126 



Fishery Bulletin 90|l). 1992 



protracted spawning season. Vitellogenesis 

 does not cease when spawning begins, but 

 rather it continues throughout most of the 

 season as the advanced stocks of yolked 

 oocytes are matured and spawned. Spawn- 

 ing frequency appears to increase near the 

 end of the season. This may cause a higher 

 daily production of eggs by the population 

 in late-March or April than in February, 

 even though fewer females have active 

 ovaries in April. 



To estimate reproductive effort of Dover 

 sole, we calculated the hypothetical weight 

 of the ovary when the entire advanced stock 

 of oocytes had completed vitellogenesis and 

 hydration had begun. The weight is hypo- 

 thetical because a Dover sole ovary never 

 contains a full complement of completely 

 yolked oocytes, since vitellogenesis of the 

 smaller advanced yolked oocytes continues 

 after a female begins spawning. To compute 

 the hypothetical weight, we assumed that 

 all oocytes completed vitellogenesis when 

 their average diameter was 1.5mm. Hydra- 

 tion begins when the advanced yolked 

 oocytes have a mean diameter of 1.3-1.7 

 mm. We estimated the gonad weight of a 

 lOOOg female with oocytes having a mean 

 diameter of 1.5 mm, using an equation in 

 which gonad weight was expressed as a 

 function of fish weight and volume of the 

 average advanced oocyte (1.5 mm diameter 

 has a volume of 1.77 mm^; California plus 

 Oregon data; Table 3). The ovary was esti- 

 mated to weigh 144g, or about 14% of the 

 body weight. In other words, the annual 

 reproductive effort of Dover sole was about 

 14% per year, and this effort was distrib- 

 uted over about nine spawnings averaging 

 about 1.6% of their body weight per spawning. Gonad 

 weight was considered to be a measure of reproduc- 

 tive effort by Gunderson and Dygert (1988); but they 

 did not adjust the gonad weight for the full complement 

 of yolk, and consequently their estimates are not com- 

 parable to these. 



Assessment of sexual maturity 



Our estimates of length at 50% mature (ML50) were 

 higher when females were taken during the spaviming 

 season than when they were sampled before spawning 

 began, regardless of the histological criteria used. Thus, 

 during the spawning season ovaries of some post- 

 spawning females had regressed far enough that they 

 were histologically indistinguishable from immature 



X 



Lli 

 Q 



< 



a. 



O 



z 



3 5 10 15 2 



MEAN DIAMETER OF ADVANCED STOCK (STAGE 3) (mm) 



Figure 12 



Optimal range of mean diameter of advanced yolked oocytes (stage 3) for 

 determining potential annual fecundity of Dover sole (shaded area, 0.86- 

 1.1mm). Open circles, Student's t as a function of the minimum mean 

 diameter of stage-3 oocytes included in the fecundity data set: when t » 2 

 (P 0.05), the mean oocyte diameter had a positive correlation viith fecun- 

 dity, indicating the stock of yolked oocytes for the season was not fully 

 recruited into stage 3. Spawning rate index (filled circles) for females used 

 in fecundity estimates; index = when no females show signs of past or 

 imminent spawning, and index = 1 when all females have postovulatory 

 follicles or hydrated oocytes. Upper panel indicates diameter range of 

 oocytes in three yolked oocyte stages and hydrated oocytes. Only stage-3 

 oocytes were used to estimate potential annual fecundity. 



females. This finding has two important implications: 

 First, it indicates that even the broadest histological 

 criteria, based on analysis of H&E sections, will not 

 identify all postspawning females; second, it means that 

 estimates of length or age at first maturity should 

 always be conducted prior to the onset of spawning, 

 when postspawning females with highly regressed 

 ovaries are rare. 



Another limit to our ability to assess sexual matur- 

 ity is that we do not know how many of the females 

 that begin vitellogenesis actually complete it during 

 the current reproductive season. Dover sole ovaries 

 with oocytes in the early vitellogenic stage occurred 

 throughout the spawning season as well as before it 

 began, indicating some females that begin vitellogen- 

 esis may not reach sexual maturity in the current 



