coast. Rainbow trout farms are distributed in all 

 parts of the country. However, fish farms which 

 produce fish for export are centered in Shizuoi<a and 

 Nagano Prefectures. 



SOME TECHNICAL PROBLEMS IN 

 FRESHWATER FISH PRODUCTION 



Many technical problems have arisen in freshwa- 

 ter fish production. While many of the same prob- 

 lems occur with all species, their relative order of 

 importance differs by species as follows: 



Carp: I- Improvement of breed. 

 Eel: 1. Stabilization of elver supply. 



2. Prevention of epidemic diseases. 

 Rainbow trout: 



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. 



Carp 



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. 



Eel 



In eel culture, elvers are collected along the 

 Pacific coast from December to April. Eel culturists 

 are often troubled with scarcity of young eels be- 

 cause of fluctuations in abundance from year to year. 

 The demand for elvers has rapidly increased in re- 

 cent years proportionally with the increase of culture 

 ponds. Three counterplans have been adopted to 

 solve the problem: I) improvement of survival 

 rate of elvers, 2) imporlation of elvers from foreign 

 countries, and 3) establishment of spawning tech- 

 niques. 



Heating apparatus to increase winter tempera- 

 tures in ponds is proving to be effective for improv- 

 ing 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 circula- 

 tion, the water is heated to a constant temperature. 

 Although this apparatus entails high costs, survival 

 rates often reach 939^. 



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 ex- 

 tremely 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 Fa- 

 culty of Agriculture, University of Tokyo. 



The prevention of epidemic disease is another im- 

 portant problem in eel culture. It is difficult to keep 

 accurate records of mortalities in eel culture, be- 

 cause turbidity of the pond water inhibits finding or 

 seeing dead fish. Attempts at estimating losses in 

 Shizuoka Prefecture have revealed a seasonal pat- 

 tern. 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 to- 

 taled 2,600 tons in Shizuoka Prefecture (Table 5). 

 This was due to a new epidemic disease, bran- 

 chionephritis, named by S. Egusa. Professor of Fa- 

 culty of Agriculture, University of Tokyo. Fortu- 



Tahle .^. — Loss of eel in Shizuoka Prefecture. 



Year 



Amount of loss 



1964 

 1965 

 1966 

 1967 

 l%8 

 1%9 

 1970 



Ions 



492 

 200 

 522 

 199 

 449 

 20.'> 

 1,639 



20 



