As the fish become older, their ovaries acquire 

 increasing amounts of connective tissue; hence, 

 they should produce fewer eggs per gram of ovary 

 with increase in age. This anticipated change is 

 not borne out, however, in table 18. The per- 

 gram-ovary yield of eggs was highly variable, 

 even among the fish of an age group. The situa- 

 tion is not surprising since the problem of finding 

 differences is compounded by the fact that the 

 ovaries were hi varying stages of development at 

 the time the fish were captured. An ovary con- 

 tains a certain number of eggs which would be 

 coming spawning season, 

 with immature ones and 

 the ovary, make up the 

 The farther along these 



spawned during the 

 These eggs, together 

 other components of 

 weight of the ovary. 



eggs develop the heavier they become; the weight 

 of the ovary increases and its per-gram yield of 

 eggs decreases. 



On the per-gram-ol-fish basis the older fish do, 

 indeed, produce fewer eggs than the younger ones 

 (except the precocious I group). The "relative 

 fecundity" tripled from the I group to the II 

 group and then declined to age-group VI which 

 had about the same value as the I group. 



On the basis of fish size it would appear that 

 fish weighing 500-600 g. produce more eggs than 

 do those whose weights fall either below or above 

 that range. The productivity of these fish may 

 be the result of age rather than size, however; 

 in the table only the II-group shad occupy this 

 range. Further information on this possibility 

 was gained from 69 II-group female shad, captured 

 in the first two quarters of June 1954; their weights 

 ranged from 339 to 733 g. They were divided 

 by 50-g. groupings (301 to 350, 351 to 400, — ), 

 and the ntio ol ovary weight to fish weight was 

 calculated for each group. (Egg counts had not 

 been made on these fish.) The ratio was highest 

 for the 500- to 600-g. fish. This finding would 

 suggest, then, that fish in this weight range 

 produce the most eggs. 



Spawning 



The single spawning site of gizzard shad that 

 I was able to find was a bar some 200 feet long 

 and covered by 2 to 4 feet of water near Put-in-Bay. 

 The bottom is topped with sand, gravel, and 

 boulders. Cladophora, Myriophyllum, and Bu- 

 tomus umbellatus (forma vallisneriifoluis) are abun- 

 dant during the spawning season. 



No gizzard shad were found on this bar in 1954 

 prior to, or after, the spawning season. To learn 

 the day-to-day variations of the numbers of shad 

 there during the 1955 spawning season, I set a 

 100-foot, 4-inch-mesh gill net nightly along the top 

 of the bar from May 9 to June 30. The catches, 

 together with the water temperatures, are recorded 

 in table 19. 



It was evident later that the female shad does 

 not spawn her entire egg holdings in one visit to 

 the bar. Consequently, some shad would prob- 

 ably have visited the bar more than once during 

 the season had they not been caught. Further- 

 more, gizzard shad exhibit greater activity at the 

 height of spawning than just prior to it or after- 

 wards. Hence, greater percentages of those 

 present in a given area are likely to swim afoul 

 of the net at this tune than at other periods. The 

 table must be interpreted with these points in 

 mind. 



Temperature clearly is important in the onset 

 and progress of spawning. Gizzard shad first 

 appeared in the net on this bar at a temperature 

 of 59° F. and were common at about 67° F. 

 Indeed, when the water temperature dropped 

 slightly (to 65° and 65.5° F. on June 12-15) the 

 numbers of shad also dropped. 



To define the spawning site more precisely, I 

 set gill nets transversely across the bar. In every 



Table 19. — Water temperatures and daily captures of shad 

 by a 100-foot, 4-ineh-mesh gill net set on a spawning site 

 in Fishery Bay in May and June 1955 



[The net was lifted daily at 9 a.m. except on June 2-11 (see text); the water 

 temperature was read at the time of lifting) 



GIZZARD SHAD IN WESTERN LAKE ERIE 



413 



