by
Vijai Srisuvantach
Manob Thangtrongpiros
Regional Lead Centre in Thailand
National Inland Fisheries Institute
Bangkhen, Bangkok 10900
Thailand
1. INTRODUCTION
Walking catfish in English, or “pla duk” in Thai, is a generic name for a number of species belonging to the family Clarridae. Five are encountered in Thailand, two of which are popular sources of animal protein, Clarias batrachus and Clarias macrocephalus, locally known as “pla duk dan” and “pla duk oui”, respectively. C. batrachus fry is easily obtained from the spawning pond. Unfortunately, C. macrocephalus do not readily reproduce themselves in captivity. However, it can be induced to breed if injected with extracts of fish pituitary glands containing gonadotropin sex hormones.
Thai consumers have a preference for C. macrocephalus but, because of bottlenecks in fry availability and slow growth, its culture is still limited in comparison to C. batrachus.
Our biologists have worked for 20 years to develop artificial breeding methods. This attempt was first successful for Pangasius sutchi and Chinese Carps in 1965. Further studies were continued. Up to now, induced spawning is a rutine work for our aquaculturists.
The purpose of this review is to summarize the induced breeding and larval rearing of C. macrocephalus practices in Thailand.
2. Characteristics and Biology
The species is closely related to C. batrachus from which it can be distinguished by the wide occipital process (Fig. 1). Body is elongate with head broadly depressed, four pairs of well developed barbells, and small eyes. Dorsal and anal fins are long without spine. Pectoral fin has a pungent spine with serrated on its inner edge. Caudal fin is not confluent with dorsal or anal fin (Fig. 2). Body color is dark brown with purplish tint and about tent transverse rows of small white spots on the side.
Fig. 1. Occipital process of Clarias macrocephalus (left) and Clarias batrachus (right).
Fig. 2. Clarias macrocephalus Günther
The species is distinguished by their ability to survive in a wide range of water conditions. It requires a relatively small area for culture and can be stocked more densely than many other species. They can live out of water for several hours or in waters of low oxygen content as they have accessory organs that enable them to breathe atmospheric air. Its range of distribution includes the areas from Indochina penninsular. Thailand and the Philippines.
The spawning season is between May and October. The female makes a small round hollow nest with grassy bottom about 30 cm in diameter and 5 – 8 cm deep in shallow water. The eggs are deposited in the nest and attached to the roots of aquatic vegetation in the nest. The male will take charge of these eggs until they are hatched out. The egg can be hatched out within 20 hours at temperature of 25 – 30°C. A female weighing 300 – 800 gm can produce between 5,000 – 10,000 eggs. The natural diet is wide-ranging, it includes worms, insects, shrimps and decayed matter.
3. Selection of Spawners
The essentials in fish induced spawning are fully ripe mature brooders both female and male. Brood fish should be carefully tended for two to three months before induced spawning operations are carried out. Males and females should be segregrated and stocked in separate ponds. Selection of spawners is one of the most important stages in induced spawning operations. It is necessary to know how to select healthy males and females in order to obtain maximum production of fry.
Determination of ripeness is an art and requires experience. To be good brooders the firsh must be more than one year old or 150 gm. Sex can be distinguished by the shape of the genital papilla (Fig. 3). The male genital papilla is pointed. The female papilla is oval shape. The following characteristics can be used as guidelines to ascertain that the female is ready for induced spawning operations. It has a bulging abodomen. It is clastic and soft to the touch. The cloaca is reddish and prominent, and the contour of this ovary can be seem on both sides of the abdomen.
4. Obtaining the Pituitary Gland
Pituitary gland contains hormone namely gonadotropin which stimulate the production of sex steroids in the gonad which responsible for the maturation of gametes. Gonadotropin is composed of follicle stimulating hormone (FSH) and luteinizing hormone (LH) which are responsible for egg development and egg ovulation respectively.
Pituitary gland of common carp, Chinese carp, Indian carp, and Panqasius sutchi can be used for induced spawning. The fish from which the hyopphysis is to be collected is weighed and placed on a shopping board. The skull is cut open with a knife (Fig. 4). After removing a piece of the skull, fatty tissue and blood are wiped off with a cotton pad. The pituitary gland can be seen after the mid-brain has been folded back by using forceps.
Fig. 3. Genital papillae of the male and female Clarias macrocephalus
5. Amount of Pituitary Gland Solution Injected
One concentration or one dose commonly used can be expressed as follow:
The gland is ground in the homoginizer: distilled water is added and the gland is again ground. A syringe is used to take up the solution for injection (Fig. 5). The female can be injected with aypophysis of Chinese carp in a dosage of 1.0 for the first injection and 2.0 for the second injection with the time interval of 6 hours. Aliquots of isotomic saline solution or distilled water is added depending on the weight of recipient, about 0.5 ml for fish weigh less than 1.0 kg of body weight and 1.0 kg body weight and 1.0 ml for fish weigh 1 – 3 kg.
The intramuscular injection is given in the area between the base of the dorsal fin and lateral line (Fig. 6).
6. Artificial Insemination
Ovulation occurs about ten hours after the second injections. The water on the female's body should be wiped off with a towel. As the abdomen is being pressed, the stripped eggs should be collected in a dry plastic container (Fig. 7). At the same time, the milt is made to drip on the eggs by grinding the testis with fingers and pouring the water through the fine mesh cloth. Eggs and sperm are mixed and stirred gently with a feather. Next, a little clean water is added and gently mixed again. After one to two minutes, water is added two or three times to cleanse the fertilized eggs. The fertilized eggs are transferred to the hatching hapa (Fig. 8). Most of the fertilized eggs hatch out within 24 hours. Figure 9 shows the location of testis.
7. Fry Mursing
After yolk resorption, usually within 2 days, the larvae are transferred from hapa to the nursery fiber glass tank. The fry develop feeding behaviour at about the same time their yolk was absorbed. The food to be given for the first 3 weeks is live moina. Usually 3 weeks old fry with the size of 2 – 3 cm are distributed to the fish farmers.
Fig. 4. Extraction method for removing the pituitary gland, showing transverse cut, mid-brain and pituitary gland location.
Fig. 5. Preparation of the pituitary gland solution
Fig. 6. Location for intramuscular injection.
Fig. 7. Artificial insemination.
Fig. 9. Transferring fertilized eggs to incubator hapa.
Fig. 8. Location of testis with the digestive track folded up.
8. Egg Development
The stages of egg development are given in Fig. 10.
Fig. 10. Egg development of Clarias macroccphalus, 70 X
