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Fishery Bulletin 108(2) 
Figure 1 
Distribution and relative abundance (kg/ha) of (A) longspine thornyhead ( Sebastolobus altivelis) and (B) shortspine 
thornyhead ( Sebastolobus alascanus) determined from the 2003 Northwest Fisheries Science Center west coast ground- 
fish trawl survey. SD = standard deviation. 
length 14.3 cm). Longspine thornyhead sex was misiden- 
tified by visual examination in 23.1% of males and 22.4% 
of females, and for shortspine thornyheads, in 9.4% of 
males and 9.3% of females. 
Average lengths of longspine and shortspine thorny- 
heads (females, males, and total) for which sex was 
misidentified were significantly lower than the lengths 
for fish whose sex was correctly assigned (Table 1). 
For shortspine thornyheads, the average length of sex- 
misidentified females was significantly smaller than 
that of males (ANOVA: df=6, F- 5.5, P= 0.02). Similar 
tendencies were seen for longspine thornyhead lengths 
but the results were not significant (Table 1). 
Determining sex for longspine thornyheads greater 
than 22 cm would eliminate approximately 80% of the 
overall error rate, but would also eliminate 50% of the 
fish whose sex was correctly determined. By proposing 
18 cm as the minimum size for examining longspine 
thornyheads in the field we eliminated approximately 
65% of the incorrectly sexed fish, while retaining >70% 
of those correctly sexed (Fig. 2A). On average, the sex 
of 50.5% of longspine thornyheads ranging in size from 
11 to 17 cm was incorrectly determined. This average 
dropped to approximately 10% for longspine thorny- 
heads at lengths from 18 to 34 cm. A similar result was 
seen for shortspine thornyheads (Fig. 2A). The average 
percentage of shortspine thornyheads with misidentified 
sex was 53.7% at lengths from 11 to 17 cm. This value 
decreased to 5.9% for larger fish (18-71 cm) (Fig. 2A). 
With a single exception, more males were misiden- 
tified as females in every size category for both spe- 
cies, and the absolute percentage of sex-misidentified 
fish decreased at fork lengths greater than 17 cm (Fig. 
2B). For longspine thornyheads the average decreased 
from 15.8% for fish 11-17 cm to 2.2% for fish 18-34 
cm length and the average percentage for shortspine 
thornyheads dropped from 24.5% to 3.0% in the larger 
size category (Fig. 2B). 
Sex misidentification in longspine thornyheads did not 
vary significantly by month from June through October 
(ANOVA: df=7, F=1.74, P=0.34; Fig. 3A). However, sex 
misidentification for shortspine thornyheads was signifi- 
cantly higher in August, with an increasing trend from 
June through August followed by a decline (ANOVA: 
df=7, F=15.5, P=0.02; Fig. 3A). 
The accuracy of sex determination varied by geo- 
graphic area for both species (Fig. 3B). The sex of long- 
spine thornyheads was more frequently misidentified 
