DRAGONFLIES AND DAMSEI.I?UES IN PONDFISH CUIvTURE. 



229 



The number of fish included in this table is large enough to give considerable weight 

 to the conclusions drawn; a thousand fish stomachs ought to furnish a fairly reliable 

 basis for judgment. Furthermore, the fish have been taken from the ponds during 

 every month in the year except December, January, and February, and thus include 

 as much of the yearly life cycle as is available. 



Very small fish do not eat ©donate nymphs. Of the largemouth black bass from 

 pond 3 only one, and that the largest of them all, had taken a damselfly nymph ; eight 

 specimens of bluegills, averaging less than 10 mm. in length, from ponds i and 2, did 

 not show any trace of ©donate food; and the five channel cats that averaged only 9 mm. 

 in length from pond 9 had eaten nothing as large as an odonate nymph. 



While these small fish refuse the nymphs, they do not refuse the eggs which the 

 dragonflies distribute so freely about the ponds. Of the 22 buffalofish from pond 7D 

 only I had eaten dragonfly eggs. But among 59 sunfish from pond 15B, 10 were found 

 to have eaten dragonfly eggs; these 10 fish averaged but 13 mm. in length, the shortest 

 being 10 mm. long and the longest 18 mm. Odonate eggs formed 55 per cent (average) 

 of the food of these 10 fish and reached as high as 98 per cent in one of them. Hence 

 for some kinds of fish odonate eggs will furnish an acceptable food, while the young 

 fry are from 10 to 20 mm. in length. 



Judging from the records of the largemouth bass, the bluegill, the common sunfish, 

 and the calico bass, young fish must attain a length of from 22 to 25 mm. before they 

 begin to eat odonate nymphs. From 25 to 40 mm. they take them in comparatively 

 small numbers; from 40 to 100 mm. they eat them in much larger quantities, and 

 often eat nothing else. In every instance where the odonate food constituted 100 

 per cent the fish was over 40 mm. in length and nine-tenths of them were over 50 mm. 

 From 50 to 80 mm., therefore, may be taken as the size of fish for which odonate nytnphs 

 will prove most serviceable as food. 



The stomach contents of a largemouth black bass, 80 mm. long, examined July 

 30, 1 91 7, consisted of three fully grown nymphs of L. Ivxtuosa, one fully grown nymph of 

 Erythemis, and the remainder of the food fragments of similar nymphs too far digested 

 for identification. 



The small percentage of odonate food in the stomachs of the common sunfish 

 and the calico bass may well be due to the small size of the fish examined. Of the 173 

 common sunfish included in the table, 96 were under 25 mm. in length and only 32 were 

 30 mm. or over in length, so that really the 10 fish which ate odonate nymphs were 

 10 out of 32 rather than 10 out of 173. Similarly, among the 143 calico bass there were 

 only 21 that reached 35 mm. in length, while 100 were between 26 and 34 mm., just the 

 size when they begin to eat sparingly of nymphs. The calico bass does not seem to 

 begin this -diet quite as early in life as some of the other fish, although the table of the 

 Madison fish shows that the adults eat damselfly nymphs in goodly numbers. If we 

 remove from the lists here given all the fish 25 mm. in length or under — ^namely, those 

 that were too small to be expected to eat odonate food — it can readily be seen that the 

 percentage of odonate feeders would be considerably increased. 



We may next consider the fish's diet from a seasonal standpoint — the examination 

 of fish food began in the latter part of June and continued through the summer and 

 fall. Both the fish and the nymphs were steadily increasing in size during the period. 

 146954°— 20 4 



