so that they will not be split apart by penetrating and freezing water at these places. 

 Inflow ditches and shunting ditches are of course to be cleaned regularly, the shut-off 

 arrangements must be cleansed and naintained in condition. 



All trees and shrubs whose settlement upon pond dams has not been prevented, must 

 furthermore be kept away. The south side in particular must remain free of all shade 

 giving trees. Every unnecessary shading of pond dams is bad, unless it is concerned 

 with maintaining coolness in trout ponds by shading. Yu'lth carp spawnins^ and nursing 

 ponds, trees are often necessary for protection against wind but they must not shade 

 the ponds. It has been previously shown that light and heat form the foundation of all 

 production and that these factors are not available in excessive strength in Central 

 Europe, 



C. Aeration of the Water for Qx:/-gen Enrichment . 



Oxygen enrichment in the water is necessary mainly in trout fisheries, where a high 

 oxygen content in the water must be continually promoted and where water in an almost 

 oxygen free state has just left the spring and must at once flow into the ponds. It is 

 also an advantage to introduce an aeration device when the water is to flow through a 

 series of several ponds, IVhere there is a high content of iron it can be precipitated 

 out by aeration and in some cases by simultaneous liming. In accordance with natural 

 conditions the oxygen enrichment should take place before the water enters the pond. 



The aeration may be practically accomplished in three ways. The most natural way 

 is to plant the water inlet with overwater plants, such as water cress, swamp cress, 

 bitter cress, brooklime ( Veronica beccabunga), water-speedwell ( Veronica anagallis ) , 

 water mints ( Mentha aquatica ), etc. The plants divide the streaming water and bring it 

 into better contact with air and at the same time considerably enrich its oxygen content 

 by their own production of oxygen. In deeper inlets it is very advisable to have an 

 abundant growth of oxygen -producing under-water plants. 



Secondly, an enlargement of the contact surface between air and water, which serves 

 for better oxygen absorption from the air, may be produced by adding turns, or if possible, 

 water falls (masoned steps, etc.) in the water inlet. Thirdly, water dispersing water 

 v/heels may be installed under the inflows in the pond itself. (These wheels may be 

 gotten from Poetzschke, Pond Estate Brake at Iserlohn in T>'estphalia. ) Similar results are 

 achieved by building in horizontal sieves or boards under the inflov;. They likewise 

 disperse the water and provide intimate agitation of water cind air. These arrangements 

 also prevent trout from leaping into the inlets. 



Serving indirectly the oxygen enrichment in the carp pond, are all measures which 

 decrease the amounts of putrefiable organic substance on the pond bottom or in the pond 

 water (bottom-cultivation, liming), which provide better illumination (elimination of 

 excessive plant growth and of trees, or which provide better growth of green submerged 

 plants (fertilization). Several methods for the increasing of the oxygen content, which 

 are concerned vdth hibernation onl;/, are discussed in Chapter X. 



D. P.emoval of Undersirable and Excessive Plant Srovd:.h in the Pond . 



The removal of production-biologically injurious plant growth in the pond includes 

 both submerged weeds and above surface plants. The latter group includes not only reeds 

 but also the total above-water vegetation or "hardy flora". 



The positive and negative significance of the water plants for the metabolic cycle 

 in the pond, has been already discussed. The complete or partial removal of the plants 

 has the purpose of permitting the least possible reaction of negative factors in favor 

 of production biological positive values. The conquest of plants therefore represents 

 interference in the result of the life processes in the pond. It will clear the way 

 for the cr,'cle of the substances v;hich will lead to the highest possible production of 

 fish animal-food. At the same time the removal, of excessive plant growth wherever it 



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