106 FISH HATCFIKRY MANAGEMENT 



TaBI.K 14. HERBICIDES REGISTERED HY IHE UNH KD S I'A 1 ES FOOD AND DRUG 

 AL. 1976; SNOW ET AL. U)64.) 



Low or moderate 



"Percentages are percent actual ingredient. 

 Consult product labels for limitations on use. 

 'Only for use by federal, state, and local public agencies. 



fish to a depth of two to three times the secchi disk visibility. Penetration 

 of oxygen below this depth depends on mechanical mixing. 



Two processes use up dissolved oxygen: chemical oxidation and respira- 

 tion. Both occur throughout the water column and in the top layer of pond 

 sediments. The first involves chiefly inorganic compounds and elements, 

 and rarely is of major significance in ponds. Respiration is the main cause 

 of oxygen depletion. All aquatic organisms respire — not only fish, but 

 plants, phytoplankton (even during photosynthesis), zooplankton, bottom 

 animals (such as crayfish), and perhaps most importantly, the bacteria that 

 live off nitrogenous and organic material. 



Over the whole year, but especially during the growing season, the oxy- 

 gen concentration in a pond is determined primarily by the balance of pho- 

 tosynthesis and respiration. For pond fish culture to succeed, photosyn- 

 thesis must stay ahead of respiration. Pond management techniques involve 

 manipulation of both components. 



Of all the physical variables that affect dissolved oxygen concentrations, 

 temperature is by far the most important. It has direct influences on the 

 oxygen balance: photosynthesis, respiration, and chemical oxidation all 

 proceed faster at higher temperatures. It has a direct influence on a pond's 



