70 FISH HATCHERY MANAGEMENT 



temperature drop from 50° to 46°F would have an increase in Flow Index 

 from 1.50 to 1.80, because the metabolic rate of the fish normally would 

 drop and the oxygen concentration would increase with a drop in water 

 temperature. The reverse would be true with a rise in water temperature. 

 Although Table 8 is useful for planning and estimating preliminary carrying capa- 

 city in a trout or salmon hatchery, it should be considered only as a guide and 

 specific Flow Indexes ultimately should be developed at each individual hatchery. 



The table is based on oxygen levels in the inflowing water at or near 

 100% saturation. If a rise or drop in oxygen occurs, there is a correspond- 

 ing rise or drop in the Flow Index, proportional to the oxygen available for 

 growth (that oxygen in excess of the minimum concentration acceptable for 

 the species of fish being reared). 



Example: There is a seasonal drop in oxygen concentration from 11.0 to 

 8.0 parts per million (ppm) in the water supply of a trout hatchery, and the 

 minimum acceptable oxygen concentration for trout is 5.0 ppm. The Flow 

 Index has been established at 1.5 when the water supply contained 11.0 

 ppm oxygen. What is the Flow Index at the lower oxygen concentration? 



(1) With 11 ppm oxygen in the water supply, there is 6 ppm available 

 oxygen, since the minimum acceptable level for trout is 5 ppm (ll ppm — 5 

 ppm). 



(2) With 8 ppm oxygen in the water supply, there is 3 ppm available ox- 

 ygen (8 ppm — 5 ppm). 



(3) The reduction in Flow Index is the available oxygen at 8 ppm divid- 

 ed by the available oxygen at 11 ppm or a 0.5 reduction (3 ^ 6). 



(4) The Flow Index will be 0.75 at the lower oxygen concentration 



(1.5x0.5). 



Table 9 presents dissolved oxygen concentrations in water at various tem- 

 peratures and elevations above sea level. The percent saturation can be cal- 

 culated, once the dissolved oxygen in parts per million is determined for 

 the water supply. 



Many hatcheries reuse water through a series of raceways or ponds and 

 the dissolved oxygen concentration may decrease as the water flows 

 through the series. As a result, if aeration does not restore the used oxygen 

 to the original concentration, the carrying capacity will decrease through a 

 series of raceways somewhat proportional to the oxygen decrease. The car- 

 rying capacity or Flow Index of succeeding raceways in the series can be 

 calculated by determining the percent decrease in oxygen saturation in the 

 water flow, but only down to the minimum acceptable oxygen concentration for the 

 fish species. 



Calculations of rearing unit loadings should be based on the final 

 weights and sizes anticipated when the fish are to be harvested or loadings 



