Fruh et al.: Accuracy of sex determination for Sebastolobus altivelis and S. aloscanus 
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from May to October when thornyheads are not re- 
productively active and gonads are in a resting state 
(Moser, 1974; Wakefield, 1990). 
The addition of sex identification for both thornyhead 
species to survey sampling protocols will improve the 
information available for management of the resource. 
To address concerns about the ability of field person- 
nel to correctly determine sex of thornyheads while at 
sea, we examined the sex of longspine and shortspine 
thornyheads in the laboratory using macroscopic ex- 
amination of gonads (as a correlate for field work) in 
contrast to microscopic techniques (for confirmation of 
results). An additional goal was to determine a mini- 
mum size below which the error rate for classification 
of sex of thornyheads in the field was judged to be too 
high by investigating the relationship between sex mis- 
identification and length, geographic area, and month 
captured. Because assessment scientists are interested 
in the actual proportion of males to females, we also 
evaluated absolute percent error after accounting for 
the portion of the error that was cancelled out by bal- 
ancing the number of misidentified males reported as 
females against the number of misidentified females 
reported as males. 
Materials and methods 
The 2003 NWFSC West Coast Groundfish Bottom Trawl 
Survey was conducted between 24 June and 23 October, 
from the area off Cape Flattery, Washington (48°10'N 
lat.) to the U.S. -Mexico border (32°30'N lat.) at water 
depths of 55-1280 m. The survey area was covered twice 
by chartered commercial fishing vessels (20 to 28 m 
length). The first sampling period was from 24 June to 
13 August and the second from 31 August to 23 October. 
A stratified random sampling design was used and the 
survey area was subdivided into adjacent cells of equal 
area (1.5 nmi long, by 2.0 nmi lat., Albers equal area 
projection). A total of 620 primary sites were randomly 
selected from cells stratified by geographic location and 
depth. The geographic allocation was based on assign- 
ing 15-25% of the cells to each of five International 
North Pacific Fisheries Commission (INPFC) statis- 
tical areas: U.S. -Vancouver (47°30'N to U.S. -Canada 
border), Columbia (43°00' to 47°30'N), Eureka (40°30' to 
43°00'N), Monterey (36°00' to 40°30'N), and Conception 
(U.S. -Mexico border to 36°00'N). The survey area was 
further stratified into depth zones with 45% of the cells 
allocated to the shallow depth zone (55-183 m), 30% to 
mid-depth (184-549 m) and 25% to the deep stratum 
(550-1280 m). Each of four chartered fishing vessels 
was assigned 155 stations to sample. 
The bottom trawl survey is a standardized fishery in- 
dependent survey and all fishing operations are conduct- 
ed in strict compliance to national protocols (Stauffer, 
2004). Vessels were equipped with standard Aberdeen- 
style nets with small mesh (1.5-inch stretched measure) 
liner in the codend. All thornyheads randomly selected 
for biological sampling were assigned a unique identi- 
fication number, individually weighed (kg), measured 
(fork length, cm), and frozen while at sea. All frozen 
specimens were brought back to the laboratory where 
fish were thawed, dissected, and examined macroscopi- 
cally to identify sex. For macroscopic examination of 
gonads, an incision was made with a scalpel on the 
ventral surface of each thornyhead from the vent to the 
base of the pectoral fin. The lateral side of the fish was 
opened to expose the gonads, and a visual identification 
of sex was based on the physical structure of the gonad- 
al tissue as described by Lagler et al. (1962). Sex was 
recorded as male, female, or unknown. For microscopic 
identification of sex, a section of gonad tissue from each 
fish was placed on a glass microscope slide, stained with 
acetocarmine solution and compressed with a cover slip. 
The stain acted on the gonad tissue by readily staining 
oocytes dark pink (Guerrero, 1974). The slides were 
viewed under a lOx power microscope (Leica DM LS2, 
Bannockburn, IL), and females were distinguished from 
males by the presence of dark pink stained oocytes. 
Accuracy of sex determination was examined in rela- 
tion to length by species, geographic region, and month 
of capture (June-October). To determine a size thresh- 
old below which sex determination should not be at- 
tempted in the field, we examined both the total and 
absolute percentage of incorrectly sexed thornyheads 
in relation to length. To avoid biasing results, we did 
not consider our ability to correctly identify female 
thornyheads at smaller sizes, as opposed to our ability 
to correctly identify males at smaller sizes. Absolute er- 
ror was calculated as the absolute value of misidentified 
males minus misidentified females divided by the total 
number examined at each 1-cm size interval, and this 
value was then expressed as a percentage. Size data 
were transformed (natural logarithm) to reduce hetero- 
geneity of variance before statistical analysis. Data were 
statistically compared by analysis of variance (ANOVA) 
by using SAS for Windows (SAS Institute, Inc., Cary, 
NC). Significant ANOVAs were followed by a nonpara- 
metric comparison of means test (Tukey’s test). Fish in 
which the gonad could not be found, stained, or micro- 
scopically identified were not included in the analyses. 
Results 
A total of 574 successful tows were completed. Figure 1 
shows the distribution and relative abundance (kg/ha) 
of thornyheads from the 2003 survey. Both species 
were concentrated in the mid- and deep depth strata 
(183-1280 m) and exhibited higher relative abundance 
north of Pt. Conception, CA (34°30'N lat.). Longspine 
thornyheads were collected in 214 tows at depths of 
328—1280 m (mean depth 802 m) and shortspine thorny- 
heads were collected in 311 tows at depths of 88-1280 m 
(mean depth 605 m). A total of 2325 thornyheads were 
collected for later processing in the laboratory. Sex was 
determined for 852 longspine thornyheads and 1148 
shortspine thornyheads. Sex was indeterminable for 
189 longspine and 136 shortspine thornyheads (average 
