250 



BULLETIl^ OF THE BUKEAU OF FISHERIES. 



the pond begin to lower the fingerlings swarmed to the surface near the outlet. At 

 the same time numbers of these water tigers appeared and attacked the fish from 

 all sides. The dytiscid larva seized the fish by its throat and plunged its powerful 

 mandibles into the flesh near the heart. After sucking the blood a moment it 

 dropped the fish and attacked another, until the destruction became enormous. 

 Similar attacks have been witnessed in the fishponds at Fairport, Iowa, but the 

 number of fish killed was much less, owing to the smaller number of dytiscid larvae 

 present. 



On May 18, 1916, 4,500 fry of Ictiohus cyprinella, the common buffalofish, were 

 placed in pond 14B, from which all the larger fish had been previously removed. 

 From the time these fry were placed in the pond repeated search for them failed 

 to reveal their presence. On July 12 the pond was drawn, but no buffalo fry were 

 found. Many dytiscid larvae, however, appeared, and it was felt that they, with 

 other enemies, might have contributed to the disappearance of the fish fry. No 

 positive proof of this could be obtained, and since the dytiscid larva does not chew 

 its prey but merely sucks its juices, it would seem as if there should have been some 

 of the dead fry floating in the pond if the dytiscid larvae were really responsible for 

 their disappearance. 



One of these attacks of a water tiger upon a small fish has been made 

 the subject of a movie film. While this particular attack was undoubtedly 

 staged for effect, it does reveal the voracity and tigerlike nature of the larva. 

 There is no objection to artificial conditions when the reality has been repeatedly 

 observed under perfectly natural conditions. Fortunately the larvae of other 

 dytiscid genera are too small to function as enemies of fish. None of them, so 

 far as known, have been reported as attacking young fish, and it is probable that 

 ordinarily they are satisfied with other food. 



Of the Hydrophilidae the larvae of various species of the genus Hydrous have 

 been captured while eating fish both in Europe and in America. The following 

 is another instance to be added to a list that is already rather long. Ten thousand 

 small fry of the buffalofish {Ictiohus cyprinella) were placed in pond 3E, June 1, 1918. 

 During the month many Hydrous larvae appeared in the pond and the buffalo fry 

 began to disappear. By watching the pond closely the beetle larvae were actually 

 captured while eating fish on July 3. The pond was then drained and the fry 

 removed and about 150 full-grown Hydrous larvse were picked up, while many 

 more larvae and pupae were obtained from pupal chambers around the pond shores. 

 In this instance, therefore, it was actually proved that the beetle larvae were respon- 

 sible for the disappearance of some of the fish. Of the remaining genera of the 

 Hydrophilidae, Hydrophilus is about the only one whose larvae would be large 

 enough to become a menace to small fish. It has not been reported as actually seen 

 committing this depredation, but it might do so xmder stress of hunger. 



Of the Gyrinidae the larvae of Dineutes americanus have been seen eating small 

 fish. In the early summer of 1916 the channel catfish {Ictalurus punctatus) in 

 pond 9D for the first time spawned and hatched a brood of young. During the 

 subsequent lowering of the pond in order to secure and remove these fry, George 

 Lay observed large numbers of Dineutes larvae attacking the young fish. Several 



