Hunter et al : Fecundity, spawning, and maturity of Microstomus pacificus 



1 1 



declined over the spawning sea- 

 son as females expended their 

 stock of advanced yolked oocytes. 

 We fitted a weighted Weibull 

 function to the combined Califor- 

 nia and Oregon data, yielding the 

 equation 



0.656 e 



-(111.5) 



(Eq. 1) 



0.8 



0.6 



I 



u 

 o 

 o 



z 

 < 



UJ 



s 



1.2 



1.0 



0.8 



0.6 



20 



where t is days elapsed since 1 

 November; P, the fraction active, 



is weighted by , and 



P(l-P) 

 pseudo r^ is 0.96. According to 

 the equation, the percentage of 

 females with active ovaries de- 

 clined from 65% at the onset of 

 the spawning season (about 6 

 December) to 40% by the end of 

 January; by the end of February 

 only 18% of the females had ac- 

 tive ovaries (Fig. 3, upper left). 



In California and Oregon, the 

 mean diameter of the stock of ad- 

 vanced yolked oocytes increased 

 steadily from December through 

 April (Fig. 4). Thus reproductive- 

 ly active females continued 

 vitellogenesis throughout most of 

 the spawning season. By March 

 or April the average advanced 

 yolked oocyte is closer to the 

 minimum size at which hydration 

 begins (diameter 1.35 mm). Thus 

 Dover sole may be able to spawn 

 at a higher rate late in the 

 spawning season, because less 

 yolk would have to be added to 

 the advanced oocytes for them to 

 attain the size at hydration. 



Only 10 females taken in No- 

 vember-December were classed 

 as spawning on the basis of their 

 ovarian histology (Oregon and California data com- 

 bined, N 949). They comprised only 1.0% of all females 

 with preserved ovaries taken during this time and only 

 1.7% of those classed as active. Clearly, spawning is 

 just beginning in November-December in California 

 and Oregon waters. The spawning rate index increased 

 from 1.3% in November-December to 12% by early 

 February; it accelerated at the end of the season with 

 spawning females comprising about 70% of all active 

 females (percent calculated by the trend line in Fig. 3, 



Hydration 



Z 

 < 



5 

 ui 



o 

 o 

 o 



1.2 



1.0 



40 

 Dec 



60 



I 



80 

 Jan 



100 

 Feb 



120 



I 



160 

 Apr 



180 200 



I May 



Hydration 



I 



40 

 Dec 



60 



I 



80 



Jan 



100 

 Feb 



120 



I 



140 

 Mar 



160 



Apr 



180 



May 



200 



ELAPSED TIME (days) 



Figure 4 



Increase in mean oocyte diameter (D) of the advanced yolked oocytes of Dover sole 

 Microstomus pacificus from Oregon (top) and California (bottom) as a function of elapsed 

 time (T) since 1 November. Data from different spawning seasons are combined; shaded 

 area indicates size range of oocytes at the onset of hydration; trend lines are Oregon 

 (D = 0.742 + 0.00282T, r' 0.658, A^ 195) and California (D = 0.761 + 0.0022T, r- 0.41, 

 N 365). 



upper right). Thus at the end of the season, most of 

 the females with active ovaries had spawned recently. 

 We believe this sharp increase in the index near the 

 end of the season is evidence for a seasonal increase 

 in spawning frequency. 



A late seasonal increase also existed in the occur- 

 rence of multiple spawning stages within the same 

 ovary. As many as five different past or potential 

 spawning stages could be distinguished histologically 

 in the same ovary: two stages of postovulatory folli- 



