186 Shelby D. Gerking 



protein utilization experiments in this 9° temperature range 

 are comparable. 



A large amount of work has been done on food conversion 

 by fishes, but most of it relates weight gain to the amount of 

 food consumed. Large variations have been encountered and 

 are due to variable fat deposition, unknown organic composi- 

 tion of the food, differences in the size of experimental fish, 

 and other factors. Most of the research has been done on a 

 variety of foods fed to young trout in order to obtain maxi- 

 mum growth rates in hatcheries. This material is not applic- 

 able to the present discussion since the fish used in the experi- 

 ments were nearly uniform in size and age. Ivlev (1939a, h, c), 

 Karzinkin (1939) and Morgulis (1919) have studied fish nutri- 

 tion by detailed analyses of the organic constituents of the 

 fish and their food, but none of these workers compared the 

 efficiency of food utilization by fish of different sizes. 



Although the effect of age and size on protein utilization for 

 growth cannot be separated, it can be stated definitely that 

 larger fish are less efficient in this respect than smaller ones. 

 Chronological age may play some part in this phenomenon. 

 It is universally true that the rate of growth declines as age 

 and size increase. In this respect fishes conform to the com- 

 mon vertebrate pattern. The protein metabolism studies 

 demonstrate that this decline is due to a decreasing ability of 

 the fish to utilize its food for growth as it grows larger and 

 older. The growth of most vertebrates stops relatively early 

 in life while that of fishes is prolonged. Although this dif- 

 ference cannot be explained at the present time, the loss of 

 growth efficiency with age is clearly exhibited by both. 



Fecundity 



Live -bearing fishes 



There are several groups of fish which give birth to well- 

 developed young, but reproduction in relation to age has been 

 studied in only one family, the Poeciliidae. In this family the 



