The practical success of artificial lake enrichment demands a consis- 

 tent increased yield of desirable fish to angler. Assuming that productive 

 capacity of a lake can be enhanced by fertilization, it is necessary to 

 examine the possible effects of f ertilizers on the principal objectiveo 

 Some concepts of increased productivity, based on hypotheses in Province of 

 Quebec (19hQ) s which do not insure practical success are — 



lo A general increase in the abundance of plants and animals to the 

 extent of complete eutrophi cation, and resulting destruction of 

 the fish populationo 



2, A decrease in the population of desirable fish owing to overcrowd- 

 ing by excessive growth of submerged flora, 



3, A decrease in total population of desirable fish by the gain of a 

 few large cannibalistic individuals, or by increase in the popula- 

 tion of coarse fish, 



iio An increase in population of desirable fish with individuals too 

 small to be utilized. 



5. An increase in population of desirable fish with individuals of 

 usable size^, but a lower yield to anglers caused by excessive 

 natural food or difficulty in angling (surface blooms, littoral 

 vegetation) » 



Other concepts could be added to this list but it is intended solely 

 to indicate effects which deserve consideration in the planning of a lake- 

 fertilization programj and in the interpretation of results, A hypothetical 

 situation demonstrates the flaws in some conclusive reasonings Suppose that 

 a lake containing a population of undersized fish was fertilized to increase 

 the size of its fish. Due consideration was given to several chemical and 

 biological factors. Fertilizers were selected and applied according to 

 recommended procedure. Post-treatment determinations showed improved chemi- 

 cal conditions and a general increase in plant life, nannoplankton^ benthos 

 fauna, or other indexes used in the study. Fish growth was carefully 

 watched but. alas, there was no change. Although the number of fish may 

 have increased owing to conditions which allowed a greater population 

 density, the investigator erroneously concluded (on the strength of his 

 data) that no benefit to fish resulted from the addition of nutidents , 



A lake is a sizable and complex environment, often reacting indepen- 

 dently or adversely to efforts in its management. The fertilization of a 

 lake poses an impressive problemo Cost of enrichment, in terms of net 

 fish yield, may be prohibitive; this is especially true of deep oligotrphic 

 lakes. Experience gained from pond fertilization shows that a single ap- 

 plication will not keep productivity high indefinitely. Rather, fertili- 

 zation requires the renewal of nutrients at least annually, especially if 

 the crop is regularly removed^ What, therefore, is the basis for the arti- 

 ficial enrichment of a lake? The stimulus must be either an explicit 



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