HATCHERY OPERATIONS 105 



may be damaged by activity of the chemical. Avoid overdoses and spil- 

 lages. Avoid use near sensitive crops and reduce drift hazards as much as 

 possible; do not apply herbicides on windy days. Clean all application 

 equipment in areas where the rinsing solutions will not contaminate other 

 areas or streams. 



Fish culturists must also be aware of the current registration status of 

 herbicides. Continuing changes in the regulation of pesticide and drug use 

 in the United States has created confusion concerning what chemicals may 

 be used in fisheries work. Table 14 lists those chemicals that presently pos- 

 sess registered status for use in the presence of food fish only, a food fish 

 being defined as one normally consumed by humans. 



Special Problems in Pond Culture 



DISSOLVED OXYGEN 



Because adequate amounts of dissolved oxygen are critical for good fish 

 growth and survival, this gas is of major concern to fish culturists (Figure 

 39). On rare occasions, high levels of oxygen supersaturation — caused by 

 intensive algal photosynthesis — may induce emphysema in fish. Virtually 

 all oxygen- related problems, however, are caused by gas concentrations 

 that are too low. 



Tolerances of fish to low dissolved oxygen concentrations vary among 

 species. In general, fish do well at concentrations above 4 parts per million. 

 They can survive extended periods (days) at 3 parts per million, but do not 

 grow well. Most fish can tolerate 1-2 parts per million for a few hours, but 

 will die if concentrations are prolonged at this level or drop even lower. 



In ponds that have no flowing freshwater supply, oxygen comes from 

 only two sources: diffusion from the air; and photosynthesis. Oxygen dif- 

 fuses across the water surface into or out of the pond, depending on 

 whether the water is subsaturated or supersaturated with the gas. Once 

 oxygen enters the surface film of water, it diffuses only slowly through the 

 rest of the water mass. Only if surface water is mechanically mixed with 

 the rest of the pond — by wind, pumps, or outboard motors — will diffused 

 oxygen help to aerate the whole pond. 



During warmer months of the year when fish grow well, photosynthesis 

 is the most important source of pond oxygen. Some photosynthetic oxygen 

 comes from rooted aquatic plants, but most of it typically comes from phy- 

 toplankton. Photosynthesis requires light; more occurs on bright days than 

 on cloudy ones. The water depth at which photosynthesis can occur 

 depends on water clarity. Excessive clay turbidity or dense blooms of phy- 

 toplankton can restrict oxygen production to the upper foot or less of wa- 

 ter. Generally, photosynthesis will produce adequate amounts of oxygen for 



