30 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



LOG W* 3.088 LOG L-4.893S 



40 50 



LENGTH IMMI 



FiGXTRE 6. — Length-weight relation for tilapla 17 to 73 

 mm. in length. 



about the same hour each time to obtain a general 

 record of the variation in oxygen content of the 

 water (appendix tables 1, 3, 4, and 5). In a 24- 

 hour series (with sampling at hourly intervals) 

 obtained at the pilot plant November 12-13, 1957, 

 we found a marked diurnal change in the con- 

 centration of oxygen in the tanks, the minimum 

 concentration occurring at about daybreak and 

 the maximum concentration at about midafter- 

 noon (fig. 7; appendix table 2). The maximum 

 concentration was certainly the result of photo- 

 synthesis by tlie algae in the tanks, and the mini- 

 mum was caused by respiration of both algae and 

 fish. Water temperatures in the tanks showed 

 a similar diurnal variation, with the occurrence of 



312 



i 



EH) 



-i — I — I — I — I — r 



-i — I — I — I — ^" 



1600 2000 2400 



TIME OF DAY 



Figure 7. — Diurnal variation in oxygen concentration and 

 temperature in tanks 1 and 2 at the pilot plant. 



maximum and minimum temperatures correspond- 

 ing very closely to tlte maximum and minimum 

 concentrations of oxygen. 



Vaas and Hofstede (1952) stated that T. mos- 

 sa?7iiica has a high metabolic rate compared with 

 carp, and that oxygen is consumed and carbon 

 dioxide lil>erated in large quantities within a short 

 time by the species. They also found that when 

 oxygen tension is low, especially during the morn- 

 ing, file fish concfintrate at the surface of the water 

 suspended in a diagonal position, sucking in the 

 well-aerated water of the surface layer with wide- 

 open mouths. 



It is suspected that some of the mortalities 

 among the adults at both plants were associated 

 with an insufficient amount of oxygen. In tank 

 3 at the pilot plant, deaths occurred periodically 

 for about 2 months in the summer of 1958. Dur- 

 ing this period, the oxygen minima occurring in 

 the early morning were consistently low, averag- 

 ing only 0.38 ml./l. Of the 10 adults that died 

 during this period, 9 were large males averaging 

 29 cm., and the single female was 23 cm., perhaps 

 indicating the greater susceptibility of the larger 

 tilapia to low oxygen concentration. 



Since our purpose in sampling oxygen was pri- 

 marily to monitor environmental suitability, we | 

 have not attempted to relate the data to fry pro- 

 duction, except in the heating experiment, where 

 low oxygen concentrations were believed to be as- 

 sociated with the poor fry production. From the 

 data it appears that, in most instances, the mini- 

 mum or near-minimum oxygen concentration in 

 the sampled tanks was sufficient to sustain the 

 tilapia. 



Hydrogen- Ion Concentration 



Determination of hydrogen-ion concentration 

 was made by the colorimetric method at the time 

 the oxygen samples were drawn. The pH values 

 of the tanks that were sampled (appendix tables 

 6-8) ranged from 7.3 to values exceeding 8.8 (the 

 color standard used at first could measure pH 

 only to 8.8). It was expected that the pH in our 

 tanks would remain on the alkaline side, since 

 city water (pH range, 7.9 to 8.3) was used and, 

 with the exception of the fry tank, the tanks con- 

 tained a 3- to 4-inch layer of calcareous beach 

 sand. 



