Table 7. — Comparison of weights of male shad at 20-mm. 

 intervals with those of females whose weights have been ad- 

 justed to include ovaries of only 1 percent of their body 

 weight 



[Based on June 1955 collection] 



oil in the eggs rather than as the heavier protein 

 of flesh. The spent fish weighed the least. Their 

 average weight was 85 percent of that of the fish 

 which would not have spawned during the current 

 season. The average percentage loss of weight of 

 mature females at spawning increased with length 

 of fish and averaged 10.7 percent. 



Seasonal Differences in the Weight of Fish of a Given 

 Length 



The weight of gizzard shad of a given length 

 varies from season to season, and this seasonal 

 variation is similar from year to year. The na- 

 ture of the seasonal changes (table 9) appears 

 from the average monthly length-weight data. In 

 June and July, when weights of the sexes differed 

 appreciably, the males and females were treated 

 both separately and combined; in other months 

 they were combined. 



Gizzard shad attained their greatest weight in 

 August-October, after which the weight declined 

 slowly and irregularly, reaching a low for the com- 

 bined sexes about May-June. (Interpretations 



are handicapped by differences of slope in the 

 monthly logarithmic lines.) During the next 2 

 months, the weights increased rapidly. The 

 weight of the female was least in May (not shown 

 in table 9), and an increase was obvious in June, 

 but that of the male reached the lowest point in 

 June. 



Studies of the gut contents indicate that gizzard 

 shad consume little food in winter and early 

 spring. During this period they subsist largely 

 on energy stored in the body tissues. Because 

 metabolic rates are low in winter, decrease of the 

 body weight is slight. As the water temperature 

 rises in early spring, the metabolic rate of the 

 fish increases more rapidly than the rate of food 

 intake and the body weight decreases more 

 rapidly. This process gains momentum with the 

 progression of spring until about May-June when 

 the lowest body weight is attained apparently 

 just before the renewed feeding begins to meet 

 the energy requirements of the fish. The occur- 

 rence of peak weight, in August-October, un- 

 doubtedly is directly related to feeding. 



Continued loss of weight in June among the 

 males can be attributed to their spawning activities. 



Table 8. — Weights of three groupings of female gizzard shad 

 of different ovarian development, June 1955 



Table 9. — Monthly variations of weight of gizzard shad as determined from the monthly length-weight equations 



WEIGHT 



402 



U.S. FISH AND WILDLIFE SERVICE 



