HATCHERY OPERATIONS 109 



vulnerable to many outside influences: a cloudy day that slows photosyn- 

 thesis; a hot still day that causes stratification; a miscalculated food ration 

 that is too large for fish to consume before it decomposes. 



Typically the summer oxygen content in a pond follows a 24- hour cycle: 



highest in the late afternoon after a day of photosynthesis; lowest at dawn 

 after a night of respiration. It is the nighttime oxygen depletion that is 



most critical to pond culturists. 



Pond managers can take several precautions to prevent, or reduce the 



severity of, dissolved oxygen problems. 



(1) Most ponds are fertilized to stimulate plankton production for natur- 

 al fish food. Suitable plankton densities allow secchi disk readings of 12-24 

 inches. Fertilization should be stopped if readings drop to 10 inches or less. 

 Special care should be taken if the pond is receiving supplemental fish 

 food, as this can stimulate sudden plankton blooms and subsequent die- 

 offs. 



(2) Because the frequency of dissolved oxygen problems increases with 

 the supplemental feeding rate, fish should not be given more than 30 

 pounds of food per acre per day. 



(3) If algicides are used to control plankton densities, they should be ap- 

 plied before, rather than during, a bloom. Otherwise, the accelerated die- 

 off of the bloom will worsen the rate of oxygen depletion. 



(4) During critical periods of the summer, the oxygen concentration 

 should be monitored. This is most easily accomplished at dusk and two or 

 three hours later. These two values can be plotted against time on a graph, 

 and the straight line extended to predict the dissolved oxygen at dawn. 

 This will allow emergency aeration to be prepared in advance. 



Dissolved oxygen problems may arise in spite of precautions. Corrective 

 measures for specific problems are suggested below. 



(1) If there has been an excessive kill of pond weeds or plankton that 

 are decaying, add 20'/o superphosphate by midmorning at a rate of 50-100 

 pounds per acre. Stir the pond with an outboard motor or otherwise mix or 

 circulate water to rapidly distribute phosphate and add atmospheric oxy- 

 gen; 1 to 2 hours of stirring a 1-acre pond should suffice. Dilute the 

 oxygen- deficient water with fresh water of about the same temperature. 

 Distribute, as evenly as possible, 100-200 pounds of hydrated lime, 

 Ca(OH)2, per acre in the late afternoon if CO2 levels are 10 parts per mil- 

 lion or higher. Then stir for another one to two hours. 



(2) Low dissolved oxygen may be caused by excessive rooted vegetation 

 and a lack of phytoplankton photosynthesis. If the pond is unstratified, add 

 P2O5 and stir or circulate as in (l) above. Add fresh water if available. If 

 the pond is stratified, which is the usual case in warm months, aerate the 

 surface waters by agitation, draw off the cool oxygen- deficient bottom wa- 

 ter, or add colder fresh water. 



