Sof et a l.: Gonadal development and changes in plasma reproductive steroids in Pleuronectes vetulus 
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Figure 4 
Mean +SE of(A) gonadosomatic index (GSI), and (B) plasma steroid con- 
centrations in female English sole at each stage of oocyte development. * 
denotes significantly higher levels than levels observed in regressed and 
spawned out fish (AN OVA, Fisher’s PLSD, 95% Cl). Numbers in parenthe- 
ses represent animals at each ovarian developmental stage. 
that females at 260 mm were three years 
of age, and males at 195 mm were two 
years of age. Other studies have shown 
that females first mature at three years 
of age, whereas males mature at two 
years of age (Smith, 1936; Harry, 1959). 
The pattern and timing of ovarian de- 
velopment observed in our study were 
similar to previous reports on English 
sole (Garrison and Miller, 1982; Kruse 
and Tyler, 1983; Johnson et al., 1991; 
Fargo and Tyler, 1994). Gonadal recru- 
descence in Puget Sound English sole 
began in early fall (September-October), 
vitellogenesis and spermiogenesis peaked 
in early winter (December-January), and 
spawning took place during late winter 
months (February and March). However, 
our findings suggest that small propor- 
tions of animals did enter vitellogenesis 
early; vitellogenic sole were found as 
early as July, whereas spawned out sole 
were found as early as October. In an 
earlier study that investigated oocyte 
development in female English sole 
(Johnson etah, 1991), the earliest spawn- 
ing females were found in January. This 
discrepancy may be due to the fact that 
the number of fish collected in early fall 
in the first study (Johnson et al., 1991) 
was small compared with the present 
study; therefore, early spawning females 
may not have been observed. Neverthe- 
less, changes in reproductive parameters 
(GSI, and plasma sex steroids) were simi- 
lar to those described by Johnson et al. 
( 1991 ) and to the reproductive endocrine 
cycles described for other species of fe- 
male flatfish (e.g. Liu et al., 1991; 
Methven and Grim, 1991; Harmin et al., 
1995). Generally, regressed fish had low 
values for the various reproductive pa- 
rameters. The onset of vitellogenesis was 
accompanied by an increase in GSI and 
plasma sex steroid levels. After spawn- 
ing, these levels declined to levels similar to those ob- 
served in regressed animals. Reproductive parameters 
at the two residential sites (Pilot Point and Tulalip Bay) 
were similar and supported the findings of Laroche and 
Richardson (1979) who observed no apparent latitudi- 
nal trend in the time of spawning in sole from the Or- 
egon coast. Reproductive parameters in fish from Uni- 
versity Point, however, were higher because a major- 
ity of sole at this site were undergoing final matura- 
tion or spawning (Johnson et al., 1991). 
It is well established that E2 stimulates the liver 
to produce vitellogenin (Ng and Idler, 1983), and in 
this study, as in Johnson et al. (1991), vitellogenesis 
in female English sole coincided with a rise in plasma 
E2 concentrations. Plasma concentrations of T, a bio- 
synthetic precursor to E2 (Kagawa et al., 1982), 
changed in parallel with the plasma E2 concentra- 
tions, although plasma T levels were lower than 
plasma E2 levels at all stages of gonadal develop- 
ment. Higher plasma T levels were observed in 
