12 



FISHERY BULIyETIN OF THE FISH AND WILDLIFE SERVICE 



8. Better physical condition of brood stock 

 than in 1958, when they were captured and trans- 

 ported from Oahu to Maui. 



9. Larger size of brood fisli, wliich may liave 

 been responsible for the larger number of young 

 produced per female (Chen, 1953; Vaas and Hof- 

 stede. 1952). 



In 1958, newly i-eleased fry were observed in 

 Februai-y, 1 month after the initial stocking of tlie 

 adults. Production remained at a low level, how- 

 ever, from Febniai-y to May. Hea\'y production 

 started in June and continued through October. 

 In 1958, the highest number of young collected in 

 any one day was "29,800 fiy on July 8th. The 

 average production per female for the year was 

 725 fry. 



In 1959, production was at a low level in Febru- 

 ary and March, but remained high from April 

 through September. The largest day's collection 

 of fry in 1959 was 40,877 fry on May 19th. The 

 average production per female for the year was 

 1,078 fry. 



MORTALITY OF YOUNG 



Many factors contributed to the loss of young 

 fish. Some of the observed mortalities were due to 

 handling, disease, and to structural failures such 

 as tank leaks. Many sources of attrition such as 

 predation by black-crowned night herons, dragon- 

 fly nymphs, and adult tilapia, and cannibalism 

 among the fry themselves, were known to exist but 

 difficult to estimate. Table 15 provides monthly 

 figures for the dead fish that were collected and 

 counted. In 1958, such losses totaled 48,806, or 

 4.5 percent of the gross production. The unob- 

 served mortalities for 1958 amounted to 82,400,^ 

 or 7.7 percent of the gross jiroduction. The esti- 

 mated total mortality was 131,200 or 12.2 percent. 

 Losses in 1959 totaled 99,209, or 7.7 percent of the 

 gross production. The unobserved mortalities for 

 1959 amounted to 71,800, or 5.6 percent, for a total 

 mortality of 171,000, or 13.2 percent. 



As far as we could determine, the brood tanks 

 remained disease free in 1958. In 1959, a minor 

 outbreak of the protozoan Trichodma caused some 

 loss of very young fry before the infected individ- 

 uals were removed. In the young fish, the major 

 disease problems resulted from infections of Tii- 



• Method of calculating this fiRure explained in section dealing 

 with Uie production of bait-size flsh. 



1 233 lost from fry-tank leakage. 



' 4,170 lost from fry-tank leakage; 35,373 died after treatment with pyridyl- 

 raercuric acetate. 



' 6,555 were lost when a hole opened in the bottom of a fry tank, draining the 

 tank. 



' 5,967 were lost in the same manner as in footnote 3. 



chodina, which were controlled by treatment with 

 0.5 23-p.m. copper sulphate or 3 p.p.m. potassium 

 permanganate. A condition diagnosed as acute 

 catarrhal enteritis, as described by Davis (1956), 

 was the cause of high mortality rates in a few of 

 the tanks. Losses from disease were minimized by 

 the fact that each of the 90 fry tanks had an 

 independent water supply and did not drain into 

 any of the other tanks. Except in brood-tank A, 

 none of the water used was recirculated in 1959. 

 This may not have been the most economical use of 

 water, but it did prevent infections from spreading 

 from one tank to another. 



On the few occasions in 1958 when the water 

 supply was interrupted by breaks in the line, there 

 were particularly serious outbreaks of disease. 

 Such water .shortages did not occur in 1959. 

 Cliemical treatment followed by a tliorough flush- 

 ing with fresh water usually brought an end to 

 the losses in 2 or 3 days. Highest mortality rates 

 occurred when the fish were 1 to 3 weeks old. 

 Once past their 4th week, tilapia seemed to be im- 

 mune to the usual disorders. 



Early detection and treatment of disease were 

 necessary to keep losses at a low level. Treatment 

 with 3 p.p.m. potassium permanganate was effec- 

 tive for fish 0-2 weeks old infected with Trl- 

 chodina, and 0.5 p.p.m. copper sulphate was effec- 

 tive for fish 3 weeks old and older. Treatment was 

 applied usually between the hours of 9 a.m. to 3 

 p.m., when the oxygen content of tlie water in the 

 fry tanks was highest, so that the young would 

 have an adequate oxygen supply during treatment. 



