HUNTER ET AL.: SPAWNING FREQUENCY OF SKIPJACK TUNA 



nuclei, and lymphocytes, and has a more filamen- 

 tous composition. 



Nonspawning (mature) 



Ovaries with many yolked oocytes and containing 

 no hydrated oocytes or postovulatory follicles were 

 classified as nonspawning. They may contain post- 

 ovulatory follicles in advanced stages of degenera- 

 tion which cannot be readily distinguished from late 

 stage corpora atretica. Elapsed time from spawn- 

 ing was more than 24 h. Also classified as nonspawn- 

 ing (mature) were females in postspawning condi- 

 tion. The ovaries of such females contained no 

 yolked oocytes, but atretic follicles (beta stage) were 

 present indicating that the ovary was active recently 

 (see next section). 



Immature 



Ovaries containing no yolked oocytes and no a 

 or ft stage atretic structures were classed as im- 

 mature. 



Atretic States 



It is well known in seasonal spawning fishes that 

 a low incidence of atresia (resorption of the oocyte 

 and its follicle) occurs throughout the spawning 

 season, but it becomes marked as the spawning 

 season closes and the remaining advanced oocytes 

 in the ovary are resorbed. During the initial atretic 

 phase (a), the oocyte is resorbed and any yolk 

 globules are broken down and resorbed by the 

 hypertrophying granulosa cells of the follicle (Bret- 

 schneider and Duyvene de Wit 1947; Lambert 1970). 

 In the next stage (/?), all the yolk is gone, and there 

 remains a small, rather compact structure with one 

 or more cavities. The structure is composed of 

 granulosa and theca cells with penetrating blood 

 vessels. Further stages of follicle resorption have 

 been described by the same authors, but the inci- 

 dence and extent of a and (5 stages have proven to 

 be the most useful in the classification of atretic 

 states of ovaries (Hunter and Macewicz 1985b). The 

 characteristics of a and p atretic structures are 

 described and illustrated for northern anchovy by 

 Hunter and Macewicz (1985b) and a atretic oocytes 

 of skipjack tuna are essentially similar. However, 

 P atresia differs from northern anchovy in contain- 

 ing numerous spherical vacuoles scattered through- 

 out the follicle. The vacuoles are the remnants of 

 the oil droplet which takes longer than yolk to resorb 

 and in H&E sections appear empty. Occasionally, 



a large beta stage follicle may be seen in which the 

 granulosa and thecal cells have proliferated. 



Listed below are the characteristics of the four 

 atretic states we used to classify skipjack tuna 

 ovaries along with what is known regarding the 

 spawning potential of northern anchovy classed in 

 these states. 



Atretic State 



Yolked oocytes present, with no a atresia of 

 yolked oocytes; p stage atresia may be present, but 

 it cannot be distinguished with certainty from late 

 stage postovulatory follicles (>24 h old). Female 

 northern anchovy in this state have a high poten- 

 tial of spawning. 



Atretic State 1 



Less than 50% of the yolked oocytes are in the 

 a stage of atresia. The frequency of spawning for 

 northern anchovy classed in this state is less than 

 half of that for females classed in atretic state 0. 

 Thus, atretic state 1 indicates a decline in spawn- 

 ing rate. 



Atretic State 2 



Fifty percent or more of the yolked oocytes are 

 in the a stage of atresia. The frequency of spawn- 

 ing for female northern anchovy classed in this state 

 is very low and indicates that cessation of spawn- 

 ing is imminent. 



Atretic State 3 



Ovaries contain p stage atresia and no yolked 

 oocytes. Such fish have completed their spawning 

 season since they have no yolked oocytes. The pres- 

 ence of p stage atresia indicates that oocyte resorp- 

 tion has taken place and thereby distinguishes such 

 recently mature but postspawning fish from imma- 

 ture females. In northern anchovy, atretic state 3 

 may persist for 30 d. 



RESULTS AND DISCUSSION 



All postovulatory follicles in sea-caught skipjack 

 were less degenerated than those observed in a 

 laboratory specimen examined 24 h after spawning, 

 indicating that all of those in the sea collections were 

 <24 h old. The fraction of mature females with post- 

 ovulatory follicles <24 h old ([55 + 18J/86, Table 2) 

 was 0.85 with the standard deviation estimated to 



899 



