f) FISH HATCHERY MANAGEMENT 



decreases — that is, less oxygen can be dissolved in water — at higher alti- 

 tudes and, more importantly, at higher temperatures. For this reason, the 

 relationship between absolute concentrations (parts per million) and rela- 

 tive concentrations (percent saturation) of gases is not straightforward. 

 Special conversion formulae are needed; in graphical form these can be 

 depicted as nomograms. A nomogram for oxygen is shown in Figure 1. 



Dissolved oxygen concentrations in hatchery waters are depleted in 

 several ways, but chiefly by respiration of fish and other organisms and by 

 chemical reactions with organic matter (feces, waste feed, decaying plant 

 and animal remains, et cetera). As temperature increases the metabolic rate 

 of the fish, respiration depletes the oxygen concentration of the water more 

 rapidly, and stress or even death can follow. Fluctuating water tempera- 

 tures and the resulting change in available oxygen must be considered in 

 good hatchery management. In ponds, oxygen can be restored during the 

 day by photosynthesis and at any time by wind mixing of the air and 

 water. In hatchery troughs and raceways, oxygen is supplied by continu- 

 ously flowing fresh water. However, oxygen deficiencies can arise in both 

 ponds and raceways, especially when water is reused or reconditioned. 

 Then, chemical or mechanical aeration techniques must be applied by cul- 

 turists; these are outlined below for raceways, and on pages 108-110 for 

 ponds. Aeration devices are shown in Figures 2 and 3. 



In general, water flowing into hatcheries should be at or near IOO"'!i oxy- 

 gen saturation. In raceway systems, where large numbers of fish are cul- 

 tured intensively, oxygen contents of the water should not drop below 80% 

 saturation. In ponds, where fish densities are lower (extensive culture) than 



Figure 2. A simple aeration device made of perforated aluminum can add oxy- 

 gen to the water and restrict fish from jumping into the raceway above. (FWS 

 photo.) 



