In this report are given an account of the life history and general habit of the Corean «snake-head» fish, Ophicephalus argus CANTOR, and its method of culture, with a short note on its fishing method.
Ophicephalus argus is the most northern species among the fishes of the Ophicephalidae. It is known to be distributed in southern Siberia, Manchuria, Corea (except in its northeastern district) and in northern China southward to the River Yan-tse. It is highly esteemed as a food fish in the countries where it abounds.
The usual habitats of the fish are the shallow ponds or swamps of rather stagnant water and muddy bottom, richly grown with water-plants. It is found also in the slow streams, but is never in the clear running water.
Air-breathing habit is well developed, accompanied by the corresponding structure of the supra-branchial chamber as is already well known, causing almost entire indifference to the faulness of water in which it lives. From the experiments carried out by the authors, it has been known that the air-respiration is absolutely necessary for this fish, except in the hibernation period when the respiratory function almost ceases. It suffocates in a relatively short time if it is forcibly kept under water so as to prevent completely the air-respiration, no matter how freely the water in which the fish is kept is changed. Whereas it can live ever quite healthy in any faul water in which no other fish can live if the free air-respiration is admitted. It can live also entirely out of water quite a long while, the duration of which depending upon the air temperature: about 24 hours at 21°-27° C; 3-4 days at 10°-15°; a week at the average temperature of 7° C; about 3-4 days at the average of 1°, often lowering below 0° C. That is, the maintenance of life out of water is shorter in higher temperature, becomes longer as it lowers, but becomes shorter again below certain limit.
Its natural food consists of various small aquatic animals, such as small fishes, frogs, water-insects, etc., which it voraciously feeds upon.
The spawning season is from June to July, sometimes extending to early August. Before spawning the pair of parent fish build a nest among the water plants in shallow- water. The nest is made out of stems and fronds of weeds in a shape of irregular circle of about one meter in diameter, floating at the surface of water. The water surface in the nest where the eggs are spawned is cleaned. The spawning usually takes place at dawn or early in the morning of a calm day. At first the female rises up to the water surface and keeping her belly upward dischanges eggs, followed by the male which taking similar attitude ejaculates the sperm, thus accomplishing the furtilization.
The number of eggs spawned at one time is 1,300-15,000, average 7,300. The number of spawning during one season by the same female fish is, by the authors' observations, 1-5. The egg is floating and non-adhesive, spherical, about 2 mm. in diameter, with a single very large oil-globule, and beautiful yellow in colour. It hatches out quite healthy at any water temperature between 18°-31° C. The incubation period lasts about 28 hours at 31 ° C., 120 hours at 18 °, and about 45 hours at the ordinary water temperature during spawning season, i. e. about 25° C.
The larva just hatched is about 4 mm. in total length. The body is almost black all-over during the whole larval stage, and looks like a young tadpole in general appearance. It floats in the nest, feebly moving about at times, until the end of the larval stage when the yolk is completely absorbed and the body length attains about 8 mm. Then it swims out of the nest in a shoal to feed upon minute organisms in the water.
When the post larval stage has set in, the black coloration of the body begins to fade, at first in the dorsal side and later downward. The air-respiration also begins in this stage. At the end of the post-larval stage, when the body length becomes about 18 mm., a beautiful greenish yellow coloration prevails over the body, leaving only a dark streak along the base of the anal fin. From this stage on the fish greedly feeds upon small crustaceans, fish-larvae or any other small animals which come across, causing very rapid growth. The parent fish eagerly guard over their brood till the end of the post-larval stage, boldy driving away any enemy, such as frog, eel or other animals, which will likely attack them.
At the stage of about 30 mm. in total length the fish attains the general aspect similar to an adult in shape, coloration and in habit. It becomes mature in two years when the total length reaches about 300 mm. The size of the adult fish commonly met with in Corea is 350-450 mm. in total length, being 2-4 years of age. The largest specimen examined by the authors is 850 mm. in total length and supposed to be over 20 years old.
In this report are also treated the individual variations of some external characters as well as the change of proportions of body parts during growth.
An interesting, but certainly ruthless method of fishing is employed by the Corean natives during the spawning season. A strong rod and line with a big gook to which a live frog is fastened is used for fishing. The fisher looks for the nest in the haunt of the fish, and finding it he will beat the water surface in the nest with the frog fastened to the hook. The watching parent will then rise up and strive to drive away the intruder, at first pushing with its head. If the disturbance is continued the parent fish grows angry, daringly bites the frog and thus becomes easily hooked. By this method both parent fishes can be hooked one after another from a single nest. Various other fishing tools are used, such as long lines, harpoons, or a kind of bamboo coop with which the fish is put upon among densely grown weeds in shallow water.
The method of culture of this fish is rather simple on account of its almost entire indifference to the change of temperature or the faulness of water. It can be also easily induced to spawn in the culture pond making use of an artifical nest which the junior author has devised. The artifical nest is a square frame of wood or bamboo, about 60 cm. long in one side, to which hemp-palm fibers are attached so as to hung down in the water (as is shown in fig. 22 on p. 71). The fish will deposite eggs in the inner space of this frame, just as in the nest which they build with water plants in their natural resort.
The eggs are collected soon after fertilized and reared in the hatching box or basin in the room. When the larvae begin to swim about in search for food, they are removed to the small outdoor pond, where they are fed chiefly with daphnians daring the early period of growth. They become 50-70 mm. long in one month, when they are trasported to fish-farms.
There is no particular difficulty in transporting the young fish for further culture.
Wooden tubs, commonly used for transporting young carp for farming, about 48 cm. in diameter and 24 cm. in depth, are also used for this purpose at our station. In this tub are received about 1,000 young fishes with about 14.5 litres, of water. The death-rate of the young-fish, thus transported for about 24 hours without changing water at the water Aemperature between 25°-28° C, is within 5%.
The transportation of the adult fish is very easy, as can be supposed from its respiratory habit.
The grown-up fish prefers living animals for food, but can be accustomed to take ,smashed fish or viscera of other large animals.
This passage was originally published under the above title in the: Bulletin of the Fishery Experiment Station of the Governmental-General of Chosen No. 3. March, 1933 (Series C. No. 1); pp. 89-91. This text is the English abstract of the Text in Japanese.
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