Fish Hatchery Management - fisheries & aquaculture

112 FISH HATCHERY MANAGIJMEN'frecombining with hydrogen and prevent the above reaction from moving tothe left, even though plants may be removing CO2 from the water. Themost important ion used for this purpose is calcium (Cu''), which usuallyis added in the form of gypsum (calcium sulfate, CaSOa).The two types of treatments may be combined. For example, sulfur,manure, and gypsum together may be effective in reducing pond alkalinity.TURBIDI'I'YExcessive turbidity in ponds obstructs light penetration; it can reduce photosynthesisand make it more difficult for fish to find food. Much turbidityis caused by colloids-clay particles that remain suspended in waterbecause of their small size and negative electric charges. If the charges oncolloidal particles can be neutralized, they will stick together -flocculate-andprecipitate to the bottom. Any positively charged materialcan help flocculate such colloids. Organic matter works, although it candeplete a pond's oxygen supply as it decomposes, and is not recommendedduring summer months. Weak acids or metallic ions such as calcium alsocan neutralize colloidal charges, and many culturists add (depending onpH) limestone, calcium hydroxide, or gypsum to ponds for this purpose.HYDROGE,N SULFIDEHydrogensulfide, H25, is a soluble, highly poisonous gas having thecharacteristic odor of rotten eggs. It is an anaerobic degradation product ofboth organic sulfur compounds and inorganic sulfates. Decomposition ofalgae, aquatic weeds, waste fish feed, and other naturally deposited organicmaterial is the major source of H25 in fish ponds.The toxicity of H2S depends on temperature, pH, and dissolved oxygen.At pH values of five or below, most of the H2S is in its undissociated toxicform. As pH rises the H2S dissociates into S: and H'ions, which arenontoxic. At pH [] most of the H2S has dissociated to a nontoxic form. Itstoxicity increases at higher temperatures, but oxygen will convert it tonontoxic sulfate.HrS is toxic to fish at levels above 2.0 parts per billion and toxic to eggsat 12 parts per billion. It is a known cause of low fish survival in organicallyrich ponds. If the water is well oxygenated, H25 will not escape fromthe sediments unless the latter are disturbed, as they are during seiningoperations. Hydrogen sulfide mainly is a problem during warm months,when organic decomposition is rapid and bottom waters are low in dissolvedoxygen.Hydrogen sulfide problems can be corrected in several ways: (1) removeexcess organic matter from the pond; (2) raise the pH of the water (seeabove); (3) oxygenate the water; (4) add an oxidizing agent such as potassiumpermanganate.