REPRODUCTION AND EARLY 

 DEVELOPMENT 



Size and Age at Maturity 



The sex and maturity of gizzard shad were 

 always determined by dissection except for those 

 fish from which eggs or milt were flowing during 

 the spawning season. Vladykov (1945) stated 

 that the males have darker fins than the females, 

 but I found that neither this nor any other readily 

 discernible external characteristic indicated the 

 sex with reasonable consistency for all age groups 

 in all seasons. Generally, however, one might 

 expect a shad that is relatively deep for its length 

 to be a female. Since the time of this study, 

 Moen (1959) sexed gizzard shad by examining 

 the urogenital opening with a probe. 



The sex of large fish was determined easily — 

 the testes were opaque white, whereas, the ovaries 

 were mostly semi translucent and light yellow or 

 pink. Sex determination was more difficult in 

 small shad. Here, the testes were whitish and 

 the ovaries almost colorless and translucent, but 

 the distinction between them became less and less 

 apparent as one examined smaller and smaller 

 gonads. After much study (comparisons of judg- 

 ments from macroscopic examination with subse- 

 quent determinations from histological sections) I 

 could frequently distinguish the sexes macroscopi- 

 cally when the diameter of the gonad was only 1 

 mm. and readily when it attained 2 mm. The 

 minimum standard length of fish for which sex 

 could be determined macroscopically with confi- 

 dence was about 120 mm. (4 or 5 months old), 

 although on occasion sex could not be determined 

 for a larger shad. 



Although a fish whose milt or spawn flows 

 during the spawning season is sexually mature, 

 the absence of this feature does not indicate im- 

 maturity. In this situation, during the spawning 

 season or any other time, different criteria must 

 be employed. Histological studies of ovaries 

 collected throughout the year have enabled me 

 to recognize a potential spawner several months 

 in advance of the spawning season and spent fe- 

 males as long as a month after spawning. More- 

 over, the relation of the histological characteristics 

 of the ovary to the gross appearance of that organ 

 made possible an estimate of maturity by gross 

 observations. In females that will not spawn, the 

 ovaries contain only minute, scarcely visible eggs, 



whereas potential spawners exhibit, in addition to 

 the minute eggs, fair-sized ones that are clearly 

 visible within the semitranslucent ovary several 

 months prior to spawning. Although completely 

 spent females have only the minute eggs that are 

 characteristic of the ovaries of fish which will not 

 spawn, their ovaries for a few weeks after spawn- 

 ing are more flaccid than those of nonspawners, 

 the minute eggs are some little distance apart, 

 and the interovular spaces have a more watery 

 appearance. 



The maturity of males could not be determined 

 in the absence of flowing milt. The presence of 

 motile sperm, obtained by lancing a testis, does 

 not assure maturity. Males examined in January 

 had motile sperm — even the I-group males, most 

 of which would not spawn in the spring. Maturity 

 in males is undoubtedly associated with structural 

 development or physiological change which per- 

 mits release of the sperm. The great scarcity of 

 I-group males at the spawning site in May and 

 June suggests that this release mechanism does 

 not usually become functional in males of that 

 age. The presence of a few of them on the spawn- 

 ing grounds during July may indicate that some 

 have matured near the end of the spawning season. 



In 1954, I found three mature I-group female 

 shad. Their standard lengths were 197, 225, and 

 236 mm. Some 25 or 30 I-group males, 190 to 

 230 mm. long, also were mature. The milt and 

 spawn produced by fish of this age were scanty. 

 The crosssection of an ovary of a mature I-group 

 female reveals few mature eggs (fig. 10). 



Figure 10. — Cross section of an ovary of a mature I-group 

 gizzard shad showing relatively few near-mature eggs 

 (July 6, 1954). 



GIZZARD SHAD IN WESTERN LAKE ERIE 



407 



