676 



Fishery Bulletin 92(4), 1994 



20 



10 



Males 



(n = 896) 



CD 



X) 



c 



ID 



o 



o 

 a 



03 

 C 



o 



O 



20 



JMMJSNJMMJSN 



Females 



(n = 2.195) 



10 



No 



samples 



JMMJSNJMMJSN 

 - 1990 - — II — -1991 - 



Figure 4 



Monthly mean gonadosomatic index for male and 

 female Atlantic croaker, Micropogonias undulatus, 

 in the Chesapeake Bay and adjacent coastal waters, 

 1990-91. Vertical bars are ranges. n=sample size. 

 Monthly sample sizes are presented in Table 2. 



also showed a small percentage of gravid and run- 

 ning-ripe females (Fig. 3). 



Sex ratios 



Atlantic croaker showed wide temporal fluctuations 

 in sex ratio. During both years, the frequency of 

 males in samples decreased in June and July at the 

 beginning of the spawning season, reached a mini- 

 mum in the period of September-October, and in- 

 creased again during November-December. Chi- 

 square tests (Table 2) showed highly significant dif- 

 ferences (P<0.01) in sex ratios between months over 

 the periods July-October 1990 and June-October 

 1991. 



Oocyte development and spawning pattern 



Monthly oocyte diameter distributions of fully devel- 

 oped females collected throughout the spawning sea- 

 son showed three main groups of oocytes (Fig. 5). 

 However, oocyte development appears to be asynchro- 

 nous; there is a large degree of overlap and no clearly 

 defined limits between modal groups. Histological 

 analysis showed that the first group, ranging approxi- 

 mately from 0.06 to 0.24 mm diameter, is composed 

 mainly of primary growth and cortical alveolus oo- 

 cytes but may include a few partially yolked oocytes 

 in the beginning stages of yolk deposition ( 0.22-0.24 

 mm diameter). The second group, ranging approxi- 

 mately from 0.26 to 0.38 mm diameter, is composed 

 of partially yolked oocytes in several stages of yolk 

 deposition. The third group, ranging approximately 

 from 0.40 to 0.60 mm diameter, is formed by advanced 

 yolked oocytes and probably represents the group 

 from which individual spawning batches will be 

 formed. 



Although frequency distributions of gonad stages 

 and oocyte diameters (Figs. 3 and 5) indicated At- 

 lantic croaker are multiple spawners with indeter- 

 minate fecundity, postovulatory follicles (POF's) were 

 identified only in recently ovulated, running-ripe fe- 



