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Fishery Bulletin 103(4) 



although they had no other vitellogenic oocytes. There 

 are three potential hypotheses to explain the fate of 

 these primary yolk oocytes. One explanation is that the 

 oocytes are spawned in the current reproductive season. 

 Maddock and Burton (1999) showed that in American 

 plaice (Hippoglossoides platessoides), a group-synchro- 

 nous spawner, the size frequency of oocytes during the 

 prereproductive season was not continuous, whereas 

 during the reproductive season the size frequency was 

 continuous. The reason for this difference was that 

 during the reproductive season cortical alveoli stage oo- 

 cytes are larger than those during the prereproductive 

 season. It is unclear, however, whether these cortical 

 alveoli oocytes will be spawned during the reproductive 

 season (Maddock and Burton, 1999). Although similar 

 to those of the American plaice, all Rikuzen sole ovaries 

 with primary yolk-stage oocytes contained no secondary 

 or more advanced stage oocytes. In addition, oocytes 

 that would be spawned in the current reproductive sea- 

 son developed beyond the secondary yolk stage before 

 the beginning of the reproductive season. Therefore, 

 primary yolk-stage oocytes occurring late in the repro- 

 ductive season might not be spawned that season. 



Primary yolk-stage oocytes were found from October 

 to August (the late reproductive to vitellogenic season) 

 (Table 3). From October to December only a small per- 

 centage of individuals possessed oocytes in this stage, 

 whereas their ratio increased from January to April. 

 These results indicate that females begin vitellogenesis 

 for the next reproductive season shortly after spawning. 

 This hypothesis is supported by reports that the vitel- 

 logenesis of flatfishes takes a long time (Yamamoto, 

 1954, 1956; Ishida and Kitakata, 1982; Zamarro, 1992; 

 Harmin et al., 1995). 



Atretic oocytes were present in low proportions from 

 March to April and in high proportions in May. The 

 mature phase of ovaries with atretic oocytes did not 



differ from that of ovaries without atretic oocytes. In 

 addition, developmental stage did not differ between 

 atretic and normal oocytes in any ovary. Therefore, it 

 seems that the primary yolk-stage oocytes observed late 

 in the reproductive season will not selectively degener- 

 ate, rather they will be spawned. 



Decisions regarding maturity and age at maturity 



POFs were present from September to January and all 

 specimens caught during this period had either oocytes 

 in the advanced yolk stage or POFs in their ovaries. All 

 specimens caught between November and December con- 

 tained ovaries with POFs, whereas they were observed 

 only in a small percentage of the specimens caught in 

 January. The spawning season lasted from September 

 to December, but almost all spawning had finished by 

 October. These results indicate that the duration until 

 resorption of the POFs ranges from a few weeks to two 

 months. For a few weeks immediately following spawn- 

 ing, the presence of POFs can be used as a criterion for 

 the differences between post- and prespawning individu- 

 als. This feature is consistent with that of other flatfish 

 in which POFs degenerate within one or two months 

 (Barr, 1963; Janssen et al., 1995). 



By noting the presence of POFs and advanced yolked 

 oocytes, we were able to classify individuals as mature 

 or immature. All but one individual caught during the 

 reproductive period were maturing or had spawned. 

 The body size of the mature females ranged from 114 

 to 237 mm SL, which corresponded to an age from 1 to 

 8+ years, respectively, whereas the immature female 

 (131 mm SL) was age 1+. These results indicated that 

 most female Rikuzen sole in this population mature at 

 2 years old, or at the latest at 3 years old, and spawn 

 every year after maturation. Almost all (99.5%) fish 

 caught commercially are adult individuals. 



