1 12 FISH HATCHERY MANAGEMENT 



recombining with hydrogen and prevent the above reaction from moving to 

 the left, even though plants may be removing CO^ from the water. The 

 most important ion used for this purpose is calcium (Ca ), which usually 

 is added in the form of gypsum (calcium sulfate, CaSOj. 



The two types of treatments may be combined. For example, sulfur, 

 manure, and gypsum together may be effective in reducing pond alkalinity. 



TURBIDITY 



Excessive turbidity in ponds obstructs light penetration; it can reduce pho- 

 tosynthesis and make it more difficult for fish to find food. Much turbidity 

 is caused by colloids — clay particles that remain suspended in water 

 because of their small size and negative electric charges. If the charges on 

 colloidal particles can be neutralized, they will stick together — 

 flocculate — and precipitate to the bottom. Any positively charged material 

 can help flocculate such colloids. Organic matter works, although it can 

 deplete a pond's oxygen supply as it decomposes, and is not recommended 

 during summer months. Weak acids or metallic ions such as calcium also 

 can neutralize colloidal charges, and many culturists add (depending on 

 pH) limestone, calcium hydroxide, or gypsum to ponds for this purpose. 



HYDROGEN SULFIDE 



Hydrogen sulfide, H^S, is a soluble, highly poisonous gas having the 

 characteristic odor of rotten eggs. It is an anaerobic degradation product of 

 both organic sulfur compounds and inorganic sulfates. Decomposition of 

 algae, aquatic weeds, waste fish feed, and other naturally deposited organic 

 material is the major source of H^S in fish ponds. 



The toxicity of H^S depends on temperature, pH, and dissolved oxygen. 

 At pH values of five or below, most of the H^S is in its undissociated toxic 

 form. As pH rises the H7S dissociates into S and H^ ions, which are 

 nontoxic. At pH 9 most of the H^S has dissociated to a nontoxic form. Its 

 toxicity increases at higher temperatures, but oxygen will convert it to 

 nontoxic sulfate. 



H^S is toxic to fish at levels above 2.0 parts per billion and toxic to eggs 

 at 12 parts per billion. It is a known cause of low fish survival in organi- 

 cally rich ponds. If the water is well oxygenated, H^|S will not escape from 

 the sediments unless the latter are disturbed, as they are during seining 

 operations. Hydrogen sulfide mainly is a problem during warm months, 

 when organic decomposition is rapid and bottom waters are low in dis- 

 solved oxygen. 



Hydrogen sulfide problems can be corrected in several ways: (l) remove 

 excess organic matter from the pond; (2) raise the pH of the water (see 

 above); (3) oxygenate the water; (4) add an oxidizing agent such as potas- 

 sium permanganate. 



