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Fishery Bulletin 90(1). 1992 



(postovulatory follicles present) or imminent spawning 

 (hydrated oocytes or migratory nucleus-stage oocytes 

 present), while the ovaries of nonspawning females 

 showed no evidence of recent or imminent spawming 

 but were capable of spawning in the near future. The 

 fraction of active females classed as spawning was used 

 as a spawming rate index. Spawning performance was 

 also assessed by calculating the mean number of spawn- 

 ing states (postovulatory follicles, hydrated oocytes, 

 migratory nucleus) per female in the spawning class. 



Females with ovaries classified as active are con- 

 sidered mature. On the other hand, females with in- 

 active ovaries could be either immature or mature 

 because an ovary may have regressed to an inactive 

 state after the female had attained sexual maturity. We 

 designed our histological classification of inactive 

 ovaries to distinguish as best as possible between 

 mature and immature conditions. Inactive females 

 were grouped into three classes (Fig. 1): immature, 

 uncertain maturity, and inactive-mature. The inactive- 

 mature class included ovaries showing clear histological 

 evidence of past spawning (postspawning subclass) or 

 past maturation of advanced yolked oocytes (major- 

 atresia subclass). Postspawning ovaries contained 

 either postoviilatory follicles and no advanced yolked 

 oocytes or postovulatory follicles and mostly atretic ad- 

 vanced yolked oocytes. The fraction of inactive females 

 identified as postspawning was used as an index of the 

 rate at which females passed from the active to the in- 

 active state during the spawning season. 



The five major histological classes of active and in- 

 active females were subdivided into atretic subclasses 

 using the first (a) and second {ft) stages of resorption 

 as defined by Bretschneider and Duyvene de Wit (1947) 

 and Lambert (1970). One can identify the developmen- 

 tal stage of the oocyte only during the a stage of atresia 

 because the oocyte is completely absorbed by the end 

 of this stage. Subsequent stages (fi, y and 6) involve the 

 resorption of the follicle. Thus, a atresia is of key im- 

 portance to fecundity studies since the oocyte class can 

 be identified. Subsequent stages may be useful for iden- 

 tifying past spawning activity. 



For ovaries containing early yolked or only unyolked 

 oocytes, classification was based solely on presence or 

 absence of the following atresia: /?, a of unyolked 

 oocytes, and a of early yolked oocytes (classes im- 

 mature and uncertain maturity of the inactive females) 

 (Fig. 1). 



Ovaries with advanced yolked oocytes were sub- 

 divided into two atretic subgroups using the extent of 

 the a atresia of the advanced yolked oocytes: minor 

 atresia, i.e., females with one oocyte to 49% of their 

 advanced yolked oocytes in a; and major atresia, i.e., 

 50% or more of the advanced yolked oocytes in a. We 

 showed in anchovy that the probability of spawning 



was very low when more than 50% of the advanced 

 oocytes were atretic (Hunter and Macewicz 1985b). 

 Therefore, ovaries with major atresia of advanced 

 yolked oocytes were considered inactive (inactive- 

 mature class) although the ovary contained some ad- 

 vanced yolked oocytes. 



Estimation of total fecundity 



We used the gravimetric method to estimate total 

 fecundity of Dover sole. Total fecundity (Yp) was the 

 standing stock of advanced yolked oocytes in the ovary: 

 Yp = Z ■ C, where Z is the ovary weight in grams, and 

 C is oocyte density (number of advanced yolked oocytes 

 per gram of ovarian tissue). We also measured diam- 

 eters of 30 of the advanced yolked oocytes in at least 

 one of the 2-5 tissue samples analyzed for each female 

 for which fecundity was estimated. Advanced yolked 

 oocytes were identified, counted, and measured using 

 a digitizer linked by a video camera system to a dis- 

 section microscope. 



We used the apparent density of yolk in whole 

 oocytes after preservation, when viewed on the televi- 

 sion monitor, to discriminate between developmental 

 stages of yolked oocytes. We defined three stages of 

 yolked oocytes: (1) only an initial layer of yolk along 

 the periphery of the oocyte, appearing as a narrow 

 band but not extending over 20% of the distance be- 

 tween the nucleus and the zona pellucida; (2) lightly- 

 packed yolk possibly extending from the periphery to 

 the nucleus with the nuclear area still evident; and (3) 

 yolk dense enough to occlude the nucleus (Fig. 2) which 

 is histologically equivalent to advanced yolked oocytes. 

 Counts of stage-3 oocytes were used to estimate fecun- 

 dity and measurements to estimate mean diameter of 

 these advanced yolked oocytes. 



Alpha atresia of stage-3 yolked oocytes were dis- 

 tinguished from other whole oocytes viewed on the 

 television screen. The yolk within these a-atretic 

 stage-3 oocytes appeared mottled and lighter due to 

 yolk liquefaction and subsequent resorption, whereas 

 in normal yolked oocytes it appeared dense, dark, and 

 in compact globules (Fig. 2). In addition, the zona 

 radiata (chorion, or membrane layers surrounding the 

 oocyte) of the atretic oocytes was indistinct and irreg- 

 ular in appearance. It was not possible to accurately 

 identify atretic oocytes in frozen, thawed, or poorly 

 preserved ovaries. Atretic oocytes were not included 

 in counts of advanced yolked oocytes used to estimate 

 fecundity. To estimate rates of atresia, we recorded the 

 number of a-atretic yolked oocytes in the random 

 sample of 30 stage-3 oocytes measured. The number 

 of a-atretic advanced yolked oocytes divided by 30 was 

 used as an index of the intensity of atresia in all females 

 used for fecundity estimation. 



