Domestic ducks should be kept away from brood ponds, as they frequently become 

 versatile spawn and brood robbers. Wild ducks like to get food from the feeding places 

 in carp ponds. To prevent this, the feeding places can be protected by simple roofs 

 made of twigs. The goosander occasionally establishes itself on hibernation ponds and 

 can best be driven off by shotgun shells or by destroying the nests in breeding time. 

 Dwarf mergansers and intermediate mergansers are rare in Germany. Fish losses from ice 

 divers, arctic puffins. North Sea divers, white storks, duck hawks, kites, sea eagles, 

 and sea swallows (terns) can seldom be complained of. Almost all fish eating birds, 

 besides their activities as fish robbers, become in.lurious to a considerable extent as 

 carriers of parasitic worms. The strap woim ( Ligvila simplicissima ) which especially 

 attacks tenches and crucian carps among the pond fishes, seldom occurs in regulated 

 management of pond fisheries. 



Of the mammalia, foxes, pole cats, and house cats are opportunity robbers. The 

 fish otter, being an exclusive fish eater, must on the other hand be caught where he is 

 found, by spring traps set up under water. The water shrew also causes considerable 

 damage in trout ponds and hibernation ponds. 



The possibility of controlling the higher fish enemies is restricted in all countries 

 by various stipulations which are sub.lected to frequent momentary changes. The regulations 

 of the Fishery Law and the "Regulation for the Protection of Animal and Plant Species in 

 Prussia" of December 16, 1929, in Mitteilungen der Fischereivereine, Bd. 34, 1930, (Communi- 

 cations of the Fishery associations, vol. 34, 1930), must be observed in Prussia. According 

 to the latter all birds in Germany are protected with the exception of the tufted grebe, 

 the fish heron and others, also the fish eagle and others, which can be killed only by 

 licensed hunters in definite seasons (fish eagle from September 1st to February 28th). 

 Fishery pests can, according to this law, be trapped "on artificial fish ponds". 



In this connection, I shall suggest as a remedy against losses by human fish thieves, 

 that, in addition to watching over the ponds, thorny twigs should be place along the jsond 

 shores and stumps should be allowed to remain standing in the ponds. Eixcessive fishing 

 and fishing with nets is thereby prevented. The management of fish growing establishments 

 in the immediate vicinity of large cities is almost always impossible in spite of all 

 these remedies. 



If there is a suspicion that fishes have been poisoned by fish-berries ( Coc cuius 

 indicus ) at the hands of thieves (staggering fishes on the surface), it is recommended 

 to send in the bait or the poisoned fishes to the Agricultural Institute for Fisheries in 

 Berlin-Friedrichshagen, Muggelseedamm 310, for examination. In regard to Injuries to 

 fishes due to water pollutions, see Chapter I, E, 4. Fish mortality caused by explosions 

 is recognizable according to recent researches of Gennerich by the frequent presence of a 

 ruptured swim bladder and frequently of wounds. 



Chapter XV 



TJISEASES OF PONn FISH AND OF THEIR BROOD 



A. Symptoms, distribution and importance of diseases. 



Direction for forwarding diseased fish to laboratories . 



Mortality among fish is not always caused by sane kind of fish disease. Quite often 

 it is caused by various kinds of deteriorations of the water. If such is the case, it is 

 easily detected. If the water, that is, its purity has become impaired, the mortality 

 will in most cases, occur quite suddenly and fish of all sizes and species will be effected. 

 Mortality, caused by some sort of a disease will afflict only some of the fish, oftwn 

 only a certain variety or fish of a certain size. In case of disease, this also is 

 recogiized by certain symptoms, such as unusual movements (continuous turning around, for 

 instance) or by a general apathy. Also, a sick fish will isolate itself from the rest and 

 in other cases — ^while the fish is still alive — such a fish may be seen lying upon its back 

 or sides. Bodily changes may become noticeable, such as discoloration, or a film-like 



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