FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 5. — Concentrations (ml./l.) of dissolved oxygen in 

 selected tanks, measured in morning and afternoon, 

 1958 



m FEB MAR APR MAY JIN JUL AUG SEP OCT NOV DEC 



Figure 7. — Average monthly sunlight recorded at Paia, 

 Maui, in 1958 and 1959. (Data courtesy of Hawaiian 

 Sugar Planters Association. ) 



of dissolved oxygen. The low (morning) values 

 averaged 1.11 ml./l. for tank A and 0.99 ml./l. 

 for tank B. The high (afternoon) values aver- 

 aged 6.62 ml./l. for tank A and 5.12 ml./l. for 

 tank B. This marked diunial variation resulted, 

 we believe, from oxygen being utilized during the 

 hours of darkness by both algae and fish and be- 

 ing restored during daylight hours by plioto- 

 synthesis. The difl'erence in tilapia production 

 between the two tanks may possibly have been 

 related to the higher oxygen concentration that 

 prevailed in tank A. 



On ^Nlarch 4 and 5, detailed sampling was carried 

 out to determine the differences in oxygen con- 

 centration at different positions and at different 

 depths in tlie brood tanks in relation to inflow and 

 outlet and direction of the prevailing winds. The 

 results are given in table 6. There were no major 

 differences between the two tanks in the afternoon. 

 Early in the morning the oxygen concentrations 

 were slightly less in tank B than in tank A. Con- 

 centrations of oxygen were higlier at the surface 

 than those midway in the tank, which in turn were 

 usually higher than those near tJie bottom of the 

 tanks. Concentrations were higher near the in- 

 flow end of the tanks than near the outlet, as might 

 be expected. 



A number of measurements (table 5) taken in 

 one of the fry tanks indicates that oxygen concen- 

 trations there were favorable at all times. The 

 high values are related, no doubt, to tlie sliallow 

 depth of the tanks, the high surface area to volume 

 ratio, and the presence of algae. 



