(5) In tranaitrtatlon of ilshes in heavily loaded tanks of relatively small 

 water surface, especially if this surface is not increased through 

 occasicmal moving about of the containers, and when the water is too 

 warm. 



As mentioned already, a sufficient oxygen content is also desirable from the viewpoint 

 of productivity . One of the most important problems — in time of danger — is to know what 

 ODcygen rate is critical for pond fish and what rate, if maintained for any length of time 

 will eventually become deadly to them. Only this knowledge allows to properly determine 

 the rational rate of oxygen. To generalize on the subject is impossible in practice — and 

 we call especial attention to i;^ — since the oxygen rate depends upon so many and constantly 

 changing factors . 



An Qocygen rate of from 3 to 3.5 mg per liter Is in general already disagreeable to 

 carp, they flee from it. At a rate of only 0,5 ng per liter, carp and also tench suffer 

 from excessive shortness of breath, which they try to alleviate through struggling for 

 air, an effort, which they cannot long survive. An oxygen rate of ftrcm 5 to 5«5 mg per 

 liter is critical — in summer — for salmon species. They begin to suffer from shortness of 

 breath at a rate of ^ mg per liter, A rate of 3 mg per liter does not suffice for any 

 length of time and when the rate drops to ft"om 1,5 to 2 mg per liter, they will die Trithln 

 a short time, A still lower rate is only tolerated by them for a very, very short time. 



For a sin^sle oxygen estimation one needs: 



(1) A bottle of uncolored glass of about 100 ccm capacity, with a properly 

 fitting glass stopper. The fish biologist uses accurately graduated 

 bottles, so-called "oxygen bottles", 



(2) A solution of manganous-chloride UnCl2 — (60 g in 100 can. aqua destillata), 

 in a dropping bottle - 8 drops ■ 0,5 cc, 



(3) Caustic soda solution — NaOH — (50 g. of caustic soda to 100 ccm. of aq. 

 dest.) in a bottle with rubber stopper (Caref\ill Strcmgly caustici) and 

 a graduated rubber-capped pipette marked to withdraw 0.5 cc. of fluid, 



Uanganous chloride solution and sodium hydroxide solution may be obtained in glass 

 tubule^ each containing 1/2 cc. of solution (measured and ready for use). The bottle is 

 filled so completely with the water to be tested, that upon Inserting the stopper, no air 

 bubble remains behind. Then by means of the pipette l/2 cc. of sodium hydroxide solution, 

 the 1/2 ccm. (8 drops) of manganous chloride solution are added. Both additions must be 

 made ctu-efully (if tubules are used, they must be dropped in with the openings on top), so 

 that the fluids sink to the bottom. The stopper is then replaced so that no bubble remains 

 xmder it. By thorough shaking a precipitate forms irtiich is white at firet, but immediately 

 changes to brown with the oxygen contained in the water, and of course, the more oxygen 

 present, the darker is the brown. If no oxygen is present, the precipitate remains white. 

 Ivory colored precipitate indicates at most 2 to 3 mg. of oxygen. A coffee broim precipitat* 

 indicates sufficient oxygen. It is best to make a compari-Bon test with a second aaaple of 

 known good water. The tests keep only a short time, 



22 rate . 



The natural reaction of &ny liquid, also pond water, may be either alkaline, acid or 

 neutral. Formerly, such tests were made with color indicators such as litmus, irtiich reacts 

 with a blue color to alkalines, with a violet color to neutral and with a vivid red color 

 to acid. Today, it is not only possible to determine the natural reactions, but to also 

 determine the degree of acidity, or of alkalinity at the same time. 



50 



