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Fishery Bulletin 108(2) 
Fork length (cm) 
Figure 2 
(A) Total percentage of sex-misidentified longspine ( Sebastolobus altivelis) 
and shortspine thornyheads (Sebastolobus alascanus) determined by compar- 
ing the gross morphological features of gonads to a section of each gonad 
subsequently stained and viewed microscopically, by size (fork length, cm); 
and (B) the absolute percent error in identifying the sex of thornyheads 
after accounting for the portion of the total error that is cancelled out by 
balancing the number of sex-misidentified males against the number of 
sex-misidentified females. 
significant differences in mean fork length for longspine 
thornyheads between the different areas (ANOVA: df= 
858, F=0.3, P=0.9), but for shortspine thornyheads, 
size was significantly larger in the Monterey and U.S.- 
Vancouver areas (ANOVA: df=1140, F= 4.7, P=0.0009), 
and large fish in the Monterey area had a higher rate 
of individuals for which sex was incorrectly determined 
than similar size shortspine thornyheads in the U.S.- 
Vancouver area. 
Discussion 
This study provides guidance for a minimum size limit 
below which sex of thornyheads should not be deter- 
mined at-sea because of high error rates. High quality 
biological information is important for management and 
modeling of thornyhead populations along the U.S. west 
coast (Fay, 2005). Fishery scientists need estimates of 
sex ratio for fish populations because shifts in these 
values can indicate overfishing on one sex or the other 
due to selective gear, differential growth rates, segrega- 
tion by sex or any combination of these (Cochrane, 2009). 
In previous studies of the reproductive biology of 
thornyheads, the longspine thornyhead spawning was 
determined to begin in January, peak in February and 
March, and continue at least through April (Wakefield, 
1990; Pearson and Gunderson, 2003; Cooper et al., 
2005). Shortspine thornyheads spawn between Decem- 
ber and May along the U.S. west coast. The onset of 
sexual maturity occurs at 17-19 cm total length (10% 
mature females) in both species and 90% are mature 
at 25-27 cm (Pearson and Gunderson, 2003). Sex of 
smaller thornyheads is difficult to determine, particu- 
larly during the summer, because of the small size of 
the gonads — size being a function of the annual spawn- 
ing cycle. Pearson and Gunderson (2003) noted that 
of 36 longspine thornyheads designated as immature 
