Nanmatsu et al.: Reproductive biology of Dexistes rikuzemus 



637 



An (86) (60) (84) (27) (114) (77) (64) (79) (66) (75) (129) (80) (90) 



Jan Feb Mar Apr May Jun 



Jul 



Month 



Aug early late Oct Nov Dec 

 Sep Sep 



Figure 2 



Annual changes in the gonadsomatic index (GSI) and body condition IBC) values 

 of female Rikuzen sole {Dexistes rikuzenius). Solid and open circles show the mean 

 values of GSI and BC, respectively. Vertical bars represent the standard deviations 

 of these means. Sample numbers are shown in brackets. 



within a day after each catch for histological observa- 

 tions and age determination, respectively. The otoliths 

 were washed with distilled water and left to dry until 

 preparation for age determination. Ovaries were fixed in 

 10% buffered formalin for 24 hours. The middle portions 

 of eyed-side ovaries of 309 specimens were extracted, 

 dehydrated, embedded in paraffin, sectioned at 8 f<m, 

 and stained with Mayer's hematoxylin and eosin (HE) 

 and periodic acid Schiff (PAS). 



Prepared sections were examined under a light micro- 

 scope. The oocytes were then divided into eight stages ac- 

 cording to the guidelines of Yamamoto (1956). Postovula- 

 tory follicles (POFs), which indicate spawning experience, 

 were also examined. New POFs are easily identifiable, 

 but those that have degenerated are difficult to distin- 

 guish from atretic follicles. In our study, only those that 

 could be easily identified were defined as POFs. Atretic 

 oocytes, namely advanced yolked oocytes that have been 

 resorbed into the ovaries, were also determined; simi- 

 larly, only those easily identifiable were defined as atretic 

 oocytes. The percentage of advanced oocytes that were 

 atretic was determined monthly for 10 randomly selected 

 2-7+ year-old fish (body size range: 143-210 mm SL). 



Maturity was classified by the stage of the most ad- 

 vanced oocyte and the presence of POFs. By observing 

 maturity and advanced oocyte diameter, we tested 15 

 ovaries for possible differences in oocyte development 

 between anterior, middle, and posterior positions in the 

 eyed-side ovary lobe, and between eyed-side and blind- 

 side ovary lobes. 



Oocyte diameter distributions in the late vitellogenic 

 maturity phase were examined; the reason this maturity 



phase was selected is described in the "Results" section. 

 The diameters of 50 randomly selected oocytes, extracted 

 from the middle portions of the ovaries, were measured 

 under a dissecting microscope to the nearest 20 /.im. 

 Potential fecundity was estimated with the gravimetric 

 method by using ovaries in the late vitellogenic maturity 

 phase. Extracted ovaries were rinsed and then weighed 

 to the nearest 0.0001 g, and only developing oocytes, 

 whose size is also described in the "Results" section, 

 were counted. 



Age was determined for all fish samples. Blind-side 

 otoliths were used for the analyses according to the 

 methods of Ishito (1964). The lateral surfaces of the 

 otoliths were polished with 1500-grit sand paper until 

 the transparent zones were visible. Ishito (1964) re- 

 vealed that one transparent zone is formed at the edge 

 of the otolith each winter and suggested that this may 

 be regarded as an annual mark. However, the most 

 interior ring appears when fish are aged 0+ (Ishito, 

 1964); therefore the number of transparent zones minus 

 the 0-year-old zone was the formula used for aging, and 

 the relationship between age and potential fecundity 

 was analyzed. 



Results 



Annual changes in gonadosomatic index 

 and body condition 



The annual changes in gonadosomatic index (GSI) and 

 body condition (BC) are shown in Figure 2. The GSI was 



