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BULLETIN OF THE BUREAU OF FISHERIES. 



GENERAL SUMMARY AND SUGGESTIONS. 



1. The larvse of three beetle genera — Dytiscus, Cybister, and Hydrous — are 

 positively known to destroy small fish under normal conditions. Ordinarily they 

 are kept in check by their enemies and do not become a serious menace to fish 

 culture, but sometimes under specially favorable conditions they breed in sufficient 

 numbers to kill the newly hatched fish fry about as fast as they appear. The 

 adults of these genera have also been known to kill young fish under somewhat 

 abnormal conditions, but even then they are much less destructive than the larvae. 

 Adult beetles belonging to each of these genera are present in most of the Fairport 

 fishponds but have never been observed attacking fish fry, although they have had 

 repeated opportunities. 



The larva of another genus — the whirligig beetle Dineutes — has been known 

 to kill fish fry under abnormal conditions. The larva of the scavenger beetle 

 Hydrophilus has been suspected of being capable of committing similar depreda- 

 tions, but there is as yet no actual proof. All the evidence at hand seems to indicate 

 that both these larvae are usually perfectly harmless. There has never been even 

 a suspicion against the adults of either genus. These are the only beetle genera 

 capable of injuring fish fry; all the others are too small. Consequently, the problem 

 is reduced practically to guarding against these genera; the others are negligible. 



2. On the other hand, beetle larvae and adults are eaten freely by the young of 

 nearly all our food and game fishes after the latter have reached a length of 25 to 

 40 mm. As the fish increase in size they eat the larvae of the five genera mentioned 

 above, even after these larvae have become full grown. Many fishes continue to 

 eat beetle food as long as they live. All beetle larvae that live in the water, there- 

 fore, and the adults of the numerous smaller species constitute a very important 

 item of fish food. 



3. It is only young fish of the year that are seriously menaced by these beetles. 

 Fish a year old have acquired sufficient ability to escape the attacks of the beetles 

 and in their turn have begun to eat freely of beetle larvae and adults. Hence, 

 beetles can do very little harm in a pond that is stocked with fish a year old or over; 

 on the contrary they contribute materially to the available food supply. Such a 

 pond, therefore, will not require watching as far as the beetles are concerned. If 

 the fish are breeding and young fry appear, however, then the pond must be 

 guarded to prevent the dangerous beetle species from becoming too numerous or 

 too ravenous. 



4. In case any of the beetles here mentioned should become obnoxious in a 

 breeding pond there are two possible methods of dealing with them. Either they 

 may be gotten rid of or the fish fry may be removed to another pond, and thus be 

 freed from them. The former would seem to be the easier method, but there are 

 strong objections to it and many obstacles that are hard to overcome. In the 

 first place, it is very hard to weed out the larger dangerous species and leave the 

 smaller harmless ones. Any means used to exterminate one species will operate 

 also against all the others. There will thus be destroyed a considerable amount 

 of excellent fish food which might otherwise have been utilized. There are only 

 three beetle species, one for each of the three genera, Dytiscus, Cybister, and 

 Hydrous, in the Fairport fishponds that are really dangerous, while there are 50 



