188 



BULLETIN OP THE BUREAU OF FISHERIES 



lengths. Several natural factors may have caused this phenomenon, directly op- 

 posed to Lee's phenomenon of apparent change of growth rate. 



Table 22. — Comparison of the averaged calculated lengths of spotted trout, derived from measurements 



of two scales from the same Jish 



[Scales measured by projection method and the total lengths expressed in centimeters. Fish collected from May, 1926, to May, 

 1927, and selected at random from fish represented in Table 21] 



Figure 31, presenting a comparison between matured male and female trout 

 according to size, clearly indicates that a larger percentage of males than females 



3.9 



7.B 



LENaTH-INCHZS 

 11.6 IS.7 



19.6 



2ie 



27.5 



10 



20 30 40 50 



LENCiTH- CENTIMETERS 



60 



70 



Fig. 31.— Size at maturity of the spotted trout. Males represented by solid line; females by dotted line. 



Total number of fish, 567 



occurs in the smaller sizes than in the larger sizes. Hence, we may assume that 

 in general the younger year classes contain a greater proportion of male fish than do 

 the older year classes. Since the males do not appear to attain the same average 

 mean length as the females (see p. 190), a greater rate of growth might be expected 

 among the latter. Such a condition would result in smaller calculated lengths 

 among the younger year classes with their higher percentage of males and a shghtly 

 slower rate of growth than among the older fish composed largely of females. While 

 it is possible to assume that the growth of males and females is identical prior to 

 spawning, and that the females merely reach a greater size (and age) than do the 

 males, the writer hardly believes such an assumption probable, since it is known that 

 in many species of fish the males are smaller than females of the same age. 



Another explanation is to assume that the smaller calculated lengths among the 

 younger age groups are the result of the occurrence of greater numbers of "runt" 

 or constitutionally slow-growing fish during the early years of life, but which are 

 ehminated from the fish stock gradually by the natural process of survival of only 

 the largest and fastest-growing fishes. The writer observed often that many spotted 

 trout in their first and second years possessed gill parasites (an isopod, Livoneca 



