TANK CULTURE OF TILAPIA 



33 



affliction of young tilapia. Outbreaks of tlie dis- 

 ease caused high mortality rates among fry about 

 2 to 3 weeks old, soon after they had started sup- 

 plementary feeding. Symptoms characteristic of 

 this disease were violent whirling or corkscrewing 

 accompanied by rapid breathing. The afflicted fi.sh 

 usually exhibited a series of these whirling move- 

 ments, then sank to the bottom of the tank and 

 stopped feeding. Cessation of feeding caused 

 many of the fry to have a "pinhead" appearance, 

 that is, a large head and shrunken body. 



These symptoms are similar to those described 

 for octomitiasis, commonly called whirling disease 

 or pinhead condition (Lagler, 1956; Davis, 1953; 

 Snieszko and Wolf, 1958) . Octomitiasis is caused 

 by the protozoan Octomitus salmom's, which oc- 

 curs in the intestine either in the flagellated form, 

 when the condition is chronic, or in an intracel- 

 lular stage, when the disease is acute. The etiology 

 of infectious pancreatic necrosis, on the other 

 hand, is still in doubt, although it has been re- 

 ported as probably caused by a virus (Lagler, 

 1956; Snieszko and Wolf, 1958). 



All of the symptoms noted here were observed 

 in one particular outbreak of disease at the pilot 

 plant. Dissection and examination of the stomach 

 and anterior intestine of six afflicted fish revealed 

 that only one tisli had a protozoan in the intestine. 

 All of the others appeared normal internally. On 

 the assumption that this disease was probably 

 octomitiasis, we started immediate treatment witli 

 Carbarsone (p-Ureidobenzene arsonic acid) at the 

 rate of 1 gram per pound of food (Davis, 1953). 

 The mortality rate decreased appreciably in the 

 next few days following treatment and by the end 

 of the tenth day had been reduced to a low level. 

 Although Carbarsone seemingly effected a cure, we 

 are not certain that Ocfomitus was the causative 

 organism, since the protozoan was not positively 

 identified. It might possibly have been infectious 

 pancreatic necrosis, or an acute infection of 

 octomitiasis caused by the intracellular stage of the 

 flagellate, or a combination of the two. Snieszko 

 and Wolf (1958) state that many cases diagnosed 

 as octomitiasis are in reality infectious pancreatic 

 necrosis. Careful microscopic examination is 

 necessary for correct diagnosis. 



Subsequent periodic outbreaks of infectious 

 pancreatic necrosis were definitely identified by 

 examination of the stomach and anterior intestine 



61935« 0—^2 S 



of diseased individuals. In most instances these 

 organs were distended and filled with a colorless, 

 opaque fluid, indicating a stoppage of bile flow. 

 As there is no known efl'ective therap\' for this dis- 

 ease, the usual procedure was to treat the fish with 

 ])otassium permanganate at a concentration of 3 

 p.p.m. to prevent secondai-j* infection of the weak- 

 ened fish with ectoparasites and to observe strict 

 sanitation measures. In this way, we believe that 

 most of the disease outbreaks were kept localized. 

 Mortality rates among the fiy were usually 

 highest during the first week after an outbreak and 

 gradually subsided during the following 2 to 3 

 weeks. The most serious outbreak of disease oc- 

 curred at the Kewalo plant soon after it was in 

 operation. A succession of infections that spread 

 among the newly emerged fry caused an estimated 

 loss of about 80 percent of the 146,776 fry produced 

 in a 3-month period. The disease was controlled 

 after 10 weeks. 



Prophylactic Measures 



Despite knowledge that the density of fish is 

 extremely important in relation to the outbreak 

 and spread of disease, we tended to overcrowd 

 our tanks on occasion. Since the amount of fry- 

 rearing facilities was limited, it was impossible 

 to avoid overcrowding during periods of heavy 

 fry production unless we discarded some of the 

 young fish. Our i-ecords indicate that several of 

 the disease outbreaks were directly associated with 

 periods of high production. "^^Hienever an over- 

 crowded condition existed, we made every effort 

 to prevent disease outbreaks by increasing the 

 rate of the inflowing water, by cropping the fry- 

 tank compartments frequently, and by obsei-ving 

 strict sanitation. 



Effective prophylactic measures are probably 

 of greater importance than control measures in 

 successful fish culture. Potassium permanganate 

 was periodically added to the water as a prophy- 

 lactic measure and, when conditions permitted, 

 tanks which held any diseased fish were 

 thoroughly scrubbed with a brush, refilled with 

 water, and copper sulfate or formalin was added 

 in high concentration. The environment thus 

 created was believed unfavorable to whatever 

 ectoparasites may have remained in the tank after 

 the scrubbing. 



