204 



BULLETIN OF THE BUREAU OF FISHERIES. 



mostraca of various kinds, with a good percentage of snails. Many of these nymphs 

 also, like those of Brythemis, had eaten large quantities of algae, and two of the larger 

 n5^phs, Argia moesta putrida, had shown cannibalistic tendencies and had eaten 

 smaller n5miphs. 



BENEFITS OF Diet to Fish Breeding. — A very respectable portion of the nymph 

 food consists of the adults and larvae of insects and Crustacea that are known to be 

 injurious to fish fry. Here belong the larvae of the diving beetle Dytiscus, the adult 

 water boatman Corixa, and the crayfish. The Dytiscus larvae are known by all fish- 

 culturists to prey upon small fish, and they have been repeatedly observed doing this in 

 these ponds at Fairport, whenever the water is drawn out of them for any purpose. 

 Corixa and the closely related genus Notonecta have proved to be serious pests in 

 European fishponds, killing so many of the young fry that they have to be exterminated 

 before the culture can go on successfully (Benecke, 1886, p. 340). 



Crayfish not only prey upon young fish but are also a great nuisance in a fishpond 

 because of their burrowing habits, and therefore anything which diminishes their 

 numbers must be looked upon as beneficial. 



Curiously enough the entomostracari genus, Cypris, which is eaten in large numbers 

 by the nymphs of L. luctuosa and the various damselflies, is sometimes less innocent 

 than it may appear. In a report on fish-cultural operations at Beaune, France, M. 

 Chabot-Karlen (1889, p. 310), said: 



The rearing of Daphnia pulex and Cypris fusca was also tried [to serve as fish food]. * * * Xhe 

 Cypris, however, were found to prey upon the young fish. Having been put in with the embryos of 

 the carp, they were often discovered to the number of two or three fixed upon the back of an alevin 

 devouring it, notwithstanding the efforts of the poor animal to shake itself free. 



Claims of Injuries of Diet to Fish Breeding. — Much of the food eaten by these 

 nymphs is the same as that of small fish, and it is often claimed that they thus diminish 

 the quantity of food available for the fish fry. But the young fish increase rapidly in 

 size, and if they are to be reared successfully larger prey must be provided for them as 

 they grow older. These odonate nymphs furnish such larger prey and are apparently 

 quite acceptable to the fish. (See p. 225.) 



Hence if the above argument against the nymphs is to prevail, it must be proved 

 that their value as food for the larger fish plus their value as destroyers of certain enemies 

 of the smaller fish does not recompense for the food they themselves consume plus the 

 few fish fry that the largest species may destroy. This leads very naturally to a dis- 

 cussion of the last statement, which is worthy of separate consideration. 



Nymphs as Fish Eaters. — There seems to be a prevailing opinion among fish- 

 culturists that dragonfly nymphs are very destructive to young fish, but when we 

 examine the testimony upon which this opinion is founded it does not prove to be very 

 satisfactory. 



Two insects were sent to Prof. C. V. Riley for identification in October, 1884, as 

 recorded in Insect Life, volume i, 1888, page 58. They came from W. L. Jones, of 

 Atlanta, Ga., who stated that the larger one, identified as a nymph of Anax junius, was 

 sent to him by a gentleman who stated that " it fastens on the carp fish and finally kills 

 it." Accepting all this as true, we must acknowledge that the evidence is rather 

 indirect and roundabout. No details are given; we are told nothing as to how or when 

 or where; and it is not even stated whether the fish was eaten after being killed, or 

 whether more than one fish was destroyed in this manner. 



