Physiological Changes with Age in Fish 183 



or not size and age have an effect on the abihty to convert 

 protein to body flesh (Gerking, 1952, 1954, 1955a, b). Longear 

 sunfish (Lepomis megalotis), green sunfish {Lepomis cyanelhis), 

 and bluegill (Lepomis macrochirus) gave similar results. The 

 methods used in these experiments were essentially the same 

 as those used to study food conversion in other animals (May- 

 nard, 1951). A weighed quantity of food was fed each day to a 

 group of fish of various sizes kept in separate aquaria at 

 temperatures of about 25° for a period of 30 to 50 days. They 

 were fed at maximum or near-maximum rates. At the end of 

 the period the fish were killed, weighed, and analysed for 

 protein. At the beginning of the period their weight was 

 known, and their protein composition was estimated by 

 averaging protein determinations by a micro-Kjeldahl method 

 on several fish collected at the same time and place as the ones 

 used in the experiments. The food was mealworms, Tenehrio 

 molitor larvae, which had also been analysed for their protein 

 content. Thus the efficiency of protein utilization for growth 

 could be determined for fish of various sizes by comparing the 

 amount retained with the amount absorbed by the gut. 

 Absorption of protein was measured by subtracting the 

 amount of nitrogen in the faeces from that consumed. Ab- 

 sorption was practically complete in every fish. 



Menzel (1957) has duplicated these experiments on two 

 Bermuda reef fishes, angelfish (Holocanthus hermudensis) and 

 red hind (Epinephalus guttatus). The former species is a 

 herbivore and the latter a carnivore. They were fed as much 

 as they would eat at three temperatures, 19, 23, and 28°. The 

 angelfish were fed algae (Enteromorpha salina and Monostroma 

 oxysperma) which were kept in the tanks with them, and the 

 red hinds wxre fed three species of small fish, Harengula 

 callolepis, Sardinella anchovia, and Anchoa choerostoma. 

 Efficiency of protein utilization for growth was determined 

 over a 21 -day period in the manner described above. 



The weight of Menzel's fish varied from 50 to 763 g. and the 

 sunfish from 7 to 184 g. Protein accumulation was used as an 



