226 
Accuracy of sex determination 
for northeastern Pacific Ocean thornyheads 
(Sebastolobus altivelis and S. alascanus) 
Erica L. Fruh (contact author ) 1 
Aimee Keller 2 
Jessica Trantham 3 
Victor Simon 2 
Email address for contact author: Erica.Fruh@noaa.gov 
' National Oceanographic and Atmospheric Administration 
National Marine Fisheries Service 
Northwest Fisheries Science Center 
Fishery Resource Analysis and Monitoring Division 
2032 SE OSU Drive 
Newport, Oregon 97365 
2 National Oceanographic and Atmospheric Administration 
National Marine Fisheries Service 
Northwest Fisheries Science Center 
Fishery Resource Analysis and Monitoring Division 
2725 Montlake Blvd. East 
Seattle, Washington 98112 
3 Husbandry Department 
Underwater World 
1245 Pate San Vitores RD Ste 400 
Turnon, Guam 96913 
Abstract — Determining the sex of 
thornyheads ( Sebastolobus alasca- 
nus and S. altivelis ) can be difficult 
under field conditions. We assessed 
our ability to correctly assign sex in 
the field by comparing results from 
field observations to results obtained 
in the laboratory through both mac- 
roscopic and microscopic examination 
of gonads. Sex of longspine thorny- 
heads was more difficult to determine 
than that of shortspine thornyheads 
and correct determination of sex 
was significantly related to size. 
By restricting the minimum size of 
thornyheads to 18 cm for macroscopic 
determination of sex we reduced the 
number of fish with misidentified sex 
by approximately 65%. 
Manuscript submitted 25 June 2009. 
Manuscript accepted 11 February 2010. 
Fish. Bull. 108:226-232 (2010). 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Accurate sex-specific data are essen- 
tial for fitting age-structured popula- 
tion dynamic models and estimating 
spawning biomass (Methot, 2000). 
Assessing sex ratio is of added 
importance if sex-based selectiv- 
ity occurs within a fishery; because 
separate management measures may 
be required for male and female fish 
(Cochrane, 2009). 
Thornyheads are a common conti- 
nental slope species and support a 
large commercial fishery (Gunder- 
son, 1997). Longspine thornyheads 
( Sebastolobus altivelis) are found 
from the Gulf of Alaska to southern 
Baja California, whereas shortspine 
thornyheads (Sebastolobus alasca- 
nus) are distributed from the Ber- 
ing Sea to northern Baja (Orr et al., 
2000). Longspine thornyheads gener- 
ally inhabit depths greater than 400 
m, have a distribution range to about 
1400 m depth (Jacobson and Vetter, 
1996), and a peak in abundance and 
spawning biomass at about 1000 m 
depth (Wakefield, 1990; Jacobson 
and Vetter, 1996). Shortspine thorny- 
heads are found from 20 m to over 
1500 m in depth, are most abundant 
in the range of 180 to 450 m, and 
the majority of the spawning bio- 
mass occurs between 600 and 1400 
m, where longspine thornyheads 
are most abundant (Jacobson and 
Vetter, 1996). The maximum size of 
shortspine thornyheads (>70 cm) is 
larger than that of longspine thorny- 
heads (—38 cm). Shortspine thorny- 
heads migrate to deeper water as 
their body size increases, whereas 
longspine thornyheads do not mi- 
grate to deeper water with increas- 
ing size. 
Identifying the sex of mature long- 
spine thornyheads and shortspine 
thornyheads by gross visual exami- 
nation is difficult when gonads re- 
gress to a resting state (Pearson and 
Gunderson, 2003) because male and 
female gonads are small, not fully 
developed, and are morphologically 
similar. Determining the sex of in- 
dividual thornyheads collected dur- 
ing the annual Northwest Fisheries 
Science Center (NWFSC) West Coast 
Groundfish Bottom Trawl Survey is 
difficult because the survey occurs 
