Some aqiiatics choose the hard bottom floor— destitute of organic life — for their abode 

 and secure their existence by preying upon other ionas of aquatic life. Ephemera vulgata. 

 for instance, belongs in tliis class. 



The most important and most numerous life forms of the pond floor are the mud organisms , 

 wliich find their sustenance in the ooze of decayed organic detritus and viiich is rich in 

 nutriments for these species. 



This diversity of the enumerated biological coinmunities niakes the catabolic cycle some- 

 what compiex, but figrire 5 gives a sufficiently clear picture of the parallel course of each 

 of the lliks in the chain of the cycle, and which finally leads up to the fish. Only a few 

 less important details could not be included in the sketch. 



The animal plankton, for instance, exists partly upon vegetable plankton but also sus- 

 tains itself with dustlike detritus of many origins. 



The cycle chain can be further complicated by an intermediary link, namely by the 

 presence of predatory plankton forms (larvae of chiraiomida) for instance. 



A continuous fluctuation of gain and loss in insects and insect larvae is caused by 

 the falling of insects into the pond, by the laying of eggs of others into its waters, and 

 also by the pupation of larvae, some of vrtiich fly off like so many of the water insects 

 themselves . 



Reference to the understanding of the catabolic cycle of the pond will occur frequently 

 in later chapters. It fonas the basis for rational exploitation, wherever the problem arises 

 of exclusive or partial existence of fish upon natural foodstuffs. 



All iiieasuies taken with a view to increase production mean nothing more than interfer- 

 ence 7d.th the natural course of the catabolic cycle. 



In order to do this in a rational manner, it has to strengthen the v^eakest link in the 

 cycle chain "since this detenaines the strength of the whole chain" (Thisnemann, 1931). 



All links in this chain are dependent upon one another and the degree of development 

 of the living links depends again upon the existing average of external conditions of 

 existence. 



Vi'e have already tlioroughly discussed the natural feeding of pond fish. It is therefore 

 almost superfluous to emphasize again that the direct catabolic cycle chains of vegetation 

 and bottom fauna are of greater in^jortance than all others, especially these of the plankton. 



The uncommon course (of the cycle) over detritus is of importance insofar as it makes 

 detritus feeders independent from the course of development of plants. Although it is 

 believed by many that an interference with the growth of plants is thereby avoided, 1 am of 

 the opinion that this is of little importance. Thei-e are still plenty of plant feeders 

 left to interfere with their grovrth, and on the other hand, the recuperative power and 

 abilities of submarine flora — or of some of their parts — is really astounding. 



The development of plankton is at tiioes very irregular and without visible correlation 

 to the productivity in fish meat. 



It may be stated here, that the sum of vegetation and bottom organisms is the only- 

 reliable means for estimating the yield of fish meat. True, Pauly has found '^hat all 

 through the surmer months there may occur a certain parsllelisn betv/een abundance of 

 plankton and greater yield in fish meat. 



Just recently investigations made by me, and by others upon my request, at the hatcher- 

 ies at Eberswalde, and also 6ther experiences, are a confirmation of these views. 



All measures to increase productivity must strive, on the one hand to improve the 



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