During the past 10 yr, considerable attention has been paid to fish culture in various countries, especially in the United States and Japan. In our country, freshwater fish culture has a long history, but the recent remarkable improvement of culture techniques has brought on a great change in production methods.
Four major species of freshwater fishes are cultured in Japan: 1) rainbow trout, Salmo gairdneri, 2) common carp, Cyprinus carpio, 3) eel, Anguilla japonica, and ayu, Plecoglossus altivelis. From 1959 to 1969, production of cultured fish for food increased from 15,000 to 52,000 tons, for a percent increase of 236. This can be compared to an increase of only 26% for wild fish from inland waters in this same period (Table 1).
Govemment statistics on fish culture in Japan classify the culture methods into four categories, namely, 1) standing-water pond, 2) running-water pond, 3) irrigation pond, and 4) net culture in lakes. Running-water ponds give the highest production per unit area for all species; net culture is second. Harvest from irrigation ponds is slightly higher than from standing-water ponds, because the larger water area of the former is more favorable to the growth of fish.
In 1968, the number of ponds utilized in various types of fish culture was 35,239 in number and 8,500 hectares in area. A total of 7,518 of these ponds were used to produce food fish. The number of fish farms classified by water area are listed in Table 2. Almost all the rainbow trout and ayu are cultured in running-water ponds, and eel in standing-water ponds. In the case of carp, both standing- and running-water ponds are used. Number and area of ponds per fish farm are shown in Table 3. These tables indicate that the fish farms in Japan are run on a small scale.
As shown in Table 4, private farms predominate in fish culture in Japan. Most of them produce vegetables and crops as well as fish. The proportion of farms engaging in monoculture is only 3% in carp culture, 28% in eel culture, 10% in ayu culture, and 6% in rainbow trout culture. The increase of monocultural fish farms will be desirable in the future. On the other hand, the reduced demand for rice may increase the culture of fish as a sideline in the rice producing area.
As shown in Figure 1, carp culture farms are distributed from Hokkaido to Kyushu regions. Most eel farms are centered in four prefectures: Shizuoka, Aichi, Mie, and Tokushima. They account for 78.6% of all eel culture farms. Ayu culture farms are distributed in the southern part along the Pacific coast. Rainbow trout farms are distributed in all parts of the country. However, fish farms which produce fish for export are centered in Shizuoka and Nagano Prefectures.
Many technical problems have arisen in freshwater fish production. While many of the same problems occur with all species, their relative order of importance differs by species as follows:
Carp: 1. Improvement of breed.
Eel: 1. Stabilization of elver supply.
2. Prevention of epidemic diseases.
1. Prevention of epidemic diseases.
2. Improvement of breed.
Ayu: 1. Mass production of fry.
2. Prevention of epidemic diseases.
Other fish species:
1. Culture of native trout fingerlings for stocking in natural waters.
2. Transplantation of foreign species suitable to the tastes of Japanese.
No urgent problems are found in carp culture at present, but in the future, improvement of the breed will be desirable. The selective breeding of strains considered to have desirable characteristics has been started by several institutions.
In eel culture, elvers are collected along the Pacific coast from December to April. Eel culturists are often troubled with scarcity of young eels because of fluctuations in abundance from year to year. The demand for elvers has rapidly increased in recent years proportionally with the increase of culture ponds. Three counterplans have been adopted to solve the problem: 1) improvement of survival rate of elvers, 2) importation of elvers from foreign countries, and 3) establishment of spawning techniques.
Heating apparatus to increase winter temperatures in ponds is proving to be effective for improving the survival of eels. This apparatus includes two main systems, heating and a circulating filter system.
In the latter, the water is filtered and recirculated back to the culture ponds. In the process of circulation, the water is heated to a constant temperature. Although this apparatus entails high costs, survival rates often reach 95%.
Recently, elvers of other eel species have been imported from various countries such as France, Philippines, and New Zealand, In 1969, 25.2 tons of elvers were stocked in Shizuoka Prefecture, of which 9 tons were imported from various countries. Foreign species have different habitat needs than native species; therefore, satisfactory results are not always obtained. Culture methods suitable for them are being developed.
Establishment of spawning techniques is an extremely difficult problem because the physiology and spawning behavior of adults and the life history of hatchery fry are not well understood. Several public institutions are wrestling with this problem, under the guidance of T. Hibiya, Professor of Faculty of Agriculture, University of Tokyo.
The prevention of epidemic disease is another important problem in eel culture. It is difficult to keep accurate records of mortalities in eel culture, because turbidity of the pond water inhibits finding or seeing dead fish. Attempts at estimating losses in Shizuoka Prefecture have revealed a seasonal pattern. That is, the mortality begins to increase in March and reaches its peak in April or May. Figure 2 shows these seasonal changes of mortality in Shizuoka Prefecture in 1968. In that year, most of the loss in the spring was due to fungus disease, while in summer bacterial gill disease caused the most mortalities.
In 1970, eel production suffered heavy losses. During the first half of the year, crop mortality totaled 2,600 tons in Shizuoka Prefecture (Table 5). This was due to a new epidemic disease, branchionephritis, named by S. Egusa, Professor of Faculty of Agriculture, University of Tokyo. Fortunately, this epidemic did not return last winter (1971).
Prevention of epidemic disease is also the most important problem in trout production. Infectious pancreatic necrosis (IPN), Vibrio infection, bacterial gill disease, and Hexamita infection are the principal diseases in Japan. Among them, IPN is most harmful. Fortunately, viral hemorrhagic septicemia (VHS) and whirling disease, Myxosoma cerebralis, have not yet been discovered in Japan. However, preventive systems against these two epidemics have been in effect since 1967. These preventive systems display the power in epidemiological surveys in other epidemic diseases. For example, they estimated the critical temperature as 12oC for outbreak of IPN.
Recently, the net culture of rainbow trout in shallow seawater has begun in the northern part of Japan. All attempts resulted in failure in southern Japan. because the water temperature is too high for rainbow trout to survive in seawater. The upper limit for survival in seawater is estimated to be 22o-23oC (72o-73oF). Water temperatures above 23oC lead to bacterial infections. An analogous phenomenon is observed in ayu reared in seawater.
Ayu is familiar to the taste of Japanese and is also the most important game fish in inland waters. Most fingerlings for culture are caught in Lake Biwa and along the seashore. Fingerlings from Lake Biwa are of good quality; however, the supply is limited by the natural standing crop. The catch from the seashore is also limited for the same reason. Attempts at artificial production of ayu fry have been started in several prefectures. At present, two systems of fry production are used, one type uses salt water and the other fresh water. At present, saltwater culture gives better results. In both types of culture, Rotifera are used as the starting feed. Brachionus plicatilis is used for seawater culture and B. calyciflorus for freshwater culture.
Vibrio and Glugea infections are the principal diseases of ayu culture. Vibrio infections annually occur in Shiga and Tokushima Prefectures. This disease also is observed in wild populations in Shiga Prefecture. Disinfection of fingerlings is widely practiced in many culture farms.
With the development of game fishing, it has been requested to propagate the populations of cold-water game fishes in natural streams. Rainbow trout are not suitable for this purpose, because these fish do not remain in the stocking area. The native trouts on the other hand appear to be very suitable for this purpose. Prompted by this information, many prefectural trout hatcheries have carried out the experimental production of native trout fingerlings. Large-scale production will be realized in various parts of the country.
Concurrent with the reduction of rice production, part of the fields will be changed to fish ponds. As this occurs, demand for a variety of cultured fish will increase. In anticipation of this situation, various foreign species have been experimentally introduced into our country. Examples are bluegill sunfish, whitefish, German carp, and channel catfish. Channel catfish attracts much attention because of the ease of production; its taste is also suitable to the Japanese people.
BROWN, E. E.
1 Freshwater Fisheries Research Laboratory, Hino-shi, Tokyo, Japan.