4 FISH HATCHF.RY MANAGKMF.NI' 



are geared to annual temperature changes in the natural environment. 

 Each species has a temperature range that it can tolerate, and within that 

 range it has optimal temperatures for growth and reproduction. These op- 

 timal temperatures may change as a fish grows. Successful hatchery opera- 

 tions depend on a detailed knowledge of such temperature influences. 



The temperature requirements for a fish production program should be 

 well defined, because energy must be purchased for either heating or cool- 

 ing the hatchery water supply if unsuitable temperatures occur. First con- 

 sideration should be to select a water supply with optimal temperatures for 

 the species to be reared or, conversely, to select a species of fish that 

 thrives in the water temperatures naturally available to the hatchery. 



It is important to remember that major temperature differences between 

 hatchery water and the streams into which the fish ultimately may be 

 stocked can greatly lower the success of any stocking program to which 

 hatchery operations may be directed. Within a hatchery, temperatures that 

 become too high or low for fish impart stresses that can dramatically affect 

 production and render fish more susceptible to disease. Most chemical sub- 

 stances dissolve more readily as temperature increases; in contrast, and of 

 considerable importance to hatchery operations, gases such as oxygen and 

 carbon dioxide become less soluble as temperatures rise. 



Some suggested temperature limits for commonly cultured species are 

 presented in Chapter 3, Table 17. 



Dissolved Gases 



Nitrogen and oxygen are the two most abundant gases dissolved in water. 

 Although the atmosphere contains almost four times more nitrogen than 

 oxygen in volume, oxygen has twice the solubility of nitrogen in water. 

 Therefore, fresh water usually contains about twice as much nitrogen as 

 oxygen when in equilibrium with the atmosphere. Carbon dioxide also is 

 present in water, but it normally occurs at much lower concentrations than 

 either nitrogen or oxygen because of its low concentration in the atmos- 

 phere. 



All atmospheric gases dissolve in water, although not in their atmospher- 

 ic proportions; as mentioned, for example, oxygen is over twice as soluble 

 as nitrogen. Natural waters contain additional dissolved gases that result 

 from erosion of rock and decomposition of organic matter. Several gases 

 have implications for hatchery site selection and management. Oxygen 

 must be above certain minimum concentrations. Other gases must be kept 

 below critical lethal concentrations in hatchery or pond water. As for other 

 aspects of water quality, inappropriate concentrations of dissolved gases in 

 source waters mean added expense for treatment facilities. 



