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



127 



season. At this time, it is an arbitrary choice to con- 

 sider as mature all females with vitellogenic ovaries or 

 only those with advanced yolked oocytes. Our analysis 

 showed that this arbitrary decision had a pronounced 

 effect on ML50 estimates. Thus the criteria for matur- 

 ity estimates should be precisely specified. It is par- 

 ticularly important to specify the minimum level of 

 oocyte development necessary for a female to be con- 

 sidered as mature. Our preferred definition of matur- 

 ity included females in the early stages of vitellogen- 

 esis with yolked oocytes as small as 0.18mm diameter, 

 and also included some females without vitellogenic 

 oocytes (maturity IV, Table 16). Those females without 

 vitellogenic oocytes had ft atresia in the ovary. We 

 believe that the presence of some /3 atresia is an in- 

 evitable consequence of the resorption of an active 

 ovary or ovulation. 



No discussion of sexual maturity would be complete 

 without mentioning the gross anatomical systems used 

 to classify ovaries, because they are the chief method 

 used by fishery biologists to measure sexual maturity 

 in marine fishes. Using gross anatomical criteria, we 

 accurately separated active ovaries (advanced yolked 

 oocytes present) from inactive ovaries (no advanced 

 oocytes) with classification errors of 1-12%. Deter- 

 mining sexual maturity is a far more difficult task, 

 however. Identification of mature females using gross 

 anatomical methods has the same problems with post- 

 spawning and early vitellogenesis criteria as histo- 

 logical methods, but the potential for bias is greater. 

 Anatomical criteria are less accurate and may be 

 detectable for shorter periods than histological ones. 

 For these reasons, differences between maturity 

 studies should be interpreted with caution, especially 

 when done by different observers, or with different 

 methods, or when sampling at different times of the 

 year. Many investigators have not been particularly 

 careful to restrict sampling to early in the spawning 

 season. The tendency will be to overestimate the ML50 

 using anatomical methods, especially when samples are 

 taken midseason. 



In an earlier paper on Dover sole. Hunter et al. (1990) 

 concluded that size at 50% mature in Dover sole from 

 central California in the 1980s differed from that of 

 Dover sole in northern California in the late 1940s as 

 determined by Hagerman (1952). Although a statistical 

 difference existed between these two data sets, we are 

 inclined to dismiss this difference, since it could be due 

 to differences in criteria and sampling times. Similar- 

 ly, Yoklavitch and Pikitch (1989) speculated that size 

 at 50% maturity of Oregon Dover sole has changed 

 because their estimate of maturity differed from Harry 

 (1959). We beheve that this difference also could easily 

 be due to differences in criteria and timing of sampling. 

 Our analysis of histological criteria for maturity clearly 



shows that differences in criteria or timing of sampl- 

 ing can produce differences in the ML50 as large as 

 any of those seen in the Dover sole literature. 



Acknowledgments 



We appreciate and thank M. Yoklavich and E. Pikitch 

 for providing original data and supplying some speci- 

 mens, and W.W. Wakefield for providing some speci- 

 mens. We thank all on shipboard who helped collect 

 Dover sole ovaries: E. Lynn, W. Flerx, R. Dotson, 

 R. Leong, E. Acuna, and D. Squires. We thank all 

 others who served on the scientific crews and the crews 

 of NOAA ships David Starr Jordan and Miller Free- 

 man; they contributed greatly to the success of the 

 cruises. Processing of laboratory specimens and estima- 

 tion of fecundities were assisted by W. Kicklighter, 

 M. Drawbridge, R. Leong, E. Lynn, D. Ramon, and 

 S. Swailes. Computer programs were written by 

 C. Vedovato, R. Young, and J. Butler. L. Jacobson pro- 

 vided suggestions on the modeling of sexual maturity. 

 Illustrations were produced by R. Allen and H. Orr. 

 K. Schaefer, J. Zeldis, and an anonymous reviewer 

 reviewed the manuscript. 



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