TANK CULTURE OF TILAPIA 



31 



The hydrof^en-ion concentration and its signifi- 

 cance to aquatic organisms lias been relegated for 

 some time to a minor position by many investiga- 

 tors (Welch, 1935 ; Odum, 1959) . It may possibly 

 be a limiting factor to some organisms, iiowever. 



Free Carbon Dioxide 



Analysis of the amount of free carbon dioxide 

 in tlie tanks was started at the Kewalo plant in 

 February 1959. TliB samples were drawn im- 

 mediately after those for the oxygen determina- 

 tion liad been drawn and fixed. The amount of 

 free carbon dioxide present was approximated by 

 titration of a 100-ml. sample to the phenolphtha- 

 lein endpoint with 0.02 N sodium hydroxide. 



AVelch (1935: p. 175) stated that carbon dioxide 

 is one of the most important substances in the life 

 of organisms, but that it should be present only 

 under suitable circumstances and in proper 

 amounts. He stated further that a small amount 

 of carbon dioxide appears to be essential for 

 aquatic animals. Doudoroff (1957) regarded free 

 carbon dioxide concentrations between 100 and 

 200 p. p.m. as fatal to moderately susceptible fresh- 

 water fishes; also, that exposure to concentrations 

 l>etween 50 and 100 p. p.m. causes immediate dis- 

 tress and may be lethal if the exposure is pro- 

 longed. He noted that even in polluted waters 

 free carbon dioxide concentrations rarely exceed 

 20 p. p.m. 



Throughout the period of sampling at the 

 Kewalo plant, the free carbon dioxide concen- 

 tration (appendix tables 9 and 10) was never 

 found to be in excess of 16.2 p.p.m. No attempts 

 were made to determine a relation between carbon 

 dioxide concentrations and fry production. Our 

 routine observations were made primarily to de- 

 tect excessive amounts of free carbon dioxide in 

 the brood and f i-y tanks. 



Alkalinity 



Chemical analyses of the water in tlie tanks at 

 the Kewalo plant included determinations of total 

 bicarbonate and carbonate alkalinity. A 100-ml. 

 .sami)le was titrated with 0.02 N sulfuric acid 

 against tiie phenolphthalein and methyl orange 

 endpoints. 



Alkalinity directly influences the biological pro- 

 ductivity of a body of water. The carbonates and 

 bicarbonates, wliich are in close chemical com- 

 bination witli carbon dioxide, are utilized by algae 



(Welch, 1935), and also act as buffers by keeping 

 the hydrogen-ion concentration close to the neu- 

 tral point (Odum, 1959). 



Our alkalinity determinations (appendix tables 

 8 and 10) were made primarily to gain a general 

 knowledge of the type of water present in the 

 tanks, as soft water has a smaller supply of these 

 ions and, tlierefore, is less productive. 



MORTALITIES 



Among the Adults 



Observed mortalities among tiie brood stock 

 amounted to 31 males and 11 females at the pilot 

 plant and 55 males and 56 females at the Kewalo 

 plant. The dead fish were carefully examined 

 and deaths were usually attributable to one of 

 the following factors: Rough handling, disease, 

 asphyxiation and, possibly, hydrogen sulfide 

 poisoning. 



While it might have been possible to reduce 

 mortalities by using more care in handling fish 

 and by proper treatment of diseased fish, it was 

 not always possible to prevent mortalities caused 

 by asphyxiation, since many factors contributed 

 to the oxygen concentration in the tanks. 



The heaviest mortality to occur in a single day 

 among the adults was experienced at the Kewalo 

 plant. A total of 7 males and 29 females was 

 found dead in the two brood tanks containing 

 brackish water (salinity-spawning experiment) 

 and the deaths were believed to have been caused 

 by either asphyxiation or hydrogen sulfide. Faint 

 odors of hydrogen sulfide gas were detected in 

 the brackish-water tanks for several da.ys before 

 the mortalities occurred, and, undoubtedly, this 

 gas was one of the contributing factors, if not the 

 determining factor in the deaths. The tanks were 

 not drained and cleaned at the time hydrogen sul- 

 fide was first detected in the tanks, however, be- 

 cause the experiment was to be terminated within 

 a few days. 



Among the Young 



Many factoi-s contributed to the loss of young 

 fish, chief of which was disease, with deaths from 

 handling judged to be of secondary importance. 

 At the pilot plant, the observed mortality in the 

 fry tank amounted to 34,784 fish, while juvenile 

 deaths totalled 4,523 fish. Although the total ob- 

 served moi'talitv among the voung amounted to 



