As just mentioned, the oxygen content must not sink below.^ to 5 milligrams per 

 liter. Tiiis caaiot be achieved by knockirig out holes. The area of these quickly freez- 

 ing holes — however nunerous they may be is much too smeill to permit sufficient penetration 

 of oxygen into the water, if there is not a simultaneous intermixing (Neuhaus). On the 

 contrarjr, the fishes are aroused from their hibernation by the strokes, swim about active- 

 ly and requii'e especially large quantities of oxj';',en, and come to the ice where they are 

 frozen fast. Ice holes should at least be sav;ed. I frequently observed, that fishes 

 perished in ponds in which holes had been ha.-nnered through the ice, and remained alive 

 in similar neighboring ponds in wliich no holes had been hamm£rsd. For the same reasons, 

 ice skating and the obtainijir; of ice on hibernation ponds must be absolutely forbidden. 

 It is very significant, that especially at the beginning of spring, when the first sun 

 ra^-s stir the carps and tenches out of "nibemation, fish mortality still occurs. At 

 this time the oxygen content tends to be at the lovrest, due to oxygen-consuming processes 

 of decay and because the prolonged ice coverage has excluded the air from the water. 



An often very effective means for enriching the v;ater with oxygen is, in contrast 

 to knocking out holes, the sweeping away of snow fron the ice surface. The light then 

 penetrates better into the water and the algae and plants, present in small amounts even 

 in vdnter, can produce ox^'-gen. In the greatest emergency, artificial oxygen can be care- 

 fully introduced in the deepest places in the pond. It has also been tried, to pump 

 water from beneath the ice, aerate it in a channel, and again let it flov; into the pond* 

 The success herewith is largely dependent on the degree of mixture of the in-running 

 pump v;ater ivith the pond water. The setting up of strongly rumbling pumps upon the ice 

 should be well considered, because the fishes become disquieted thereby, A pond fishery 

 in Saxony, v^hose feeder is the Spree River which becomes strongly polluted in the winter, 

 pumps the water of the hibernation pond into higher placed purchased reservoirs, from 

 which after thorough aeration it flows back into the pond through several open 100 meter 

 (32S ft.) concrete channels. In this way only little water needs to be taken from the 

 Spree, 



It is probably self evident that fishes which are closely crowded in hibernation, 

 must be completely healthy. Host illnesses of i/t-, ole pond fish stocks may be caused in 

 hibernation. Fishes which are suspected of disease must therefore be hibernated separ- 

 ately from healthy fishes. Also a few large parasites (fish- leeches, carp lice, see 

 Chapter XV, E, 2) which do not produce injurious effects in the summer, can lead to 

 emaciation and to secondary illness by continuously disquieting the hibernating fishes. 

 Injuries caused by fishing out, make the fishes in hibernation very susceptible. They 

 are also often the cause for later illness. 



The nutritional condition of the liibemating fishes is therefore very important, 

 because it likewise determines the degree of resistance power. All fishes must go into 

 hibernation in a well nourished condition if they are to endure the Avinter well. To 

 prevent emaciation the v/ater must not be too warm nor the through current too strong 

 during the hibernation, as has already been mentioned. In warm winters, according to 

 experience, the weight loss in the hibernation pond is the greatest. Regarding the 

 height of the weight loss, see Chapter I, C. Since small fishes emaciate particularly 

 rapidly (see Chapter I, B), yearling carps, and yearling and two year tenches are to be 

 fed until the beginning- of frosty v/eather, inasmuch as they are not hibernated in large 

 ponds. Wilier in 1926, confirmed this experience by an experiment. In water temper- 

 atures of L to 8°C, yearling carps still devoured lupine groats very eagerly. At 

 temperatures lower than /V°C, the desire to eat subsided somewhat, but stopped only 

 close to the freezing point (see also Chapter 1, B). The losses of these yearling 

 carps weighing only 3 to 5 grams and fed in the hibernation pond amounted to onl;^ 10 

 percent at the end of the v.lnter, while in other fisheries T/here no feeding was done, at 

 least 20 percent of yearling carps were lost in the same winter. According to this 

 experiment also, the feeding of yearling carps can be carried out in the hibematicsi 

 pond, and is to be recommended. 



iloreover the fishes also find some natural nutrition later in the hibernation 

 pond. In the beginning of spring this natural nutrition and also the artificial feeding 

 to the starting of frost are apparently of great significance in the prevention of the 

 contagious ascites disease (see Chapter X7, D, 2), which starts with an intestinal 

 inflammation and is one of the v/orst epidemics of pond fishes (Schaeperclaus, 1930). 



