Physiological Changes with Age in Fish 185 



eat extraordinary quantities or algae with a much higher 

 protein content than he used. Probably the "herbivorous" 

 fish depends to a large extent on the invertebrates living in 

 association with plants for the protein required for growth. 

 The results of the experiments on angelfish are. therefore, 

 not comparable with the others. 



The experimental results on the four carnivorous species 

 are similar. In every case the efficiency of protein utilization 

 decreased as the size of the fish increased. Efficiency was very 

 high in the smallest fishes. For example, green sunfish (body 

 weight = 7'1 g.) were 39*7 per cent efficient in using 

 protein for growth; longear sunfish (9-1 g.) were 33-3 per 

 cent efficient; bluegills (7-7 g.) 38-0 per cent; and the red 

 hind (about 227 g.) 32-1 per cent. In contrast, utilization 

 among the largest specimens was 20-0 per cent (48-5 g.), 4-7 

 per cent (103-3 g.), 23 • 6 per cent (184-0 g.), and 22 • 7 per cent 

 (612 g.), respectively, for the four species. The value for the 

 largest longear sunfish departs considerably from the others. 

 This specimen was as large as any ever observed in the creek 

 where it was captured near Bloomington, Indiana, and it was 

 probably over six years old. The other three species are not 

 represented by individuals of maximum size or age. This may 

 indicate that extremely large individuals have a very low 

 efficiency of protein utilization for growth. More evidence is 

 required to establish this point, however. 



The shape of the graphs is not consistent. A linear relation- 

 ship expresses the relation between protein efficiency and dry 

 weight in the longear sunfish, but it is curvilinear in the other 

 species. The inconsistency is due to individual variation and 

 the difficulty in establishing the initial protein content of the 

 experimental fish from analyses of sample fish. 



Menzel answered an important question with respect to the 

 effect of temperature on protein utilization. The red hind ate 

 only slightly more at 23° than at 19° but ate about twice as 

 much at 28°. This great difference in feeding rate did not 

 alter the efficiencies of protein utilization, however. Thus all 



