Table 10. — Annual differences of weights of gizzard shad in 1952-66 

 [Years compared by pairs; data limited to months for which records are available for both members of the pair] 



Annual Differences in Weight 



The study of the annual differences in the length- 

 weight relation of gizzard shad in 1952-55 was 

 based on data for only those months during which 

 adequate collections were made in at least two of 

 these years. As was brought out in the previous 

 section, the weight of shad varies with the season. 

 Since samples were not adequate for determination 

 of length-weight relations for every month of each 

 year, the use of yearly data would result in a bias 

 in favor of the year during which the specimens 

 were obtained in the months of greatest weight. 

 Consquently, the paired comparisons for 1952-55 

 were based only on data from months common to 

 both years being compared (table 10). 



Shad were heaviest in 1952 and lightest in 1953. 

 The rating of years ran: 1952>1954>1955>1953. 



LENGTHS AND WEIGHTS OF AGE 

 GROUPS 



The average size of the members of an age group 

 varies with time of capture within the year along 

 clearly seasonal lines. The monthly average 

 lengths of shad show little growth in early spring, 

 rapid growth in summer and early fall, and 

 apparently a cessation of growth in winter for 

 both sexes and all age groups (table 11). The 

 course of growth becomes more apparent when 

 portrayed graphically (fig. 8). After about June 

 of their second year of life female shad were 

 rather consistently longer than males of corre- 

 sponding age. 



The average monthly weights of shad showed, 

 in general, a gradual decrease from the first of the 

 year until June (table 11; fig. 9). This decline 



Figure 8. — Lengths of gizzard shad at capture in each 

 month by age and sex. Dots are empirical averages of 

 standard length. Open dots represent females; solid 

 dots, males. Curves were drawn by inspection. 



was followed by a rapid rise in late summer and 

 fall. Seasonal variation of the length-weight 

 relations (see earlier section) contributed to these 

 changes. The female shad were heavier than 

 males after about June of their second year of life. 

 The average monthly lengths of the O-group 

 shad warrant special comment. Because some 

 gizzard shad can be found in the spawning state 

 in western Lake Erie from early June to mid- 

 July (see section on length of spawning season), 

 every sample of young shad collected usually 

 contained fish hatched over a period of time, the 

 extremes of which often differed in age by as 

 much as a month or more. The successive col- 

 lections continually yielded low average changes 

 in length because of the great percentage of the 

 newer, smaller shad constantly added to the 

 population. This bias was aggravated by the 

 tendency of the larger O-group fish to move off- 



GIZZARD SHAD IN WESTERN LAKE ERIE 



403 



