112 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 26. — Growth exhibited by lake herring that were the 

 same length at the end of different years of life 



[Total length in inches] 



I 

 I- 

 O 



z 



111 



T 

 T I ME 



T+l 



Fic.uBE 6. — The effects of differences in age (time of 

 hatching) on the amount of growth during a later grow- 

 ing season. 



T, which may mark the end of any growth period, 

 fish A has attained length ac and fish B has at- 

 tained tlie lesser length ab. During the interval 

 from T to T+l fish A, which had expended more 

 of its ability to grow at time T, added the incre- 

 ment of length /fif which is less than the increment 

 ef added by younger fish B. This explanation of 

 growth compensation as an apparent phenomenon 

 is based on tlie premise that all fish have the same 

 growth curve. Under this concept, the growth 

 of fish during a particular time interval depends 

 principally on the size it had attained at the 

 beginning of the interval. The data presented in 

 table 26 shows that this assumption is essentially 

 correct, for fish of the same lengtli at different ages 

 tend to grow the same amount in tlie following 

 year. 



In a review of Hodgson's (1929) treatment of 

 growth compensation, Ford (193.3) offered the 



criticism that Hodgson employed curves of iden- 

 tical shape, although it is well known that growth 

 may vary from individual to individual. Ford 

 demonstrated that Hodgson's explanation could 

 be supported from comparisons of growth along 

 dissimilar curves starting at different points along 

 the time axis. 



From the hypothetical curves of figure 7 it can 

 be shown that dissimilar curves starting at the 

 same point on the time axis will also exhibit growth 

 compensation. This compensation depends on the 

 fact proved earlier that size, not age, at the start 

 of a period of growth determines the amount of 

 growth that will be made during the period. The 

 form of three of the five curves of figure 7 is iden- 

 tical with that of the growth curves of figure 6, 

 namely, OaCQ, OC, and Oabf. The curves Ocg and 

 06/ represent individual fish ^4 and B whose growth 

 up to time T departed from the typical. Fish A 

 grew more (ac) and fish B grew less (ab) at time T 

 than the typical fish which would follow curve OC. 

 Since, however, length is more important than age 

 as a determiner of growth within a period (table 

 26), fish A may be expected subsequently to grow 

 along the curve eg or, in other words, to follow 

 the same course as a normal fish hatched at time 



