

1.5.3 Feed

The key to successful farming of sea-horses is a suitable feed which guarantees good growth and development. The favourite food items of sea-horses are small crustaceans and their larvae, such as Paracalanus sp., Sckmakeria sp., Acartia sp., Oithona sp., Neomysis sp., Palaemone sp., Periclimenes sp., etc. Thus, farming of sea-horse should include the artificial culture of these or similar food organisms.

In case of shortage of live diet, other feed stuff such as dried shrimp or fresh fish meat can be used as a supplementary diet. Experiments have shown that dried or salted shrimp meat give better results than fish meat (Table 1.3).

Table 1.3. The relationship between diets and growth of H. trimacutus and H. kuda.

1.5.4 Maintenance

The culture stocking density varies with the size of the animals. Newly hatched larvae can be reared at a density of 3000/m³, which should be reduced to 200–300/m³ when they reach a body length of above 6 cm.

The rate of water exchange depends on the season, climate and other environmental conditions. During summer, the daily exchange rate should be at least 30% of the total volume. In winter the rate can be reduced to 20–30% every two to three days.

Constant monitoring of the water quality is the most important routine activity. The suitable water quality parameters for farming sea-horse are as follows:

1.6 Harvesting and Processing

In China, sea-horses are used as a popular tonic medicine. The body size of sea-horses is the main quality indicator, and therefore rearing bigger species is more profitable than growing small ones. Harvesting is usually carried out after one year.

After harvesting, sea-horses are soaked in freshwater for several hours, washed, and then dried under the sun.

The price of dried sea-horse in China or in the Southeast Asian region is about US$ 400 per kg.