Fruh et at: Accuracy of sex determination for Sebastolobus altivelis and S. alascanus 
231 
females in the field on the basis of gross morphological 
features, nine were actually males. 
Correct visual identification of sex for both shortspine 
and longspine thornyheads increased in fish longer than 
17 cm. Overall accuracy is greater for shortspine than 
for longspine thornyheads, and greater for females than 
for males, and this accuracy is related to size in both 
instances. For both species, 18 cm was selected as the 
lower limit for determining the sex of thornyheads in 
the field because the majority of sex-misidentified fish 
fell below this value. In 2003, 66% of the longspine 
thornyheads and 90% of the shortspine thornyheads 
measured in the field throughout the survey period were 
greater than 17 cm. The selected size falls within the 
range of lengths noted for the onset of sexual maturity 
in both species. 
Because the survey is conducted after the completion 
of the spawning season for longspine thornyheads (Janu- 
ary-April), the samples are collected exclusively during 
the reproductive resting stage. Sex misidentification was 
relatively constant for longspine thornyheads through- 
out the sample period and there were no significant 
differences among months. Sex misidentification was 
greater for longspine than for shortspine thornyheads 
for each time period. The lower rate of sex misidentifica- 
tion for shortspine thornyheads may be related to their 
longer spawning season (December— May). Differences 
in the reproductive cycles of the two species resulted 
in the cessation of spawning coinciding with the start 
of the survey sampling for shortspine thornyheads and 
may partially explain the observed overall lower rate of 
sex misidentification for this species. The middle of the 
reproductive resting-stage period correlated with high 
levels of sex misidentification for both species, although 
only for shortspine thornyheads was the difference sig- 
nificant (in August). 
The differences in sex misidentification among geo- 
graphic areas are more difficult to explain. Sex of long- 
spine thornyhead was more frequently misidentified in 
the U.S. -Vancouver and Columbia areas. Samples in 
these areas were collected primarily in June and Sep- 
tember, the periods with the highest rates of sex mis- 
identification. The lack of any significant differences in 
mean length for longspine thornyheads between INPFC 
areas indicates that the higher rates of misidentification 
of sex farther north were not a function of size, but were 
related to the timing of the annual spawning cycle at 
differing latitudes. 
Shortspine thornyhead samples collected in the Eure- 
ka, Columbia, and U.S. -Vancouver areas (i.e., those with 
significantly lower rates of sex misidentification) were 
primarily taken in June, July, and September when the 
rate of sex misidentification for shortspine thornyheads 
was lowest. Additionally, there were significant differ- 
ences in the lengths of shortspine thornyheads among 
areas, indicating that the lower rates of sex misidentifi- 
cation in the U.S. -Vancouver area may also be partially 
related to size (although similar size differences were 
not observed in the Eureka and Columbia areas). Be- 
cause differences in geographic area were related to size 
0 ) 
O 
q3 35 
Q_ 
30 
25 
20 
15 
10 
5 
0 
Conception Monterey Eureka Columbia US-Vancouver 
Figure 3 
Percentage of sex-misidentified longspine ( Sebastolo- 
bus altivelis) and shortspine (Sebastolobus alascanus ) 
thornyheads determined by comparing the gross mor- 
phological features of gonads to a section of each gonad 
subsequently stained and viewed microscopically (A) 
by month, and (B) by geographic area as defined by 
the International North Pacific Fisheries Commission 
regions: Conception (U.S. -Mexico border to 36°00'N lat.), 
Monterey (36°00' to 40°30'N lat.), Eureka (40°30' to 
43°00'N lat.), Columbia (43°00' to 47°30'N lat.), and 
U.S. -Vancouver (47°30'N lat. to U.S. -Canada border). 
for at least one thornyhead species and the differences 
in seasonal determination of sex were variable, we rec- 
ommend that sex determination of thornyheads <18 cm 
not be attempted in the field. This is likely a conserva- 
tive estimate because identifying sex in fresh specimens 
at sea is somewhat more reliable than examining frozen 
and thawed specimens in the laboratory. The approach 
described here establishes a protocol for determining 
a minimum size for at-sea sex identification of thorny- 
heads, but may be applicable for use with any species 
where ambiguity may exist in correctly identifying the 
sex of fish at smaller sizes, within different regions, or 
across spawning or other seasonal cycles. 
Acknowledgments 
We thank the captains and crew of the fishing vessels 
Ms. Julie , Excalibur, Captain Jack, and Blue Horizon for 
their effort during the 2003 NWFSC West Coast Ground- 
fish Bottom Trawl Survey. We also thank the biologists 
who participated in this study, including K. Bosley, J. 
Buchanan, D. Kamikawa, and V. Tuttle. 
