HOWELL: SEASONAL CHANGES IN YELLOWTAIL FLOUNDER OVARIES 



were formed as either early or late maturing oocytes 

 ceased to develop and began to be resorbed. Even in 

 the samples where most abundant (January and Feb- 

 ruary), they accounted for <2% of the oocytes ex- 

 amined. The persistence of corpora atretica and 

 corpora lutea in certain species has led some inves- 

 tigators to conclude that they are the source of 

 ovarian hormones (see review by Ball 1960). The very 

 small percentages of corpora atretica seen in this 

 study suggest that in yellowtail flounder these struc- 

 tures either disappear rapidly or are formed very in- 

 frequently. Similar low percentages have been 

 reported in numerous other teleosts (Wheeler 1924; 

 Yamamoto 1956a; Barr 1963; Davis 1977). Resorb- 

 ing oocytes (Regressing Type II) were observed even 

 more infrequently, and only in postspawning fish. 

 Because of their scarcity in the samples, no percent- 

 ages were calculated. The infrequency of the two 

 types of regressing oocytes indicates that the vast 

 majority of oocytes which reach the vitellogenic phase 

 continue to develop and are released during spawn- 

 ing. Those few which cease to develop or remain 

 in the ovaries after spawning are quickly resorbed. 



These data indicate that the development of a fully 

 mature yellowtail flounder egg requires 2 yr. During 

 the first year, which begins after spawning, oocytes 

 pass through the previtellogenic phase which in- 

 cludes oogonia, early perinucleolus, resting, and late 

 perinucleolus developmental stages. In the second 

 year of development, oocytes pass through the 

 vitellogenic phase and are then released. This phase 

 begins as late perinucleolus oocytes, now about 1 yr 

 old, develop into early maturing oocytes during the 

 spring and summer. By the fall of their second year 

 the oocytes have accumulated large amounts of yolk 

 in the cytoplasm and have reached the late maturing 

 stage. Over the following winter and early spring 

 months they continue to accumulate yolk and in- 

 crease in size. During the spawning season, batches 

 of late maturing oocytes enter the hyaline stage and 

 are then released. This apparently occurs intermit- 

 tently throughout the breeding season until virtually 

 all late maturing eggs become hyalinated and are 

 released. 



At any given time then, there are two populations or 

 year classes of oocytes present within adult yellowtail 

 flounder ovaries. These include a population of 

 small, previtellogenic oocytes which develops over 1 

 yr, and a second population of larger vitellogenic 

 oocytes which are recruited from the previtellogenic 

 population. Members of the vitellogenic population 

 are in their second year of development and will ma- 

 ture and be released in the upcoming breeding 

 season. Ovaries such as this have been described as 



"group synchronous" (Wallace and Selman 1981). 



The events described in the process of oocyte 

 maturation fit closely with seasonal changes in both 

 GSI and the macroscopic appearance of the ovaries. 

 Maturity Stage II (Developing virgin, recovering 

 spent) ovaries were seen from May through October. 

 These ovaries contained only oogonia, early and late 

 perinucleolus, and early maturing oocytes. The 

 relatively small size of these oocyte developmental 

 stages (<150 /im diameter), and the absence of late 

 maturing and hyaline oocytes, resulted in these 

 ovaries being relatively small and translucent. As ex- 

 pected, in postspawning months when fish with 

 ovaries of this type accounted for >50% of the sam- 

 ple (June-September), mean GSI values were very 

 low. Stage III (Developing, maturing) fish were ob- 

 served from September through May. Due to the 

 presence of large early and late maturing oocytes, 

 their ovaries were enlarged, yellowish in color, and 

 granular. As the percentages of fish in this maturity 

 stage increased from September to October, mean 

 GSI values increased. From November through at 

 least February, all fish had ovaries in this maturity 

 stage. Mean GSI values rose during this time due to 

 the increasing size of the late maturing oocytes pres- 

 ent. Ripe fish (Stage IV) were present from April 

 through June. Mean GSI was highest in April since all 

 fish examined were either still maturing or ripe. The 

 first spent fish (Stage V) were found in early May, and 

 by the end of May 50% of the fish had spawned. Loss 

 of eggs through spawning caused these ovaries to be 

 flaccid and bloodshot, and mean GSI values de- 

 creased as the spawning season progressed. 



ACKNOWLEDGMENTS 



Special thanks to Saul B. Saila, H. Perry Jeffries, 

 and William H. Krueger for critically reviewing the 

 manuscript. David Kesler, Nancy Bray Thompson, 

 Robert Kleckner, David Bengtson, Harold Pomeroy, 

 and Gary Davis all provided valuable assistance in 

 data collection. I would also like to thank Paul Yevich 

 and the histopathology unit at the Environmental 

 Protection Agency's Narragansett Laboratory and 

 the staff of the Point Judith Fisherman's Cooperative 

 for their assistance and cooperation. The suggestions 

 made by the three anonymous reviewers and the 

 sample collection by the National Marine Fisheries 

 Service are gratefully acknowledged. 



LITERATURE CITED 



ABLE, K. W. 



1978. Ichthyoplankton of the St. Lawrence estuary: Com- 



353 



