covering of the skin, or blisters, or swellings, or spottings, etc. Inflation of the 

 belly, a great accuraulation of parasites, etc, niay be apparent, and in cases of severe 

 affliction the eye revolving reflex nay be absent. 



A really clean-cut distinction betv:een general mortality caused by v/ater conditions, 

 and mortality due to disease is not always possible. I have found fron experimental 

 observations that v/ater deterioration may cause tj-pical diseases of certain varieties of 

 fish, lasting for da^^s, sometimes for viseks, leading eventually to either death or 

 recovery. Such an occurrence may take place, for instance with the sudden appearance of 

 free chlorine (up to 1 nilligram per liter) in the water and wi.ich will lo^/'er the pH 

 value to about 5. (Ebeling and SchrHder, 1929, Sctiaeperclaus, 1926). 



As director of the division for Fish Diseases of the Prussian Institute for Fishery 

 in Berlin-Friedrichshaj^en, I have repeatedly found that fish diseases occur especially 

 frequently in pond operations. Therefore fish diseases are of the highest practical 

 significance for everj' pond manager. But, it is also known that the fish diseases have 

 quite general and specially broad and favorable possibilities of origin and dissemi- 

 nation. The reasons for this are: 



(1) Extraordinarily good possibility of dissemination of fish-parasites and 

 disease instigators in the water (in contrast to air which offers much 

 greater obstacles to the parasites of land animals in transferring from 

 one host to another). 



(2) Possibility of chemical alterations in the living medium, the water (very 

 rarely given in the air), which alterations alone can become causes of 

 disease, and can very frequently become the "cause for aggravation of 

 disease", by unfavorably influencing the course of the disease. Also, 

 the necessary handling in the catching gives with fishes a "cause for 

 aggravation of disease", vmich is- not the case to the same degree with 

 other animal species. 



The special reasons for the strong dissemination of fish diseases in pond fisheries 

 are: 



(1) The enlargement of the fish stock density in ponds, which enters with 

 intensification of operation, and which favors the spread of the disease, 



(2) The living conditions becoming poorer due to the intensification 

 (especially of the frequent fishing out, hibernation in small ponds, of 

 feeding, etc.). 



(3) The increasing danger, when graving single or fewer kinds of fishes, of 

 the beginning and rapid spreading of special diseases peculiar to a 

 species. 



Also in horticulture it is well known that pure stock plants are in greater danger 

 from parasites than are the mixed stocks of the meadov;. 



The pond manager can only gradually accumulate sufficient experience for the recog- 

 nition and control of frequently recurring fish diseases. The purpose of the following 

 compilation of the most frequent and economically important diseases of the pond fishes 

 can therel'ore only be to support the pond manager in this direction. 



In every case, in T*hich a fishbreeder is not absolutely sure of the kind of a 

 disease present in hie ctock, or is at a loss of how to best ccmbat it under existing 

 conditions, the consultation and advice of an expert or specialist becomes imperative, 



':he large fishery institutes, like the National Institute for Fishery in Berlin- 

 Friedrichshagen, Lluggelseeda-nm 310, and the Siological Experimental Institute for 

 Fishery in Uunich nre especially qualified to advise and aid in new kinds of and 



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