that the degree of illness through D. vastator depends in the most part upon the general 

 conditions of environment and nutrition for the brood, in short, upon the resistance 

 power of the brood. The mass mortality toward the end of the nursery period are ex- 

 plained simply frcM a shortage of nutrition, caused in part by the flight period of 

 Chironomus flies or midges (see Chapter IV,B,2). The disease becomes dangerous almost 

 exclusively only to carp brood of 2 to 6 cm. (0.79 to 2,37 inches) and occurs mostly at 

 the end of June and in July. There may be a few isolated hannless worms but also 

 several hundred deadly active worms parasitic on one small carp. The gill covers are 

 often visibly spread, the gill rims appear gray. D. anchoratus. Costia and other gill 

 parasites very frequently contribute to rapid aggravation of the disease. As Thunder 

 showed for the first time, the gill lamellae react to the dactylogyrus attack by form- 

 ing thin processes up to 1.8 mm. in length, '."under's assumption, that this causss the 

 disappearance of the parasites, seems doubtful to me, however. With carps of over 6 to 

 7 cm. (2.36 to 2.75 inches) length, D. vastator probably no longer pH^ys any practical 

 part as a disease instigator, as I have repeatedly observed. 



Based on these above mentioned conclusions, the follcuving measures for the control 

 of dactylogyrus are to be recommended: 



(1) Bathing of the spawners in a 2.5 percent solution of table salt 

 (see Chapter XV,E,1) before setting them in the spawning ponds and 

 the immediate removal after the egg laying. 



(2) Water feeding of the brood extension ponds from nonstocked waters, if 

 possible not out of carp ponds. Keeping out of invaders, which are 

 mostly parasite carriers, 



(3) Good care and correct stocking of tlie brood ponds, especially of the 

 carp brood nursery ponds (see Chapter IV,B,2), for the provision of 

 an excess of natural nutrition. The brood must have grown to a length 

 of at least 5 to 6 cm, (2.0 to 2.36 inches) within four weeks. Immedi- 

 ate transfer of the extended brood in case of food shortage. 



(a) The bathing of the brood in salt baths (see Chapter XV, El) is quite 

 useless, because it is hardly possible to carry out practically. If 

 bathing is done, the brood should be placed in the best ponds immediate- 

 ly thereafter, 



StrapYfoiTa attack and attack by several other parasitic worms . Although strapworm 

 larvae "(Ligula simplicissima ) occur mainly in fishes out of lakes, they are also not 

 exactly rare in larger fisheries vd.th tenches and crucian carps. They also occur occas- 

 iorially in carps. They live between the viscera of the abdominal cavity, retard the 

 grov.'th and make the fishes more or less unsalable. The principal hosts of the strap- 

 worms are fish-eating water birds, Within two days the worms become sex ripe in the 

 birds ' intestines . The ciliated larvae creep out of the eggs which are carried into 

 the water with the bird feces. The second larval stage lives in copepods, the third in 

 the abdnmi nal cavity of the fishes. The methods of control are the control of fish- 

 eating water birds and refraining from feeding sick fishes to water birds, 



A similar, but annulated strapworm ( Schlstocephalus dimorphus ) lives only in the 

 three-pointed stickleback. 



The second stage of the strapv/orm Triaenophorus , whose principal host is the pike, 

 has occasionally led to liver diseases with trout, the so-called cysted liver. 



The verj'^ frequent Acanthocephala (Echlnorhynchus and Keorhynchus) abo-at 0.5 to 3 an. 

 (0.2 to 1,2 inches) long, whose larvae live in the food animals of fishes, when present in 

 great abundance in the trout intestine can cause intestinal inflammations, 



?ish leech attack . The fish leeches, irtiich are about 4 to 5 cm. (1.57 to 2.0 inches) 

 long, are true blood sucking parasites. They live upon the skin of fishes and of batrach- 

 ians. Viith both terminal suction cups, they attach themselves so firmly to the fish that 



209 



