10. Organic matter is necessary for a high productive capacity. 

 It occurs in solution^ in suspension, and deposited in the 

 bottom. These complex compounds result from excrax,ory proces- 

 ses and decomposition of plant or animal matter. They are the 

 source of energy for bacteria and become transformed into usable 

 nutrients by the saprophytic action of such organism,s. The ratio 

 of carbon to nitrogen has been related to levels of bacterial 

 activity, 



11. The addition of nutrients to directly increase production is the 

 common approach to aquatic fertilization. It concerns the selec= 

 tion and application of mineral or organic fertilizers to best 

 suit environmental needs ^ in order to attain highest productivity, 

 Availabilityj cost^ amount, mode of action, method, and period- 

 icity of application of fertilizers are factors to consider, A 

 second approach considers nutrient limitations due to lonavail- 

 ability. This theory operates in the fish=cuJ.tural practices of 

 draining and liming_, which mobilize nutrient md. erials held by 

 chanical retention in reduced bottom soils or inactivation due to 

 adverse environmental conditions. As a direct approach^ nutrient 

 liberation exists only in theory, 



12. Fertilizers cause a general increase in water productivity, and 

 various indexes (both biological and chemical) have been applied 

 to measure their effects. Such measurements are subject to the 

 vagaries of man and nature^ and must be viewed critically. Enrich= 

 ment may not aid the desired end of fish yield because of undesir- 

 able changes (lowered pH, toxicity, oxygen depletion) or diversion 

 of nutrient matter into nuisance animal and plant growths, 



13. Factors affecting fish yield, such as suTxlight_, temperature, rate 

 of water exchange, fish populations, and plant- growths, are con- 

 trollable to a certain extent in a culture pond. Control plus 

 optimum dimensions result in high productive capacity and render 

 such ponds very profitable for fertilization. Cultural fertili- 

 zation is said to be more economical than artificial feeding, the 

 natural nutrition enabling greater population density and result- 

 ing in hardier fish. Recommendations as to type and quality of 

 fertilizer for general enrichment can be found in the literaturej 

 but do not apply in all instances. Nutrients are added before or 

 during the growing season and must be renewed at least annually 



to svistain yields, 



111. Draining, liming, and proper stocking should be considered in 

 order to realize the greatest productive effects of fertiliza- 

 tion. Draining facilitates crop removal and control of undesir- 

 able plants or animals. Winter fallow oxidizes bottom soils, 

 thereby activating nutrient substances o Liming, strongly recom- 

 mended by European culturLsts, kills disease instigatorsj raises 



29 



