Figure 9. — Weights of gizzard shad at capture in each 

 month by age and sex. Dots are empirical averages. 

 Open dots represent females; solid dots, males. Curves 

 were drawn by inspection. 



still substantial. The principal cause of the 

 discrepancy was probably the selective sampling 

 of age-group I. Many of this group were too 

 small to be caught by the gill nets and the trap 

 nets. At the higher ages the agreement between 

 empirical and calculated lengths was good. The 

 lengths calculated for each year of life from 

 progressively older shad show a similar close 

 agreement except for the first year of life. 



No year classes exhibited consistently fast or 

 slow growth in length, nor did growth appear to 

 have been outstandingly good or bad in any 

 particular calendar year. 



Compensatory Growth in Length 



It has been observed frequently that although 

 the larger of the young fish maintain a length 

 advantage over the smaller ones during subsequent 

 growth, the smaller young fish grow more rapidly 

 during subsequent years than the larger ones and, 

 thus, reduce progressively the original length 

 difference. 



To study this "compensatory growth" in gizzard 

 shad, I tabulated the annual increments of growth 

 of Ill-group fish, arranged by 20-mm. groupings 

 of length at the end of the first year of life (table 

 14). The sexes were treated separately, and only 



fish caught in 1954 were used. In both sexes the 

 annual increments generally varied inversely with 

 first-year length; consequently, the difference in 

 length between the shortest and the longest 

 first-year fish diminished progressively to a mini- 

 mum in the third year. 



Table 13. — Comparison of the average empirical standard 

 length of gizzard shad with average length calculated for 

 each year of life for each age group for each sex 



[Fish of the same age were comhined without regard for year classes. Em- 

 pirical lengths are given only for shad captured between the end of the 

 growing season and the time of annulus formation. Number of fish in 

 parentheses] 



1 Unweighted means. 



Table 14. — Average annual increments of length of Ill- 

 group shad of the 1954 collections arranged by 20-mm. 

 groups of first-year calculated length 



' Between shortest and longest first-year groups. 



406 



U.S. FISH AND WILDLIFE SERVICE 



