Nanmatsu et al : Reproductive biology of Dexistes nkuzemus 



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Fecundity 



The potential fecundity of group-synchronous spawning 

 fish can be determined prior to the spawning season 

 (Takano, 1989). In Rikuzen sole, oocyte-stage composi- 

 tion became discontinuous beyond the late vitellogenic 

 maturity phase, when a gap was found between secondary 

 or tertiary yolk stages and the late perinucleolus stage. 

 Oocyte diameter distributions in late vitellogenic maturity 

 phase ovaries revealed that oocytes could be divided into 

 small (less than 200 jim) and large (more than 300 jmi) 

 scale groups. Taking into account the oocyte diameters 

 observed in the histological sections, small-scale group 

 oocytes corresponded to cortical alveoli or less advanced 

 stage oocytes, whereas larger oocytes corresponded to 

 secondary yolk or more advanced stage oocytes. 



The occurrence of atretic oocytes was highest in May 

 and became lower as the season progressed until the 

 end of the spawning season. These phenomena may cor- 

 relate with both annual feeding cycles and maturation. 

 Ogasawara and Kawasaki (1980) showed that in the 

 Sendai Bay population, Rikuzen sole feed actively for 

 a few months after spawning and then feed passively 

 for the next few months. Gut-content weight began to 

 increase again in June. In our study area, BC increased 

 from about May, corresponding to the time when the 

 oocytes begin to mature rapidly. As described before, 

 vitellogenesis in this species takes a long time. Because 

 oocytes are metabolically active in the season when the 

 energetic condition of Rizuzen sole is still recovering, a 

 higher proportion of atretic oocytes occur during this 

 period. 



Potential fecundity may not correspond to annual fe- 

 cundity because of the presence of atretic and residual 

 oocytes (Witthames and Greer Walker, 1995; Kurita et 

 al., 2003). Therefore, we examined the potential fecun- 

 dity of fish in the late vitellogenic maturity phase just 

 before the spawning season. The frequency of occurrence 

 of atretic oocytes may be underestimated because these 

 oocytes have shrunk and are smaller than the maturing 

 yolked oocytes. In addition, atretic oocytes may occur in 

 the ovaries during the premature maturity phase. How- 

 ever, in our samples a low percentage of atretic oocytes 

 were observed. Only a small percentage of premature 

 ovaries were found on or before the reproductive season; 

 this finding seems to indicate that the oocytes of this 

 species take a short time to develop from the tertiary 

 vitellogenic stage to maturation. These results make 

 clear that potential fecundity differs from annual fecun- 

 dity, but the extent of this difference was nevertheless 

 relatively small in the samples. Moreover, ovulated, but 

 not spawned oocytes were observed in the maturing 

 and spent ovaries; these oocytes have the potential to 

 cause an overestimation of annual fecundity. However, 

 the frequency of ovaries with residual ovulated oocytes 

 was small; therefore, such oocytes may not seriously 

 influence annual fecundity, as with the case of Dover 

 sole (Microstomus pacificus) (Hunter et al., 1992). 



Vitellogenesis in American plaice was seen to begin 

 soon after spawning ( Zamarro, 1992), as with Rikuzen 



sole. Separation of oocyte diameter in this species oc- 

 curs approximately three months before the start of 

 the spawning season. In Rikuzen sole, potential fe- 

 cundity was determined as being much closer to the 

 reproductive season. Reproduction occurred from early 

 September, but occurrence of the maturity phase in 

 August varied largely among individuals. The potential 

 fecundity of almost all fish (85%) could be determined 

 until August. These results indicate that certain condi- 

 tions and measurements are necessary when examining 

 potential fecundity without histological methods. 



Potential fecundity became determinate for the first 

 time at maturity during the late vitellogenic phase. 

 Some of the maturity phase ovaries contained second- 

 ary yolk-stage oocytes and all contained tertiary yolk- 

 stage oocytes. The secondary yolk-stage oocytes ranged 

 in diameter from 260 to 440 jim — a range that does not 

 overlap with the diameter range of primary yolk-stage 

 oocytes (180-220 f<m). Therefore, to measure potential 

 fecundity without histological observations, it is first 

 necessary to clarify the division of oocyte diameter into 

 large- and small-scale groups in order to identify de- 

 terminate fecundity. In ovaries that contain large- and 

 small-scale oocytes, only oocytes greater than 260 fim 

 in diameter but that do not experience ovulation be- 

 tween May and August are targets for potential fecun- 

 dity measurements. This method will make it easier to 

 measure the potential fecundity of this population in 

 the future. 



Potential fecundity increased curvilinearly with SL. 

 The body size of the females continued to grow even 

 after maturation; the most fecund individual had 15 

 times more maturing oocytes than the least fecund one. 

 Potential fecundity also increased until age <6+ years 

 but decreased in individuals at 27+ years. One reason 

 that older fish have less potential fecundity is a lesser 

 energetic condition with senescence. Fecundity has been 

 also reported as declining with age in other fish. As 

 American plaice in the tail of the Grand Bank of New- 

 foundland become older, the number of eggs produced 

 by females decreased (Horwood et al., 1986). Orange 

 roughy, mature first at 25 years old and live for more 

 than 100 years; their fecundity increases from an age 

 of 25 to 60 years old, then decreases in individuals 

 aged over 60 years old (Koslow et al., 1995). Fecundity 

 is positively correlated with BC in the orange roughy. 

 The oldest Rikuzen sole to appear in that study area 

 was 10+ years (Ishito, 1964) and body growth almost 

 finished by age 6+ years (Fig. 6). In addition, both the 

 BC and relative fecundity of fish over 7+ years were 

 lower than those of fish from 4 to 6+ years. 



Spawning stock biomass (SSB) has been used to ex- 

 amine the relationship of spawning fish and recruit- 

 ment; however, recent studies have indicated that SSB 

 is not always linked to reproductive potential, mainly 

 because age composition and nutrient conditions also af- 

 fect fecundity (Hunter et al., 1985a; Trippel et al., 1997; 

 Marshall et al., 1998). Our study shows that relative 

 fecundity is positively correlated with body length. In 

 addition, both relative and potential fecundity increase 



