The first task is well known through the experience of all fishbreeders, that ponds 

 of heavy soil, rich in food stiiffs (eutrophic bottoms) yield larger crops of fish than 

 ponds of lighter, i.e., poorer soil (oligotrophic bottoms). Sour (acideous) bottoms do 

 not supply any nutritional matter but deprive the water of calcium and other substances. 



It is for these reasons, that ELn . K."\uaann favors a regional division of pcsid culture, 



since ponds in oligotrophic soil make different demands upon the fishbreeder — ^vdth regard 



to treatiiient and ijnprovemtnts — than ponds in eutrophic soil. I should like to say that the 

 facts have gone ahead of ids ideaa» 



In Germany, at least, tlie methods of chalking and of fertilizing (orgcinically and 

 inorganically), of stock raising, of hibernation of fishes, etc. vary in the different 

 regions of the land. Trout culture in Hannover is handled differently from trout culture 

 in the central mountain regions and in the regions of the lower alps. Of course, it must 

 not be overlooked that these important regional factors are not brought about by geological- 

 miJieralogical conditions alcHie. The economical environment also pla/s a role in those 

 matters. 



Kith regard to the influence of geological differences upon the natural productivity, 

 opinions still differ. According to some, the difference of the water under different 

 soil conditions is of preponderance, while according to others the conditions of the soil 

 itself is the most important factor. 



The same differences of opinions erist with regard to the matter, quoted above under 2. 

 Personally, I am of the opinion that th6 differences in viewpoints are partly caused 

 through insufficient studies in these matters, and partly through all too theoretical 

 reasoning. 



The colloid content of the soil, especially of the mud layer is undoubtedly a controll- 

 ing factor for productivity. Colloid containing soil fixes or chemically binds the nutrition- 

 al matter, created within the metabolic cycle, or the alimentary substances introduced from 

 the outside. Due to its action, they become gradually reintroduced into the cycle on calcium 

 precipitation. 



Some experiences, such as the high production rate of new ponds, lacking in mud layers, 

 seem to conti-adict the generally held opinions with regard to the importance of colloid 

 ecu tents. 



But on the other hand, it is a fact that old ponds with a good laj-er of foul mud are 

 more productive than equally old ponds without such layers. These mud la^ners quite often 

 not cxily owe their productivity to the high <'olloid contents of the soil, but a good mud, 

 formed from out of decayed matter (algae and fine detritus "gyttja" contains q\iite an amount 

 of precipitated chalk, aside from absorbed nutriments. 



In three neighboring ponds of a hatchery, where the inflow of water had an A.C.C. of 

 3.3 and a pH rate of 1*1, were found the figures, quoted in table 9* 



59 



