After deducticxis made, according to these figures, we find the followtag economic values 

 of the different constituents of foodstuffs for fish (according to Keller): 



1 gram carbohydrate (nitrogen- free, digestible extracts and fiber 3.76 kcal. 



1 gram fat (digestible raw I'at) 8.57 " . 



1 gram protein (digestible raw-protein).... ^.63 ■* . 



I have calculated these figures in the accompanying tables, although I realize that thsy 

 will not be correct in certain isolated cases for pond fishes. 



The necessary digestibility of ccnsujned proteins has been siafficiently referred to. Be 

 it said, though, tnat the calculated average digestibility of a food product can be consider- 

 ably lowered, up<ai occasion, through boiling or steaming it, or by exposing it to excessive 

 heat while undergoing dehydration (lupines, fish-meal, blood-meal and meat-meat, etc.). 



By abuxidant feeding, fats may be converted — outright and almost unchanged — into "body 

 fat". The correctness of this was lately proven by Wieber in regard to eels . 



The "pointhead", feeding upon fatty, aquatic organisms has a fat content of 27 per cent, 

 as against 12 per cent in the fish-eating "truncate" (Breitkopf, "large head" eel). And in 

 conformity with the oiliness of the fat of aquatic organisms, the body fat of the "point- head" 

 (Spitzkopf eel) is richer in cleln than the fat of the "truncate" (Breitkopf eel). 



This explains, perhaps, why the flavor of maize is conveyed, so to speak, upon maize- 

 fed carp. In regions where the taste of maize is disliked, such fish become less aiarketable. 



But Willie the iaodjinamlc law — of food conversion — is applicable generally; the metabolic 

 reactions of the individual foodstuffs may be specific in the metabolism of fishes. Also, 

 the amount of fat deposits depends lyjon certain individual factors, the age of fishes, for 

 instance. 



Czensny made pond fertilization tests at Sachsenhausen. He found that the percentage 

 of fat (expressed in calories) rose in carp of frcm 500 to 600 grams weight from about 20 

 to 30 per cent^ in carp of from 1,300 to 1,400 grams weight, Naumana found the following 

 figures for fat ccmtents: 



Lusatia carp (yearlings) 1.09$ 



Galician " (3 to /^ years old) 13.81^ 



The fat rate rises in proportion to the amount of satisfaction of the nutritional 

 requirement, as is shown by the following table for 3 and A years old Galician carps in 

 various nutritional circumstances. 



Stock increased by 50% .... fat rate 8.49% 



" reduced by 50$ " " 13.36$ 



Normal stock rate but intensive feeding 13.81$ 



The calories produced are therefore not always reliable as a true picture of the 

 qualities of certain foodstuffs, Ush are even less liable to tolerate large amounts of 

 fat than warm-blooded animals. 



After consideration of pond fish requirements as to the qualitative composition of 

 inportant organic food constitxients and their mutual ratios, we nov/ turn to quantitative 

 consideration, in order to calculate sooe accurate figures, concerning the required total 

 anounts in proteins, carbohydrates and fat. 



These figures are far more difficult to calculate and compare in fish, than is the 

 case with warm-blooded animals, since the metabolism of fish depends upon a number of 

 variable factors (such as size and teaperature) and some of which are difficult to deter- 

 mine and conq)are, such as the rate of locomotion, for instance. 



23 



