« Marketable eel 



O O Fingerling 



Apr May Jun Jul Aug. Sep.Oct. Nov. Dec. Jan. Feb. Mar 

 Month 

 Figure 2. — Seasonal change of loss in eel culture ponds [n 

 Shizuoka Prefecture, in 1968. 



nately, this epidemic did not return last winter 

 (1971). 



Rainbow trout 



Prevention of epidemic disease is also the most 

 important problem in trout production. Infectious 

 pancreatic necrosis (IPN). Vibrio infection, bacter- 

 ial gill disease, and Hexainita infection are the prin- 

 cipal diseases in Japan. Among them. IPN is most 

 harmful. Fortunately, viral hemonhagic septicemia 

 ( VHS) and whirling disease, Myxosomci cerehralis. 

 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 sur- 

 veys in other epidemic diseases. For example, they 

 estimated the critical temperature as 12°C for out- 

 break of IPN. 



Recently, the net culture of rainbow trout in shal- 

 low seavvater has begun in the northern part of 

 Japan. .\\\ attempts resulted in failure in southern 

 Japan, because the water temperature is too high for 

 rainhou trout to survive in seawater. The upper limit 

 for survival in seawater is estimated to be 22°-23°C 

 (72°-73°F). Water temperatures above 23°C lead to 

 bacterial infections. An analogous phenomenon is 

 observed in ayu reared in seawater. 



Ayu 



Ayu is familiar to the taste of Japanese and is also 

 the most impoilant 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 artifi- 

 cial production of ayu fry have been started in sev- 

 eral prefectures. At present, two systems of fry pro- 

 duction are used, one type uses salt water and the 

 other fresh water. At present, saltwater culture gives 

 better results. In both types of culture, Rotifem are 

 used as the starting feed. Bnicliioniis plicatilis is 

 used for seawater culture and B. calyciflorns for 

 freshwater culture. 



Vibrio and Gliii>ea infections are the principal dis- 

 eases of ayu culture. Vibrio infections annually 

 occur in Shiga and Tokushima Prefectures. This dis- 

 ease also is observed in wild populations in Shiga 

 Prefecture. Disinfection of fingerlings is widely 

 practiced in many culture farms. 



PRODUCTION OF 



NATIVE TROUT FINGERLINGS FOR 



STOCKING IN NATURAL WATERS 



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 pre- 

 fectural trout hatcheries have carried out the ex- 

 perimental production of native trout fingerlings. 

 Large-scale production will be realized in various 

 parts of the country. 



TRANSPLANTATION OF 



FOREIGN SPECIES SUITABLE TO THE 



JAPANESE TASTES 



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. Chan- 

 nel catfish attracts much attention because of the 

 ease of production: its taste is also suitable to the 

 Japanese people. 



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