59 



Prolonged treatment 



The fourth method of treatment, namely prolonged treatment, 

 is based on the theory that a long exposure to a dilute solu- 

 tion of disinfectant is more efficacious and less toxic than is 

 the short, concentrated handdip. Furthermore, it may be applied 

 vdthout handling the fish, a factor which is not serious if the 

 fish are carefully treated from troughs but may become very much 

 so when the fish are in ponds [concrete] where a seine musi:, be 

 used and a large number of fish are involved. Prolonged treat- 

 ment of fish, either in ponds or troughs, does obviate this 

 very objectionable feature. Unfortunately, prolonged treatments 

 must still be regarded as in the experimental stage and while 

 they are, theoretically, far superior to hand dips, we have much 

 to learn concerning their practical application. 



Prolonged treatment originally consisted of adding to the 

 inflowing water by means of some convenient apparatus, suffi- 

 cient dissolved disinfectant at a uniform rate to maintain a 

 constant concentration of disinfectant over a definite period 

 of time, usually one hour. This method of treating the inflow- 

 ing water is subject to an inherent inaccuracy due to the di- 

 luting influence of the residual water in the pond at the time 

 treatment is started. This inaccuracy is not serious in the case 

 of troughs or small raceways which may be drained practically 

 to dryness and which fill rapidly, but it becomes progiessively 

 more so as the size of the body of water to be treated is 

 increased. For the treatment of the larger types of fish- 

 cultural equipment such as circular pools and raceways, the 

 most recent development in prolonged treatments is essentially 

 identical to the method of salting described except that the 

 disinfectant-concentration used is much weaker so that the 

 fish may be safely exposed for a period of several hoxirs during 

 which time, the water is recirculated in a closed system from 

 the lower end of the pond to the upper by means of a centrifugal 

 pump. This assures adequate aeration during the time when the 

 inflowing water is stopped. 



When to treat 

 In any method of treatment, time is of vital importance. 

 Disease rapidly lowers the vitality of small fish and although 

 today they may withstemd the rigors of treatment, tomorrow may 

 find them too weak. The f i sh-culturist must maintain strict 

 watch on his stock. Many external parasites give early warning 

 of their presence which is evidenced by the fish refusing to 

 eat, scratching themselves or assuming a characteristic bluish- 

 gray sheen. Fungus, of course, is an excellent indicator of 

 trouble but it usually does not appear until after the tell-tale 

 rise in the daily losses which are the surest proof that trouble 

 is present. 



