These two fundamental requirements overlap each other in many instances, so that it is 

 almost impossible to deal with them separately. It is for this reason that we will discxiss 

 here the requirements for the different factors. 



The pood-cultural most important properties of water ; depth . shore conditions, 

 movement of the water , in their r elations to the factors of llt;ht and warmth . 



The relatively high productivity of the pcnd is due — in greater part — to its shallow- 

 ness, which allows the penetration of light down to its very bottom and at the same time 

 facilitates the rapid warming up procees cf the whole mass of water. Based upon these 

 theoretical considerations, the depth of the water — in practice — has quite often been 

 calculated too lam, with resulting bad shore infringeoents and which led to costly upkeep. 

 BccessivB shallowness "of pcnds is even ic^sractical from the strictly productive-biological 

 viewpoint. 



The investigatiotiB of Ruttner on lakes have shown that the assimilative functions of 

 submerged water plants, i.e. their productivity does not lie directly under the surface of 

 the water but in a depth of from 0.3 to 1.00 and even 2.00 meters. These investigations 

 have also shown that the assimilative functicxis are regulated more strongly by the thermic 

 factor, in a depth of from 1 to 2 meters, than by the factor of light, i.e. during daylight 

 hours. 



k third reason for not choosing below 1 meter depth in a pond is the fact that temper- 

 ature changes are more pronounced in shallow waters, which of course reacts unfavorably 

 iyx)n productivity conditions. 



Also, the greater the contact surface between soil and water, the more nutritional 

 matter will be extracted from the soil, which means, of course, that the possibilities of 

 existence — for flora and fauna — are vastly increased. It also facilitates a better wamirg 

 up of the water. This explains the relatively higher productivity of smaller ponds and of 

 all ponds where the proportions between shore line and water surface, the so-called shore 

 quotient are especially great. 



I need not mention here again the productivity-increasing influence of light and warmth 

 and the productivity-lowering influence of shady shore trees, surface and submarine flora in 

 carp ponds. But I will mention here — that aside from phytoplankton — turbidity of the water 

 can also be caused through the rummaging feeding habits of carp through which the bottom 

 of ponds is often greatly stirred up. 



Motion of the water, especially in ccnbinaticxi with renewal of water is also favorable 

 to productivity, provided this rnotioi is not too strong nor of unfavorable influence upon 

 the temperature. In small trout ponda, a "through current" of Trater is absolutely necessary 

 for hy^enic reasons. Trout love a continuous "through current" of a certain force, but it 

 is not absolutely required where the stock is kept at a rational rate. 



On the other hand, such a "through current" may prove altogether Injurious in case of 

 certain diseases (Gyrodactilus). In large carp ponds, a too strong motion of the water may 

 be dangerous to the dama on account of wave action. 



The "through current" in trout ponds touches upon the space factor, which is very 

 important in trout culture. Miller has shown that the size of hatching baskets plays a 

 great role in the hatching process. Demoll has demonstrated that in the fattening process, 

 ponds of 12 by 2.2 meters show less results in growth and- worse utilization of food than is 

 the case in ponds of 28 by 5 meters (at 1 meter water depth). 



Now, the space factor is not something uniform (absolute); it must be divided again 

 into separate factors. li-ee-swimming fish are influenced by: 



(1) The total siae of a pond, the so-called "run-out factor" (Auslauffakton) as 

 Wilier calls it. If this factor becomes too small, it will restrict the 

 liberty of movement. In this respect we wish to call attention to the fact 



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