SOME PLANTS OF IMPORTANCE IN PONDFISH 



CULTURE. 



By Emmbxink Mookk, Ph. D. 



Contribution fronf the U. S. Fisheries Biological Station, Fairport, Iowa. 



INTRODUCTION. 



It is a matter of importance in the production of pondfish to con- 

 trol the growth of aquatic vegetation. This can not profitably be 

 done until it is known what i)lants enter directly into the fish dietary 

 or contril)Ute indirectly to the support of the various animal forms 

 upon which fish feed. 



There is little precise knowledge of the natural food of the ad- 

 vanced fry and young fingerlings of our ponds and streams. Nearly 

 all of the examinations of the food content of fishes refer to the ad- 

 vanced fingerling stages or to adults. The importance from an 

 economic standpoint of securing information about the natural 

 forage of very young fish is seen at once. So far as we know, only 

 a small percentage of the fry reach maturity, and by more or less 

 vague explanations the failure has been referable to our lack of 

 knowledge of the food relations in their cnvironmeut. 



This investigation, conducted at the U. 8. Fisheries Biological 

 I-iaboratory, Fairport, Iowa, refers primarily to the aquatic vegeta- 

 tion in the food of fish which are reared in ponds and considers the 

 problem froui the following standpoints: What plants contribute 

 directly to the food of the advanced fry and fingerlings? What 

 plants contribute indirectly to their food by providing forage forthe 

 various animal forms upon which fish feed? The investigation 

 covers the period of the summer months during two seasons, June 

 20 to August 31, 1917 and 1918. 



The data presented illustrate the dependence of the young fish on 

 food, mostly animals, which in turn feed on plants. The determina- 

 tion of these plants, which are the basic source of the food suppl}' 

 in the ponds, forms the chief contribution of this paper. Numerous 

 examinations have been made of the food content of young fish, in 

 which the direct use of plants by them is revealed. The results have 

 been fornnilated into tables (p. 14) which supplement the data al- 

 ready at hand in the researches of Forbes (1880), Pearse (1918), 

 Reighard (1915), and others, and indicate plant values among the 

 flowering plants, the filamentous algaj, and various microscopic 

 plants. 



The method pursued has boon to study the plant population and, 

 correlatively, the contents of the digestive tract in various species of 



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