- Because of large declines in abundance of many Eastern Pacific rockfish populations (Genus Sebastes), there has been an increasing effort to improve our understanding of the role of spawning population characteristics and individual reproductive success in recruitment variability and population dynamics. Current methods for assessing sex and maturational status in rockfishes are lethal, so fishery managers would benefit from a non-lethal method to assess the reproductive potential of rockfish populations. I developed a biochemical assay to determine sex and stage of female maturity in black rockfish (Sebastes melanops) using a blood plasma indicator vitellogenin (VTG). The specific objectives of this research were to: 1) describe the annual hormonal and gonadal changes associated with reproduction and 2) describe the timing of reproduction in female black rockfish of different age classes.
Estradiol-17β (E2) and VTG concentrations were compared with histologically determined ovarian stages of female maturity (i.e. pre-vitellogenic, early vitellogenic, mid-vitellogenic, post-vitellogenic, oocyte maturation, and post-spawned) from 325 female black rockfish sampled by hook-and-line from the central Oregon coast from September 2003 to January 2005. I also measured E2 and VTG in male fish from the same time period. VTG concentrations detected in field-collected blood samples reflected the seasonal cycle reported in previous studies. Mean concentrations of VTG in mid-vitellogenic, post-vitellogenic, and females undergoing oocyte maturation can be used to distinguish between pre-vitellogenic females and early vitellogenic females during the spawning season (P<0.05). Females can be distinguished from males based on VTG concentrations (P<0.0001), with the exception of pre-vitellogenic and post-spawned females (immature fish and mature female fish in non-reproductive summer months). Mean E2 concentrations were not as accurate for identification of sex and each individual stage of female maturity, although mid-vitellogenic and post-vitellogenic females had significantly higher concentrations of E2 than pre-vitellogenic females (P<0.05). Mean concentrations of E2 in mid-vitellogenic females, post-vitellogenic females, and females undergoing oocyte maturation were higher than in males (P<0.0001).
A Discriminant Function Analysis (DFA) identified VTG as the best predictor for predicting sex from plasma samples, with, 82% of fish correctly classified by sex. Of the 100 females and 30 males examined, 68% of females and 97% of males were correctly classified. The DFA choose plasma VTG, total length, and weight as the best predictors of stage of maturity in females and some stages could be distinguished from each other with ≈65% accuracy.
The second goal of this research was to use the biochemical assay to investigate the differences in timing of reproduction in young and older females in this species. In the fall (September-November) females age 5-8 made up 43 % of early and mid-vitellogenic (stage 3 and 4) females, where as, age 9+ females made up 100% of the females that were post-vitellogenic (stage 5) and undergoing oocyte maturation (stage 6). Females’ ages 6-8 captured in the winter months (December-February) made up 90% of the mid- and late vitellogenic (stage 4 and 5) and 100% of post-spawned (stages 7 and 8) fish. In the spring (March-May) 40% of post-spawned females were age 8 and 60% were age 9+. These results suggest that older females have more of a protracted spawning period than younger fish. Females from all age categories showed a similar trend of hormone and protein cycling. Concentrations of E2 and VTG increased throughout September-December, peaked in February, decreased and remained low until the end of August. Concentrations of E2 for early vitellogenic (stage 3) fish of the age 9+ were significantly higher than in females ages 3-5. Females’ age 9+ undergoing oocyte maturation (stage 6) had higher concentrations of E2 than females ages 6-8. Early vitellogenic (stage 3) and mid-vitellogenic (stage 4) females ages 6-8 and 9+ had higher concentrations of VTG than in females ages 3-5. Small samples sizes were prohibitive in comparing VTG and E2 concentrations between all age groups and stages of maturity, and I did not find conclusive evidence of differences in seasonal VTG or E2 levels among age groups. Potentially, this newly developed vitellogenin assay may be used for other rockfish species, providing a non-lethal assessment tool to differentiate between sex and stage of maturity in females.