50 
Fishery Bulletin 110(1) 
halibut ( Reinhardtius hippoglossoides) may aggregate 
on the basis of sex in association with environmental 
factors, latitude, and depth (Gorchinsky, 1998). 
In addition to the potential disparity between survey 
and commercial sex ratios, spatial aggregation may 
affect other indices, such as cohort-specific selectiv- 
ity and abundance estimates. Longline catches may 
yield an imperfect reflection of assemblage structure 
simply owing to the nature of the gear. Once fish be- 
come hooked they preclude the capture of other fish, 
even if those arriving later are better competitors, 
a process that at high relative abundance can lead 
to “gear saturation.” Although we may view longline 
catches as a snapshot of the underlying population, 
they are really an integration of potentially changing 
assemblage structure over soak-time. The influence 
of hook size and spacing, factors that do not vary in 
time and space, upon selectivity have received consid- 
erable attention (Skud and Hamley, 1978; Lpkkeborg 
and Bjordal, 1992; Woll et al., 2001), whereas social 
interactions have received less. However, both labora- 
tory (Stoner and Ottmar, 2004) and field (Lokkeborg 
et ah, 1989) results have shown that hook occupancy 
patterns can be affected by behavioral interactions as 
well as by relative abundance. As such, the apparent 
selectivity of any particular demographic segment may 
vary depending upon the local density of competitors 
or upon the timing with which competing population 
segments arrive at the gear. Selectivity may vary in 
space and time even when underlying abundance of 
the target demographic remains relatively constant, or 
a single selectivity pattern may arise from a variety 
of different underlying population structures. With 
respect to abundance estimation, spatial stock struc- 
ture may influence catchability where static surveys 
overlap mobile aggregations ( sensu Dew and Austring, 
2007). For halibut in the eastern Pacific, this could 
be relevant for the use of surveys to estimate abun- 
dance of prerecruit age classes, for which commercial 
CPUE is generally lacking, especially if the tendency 
to aggregate changes with age. Halibut distribution is 
highly spatially structured at young ages (Norcross et 
ah, 1995), whereas use of a static survey design as- 
sumes relatively homogenous distributions of indexed 
ages. 
Conclusion 
In conclusion, analysis of Pacific halibut aggregation 
on a variety of spatial scales warrants more attention; 
in particular, on scales approaching those at which 
the survey and fishery are prosecuted, such as among 
survey stations, within the setting patterns of longline 
sets of commercial vessels, and among adjacent fish- 
ing grounds. In the meantime, methods that do not 
rely upon the assumption of demographic concordance 
between survey and commercial catches should be 
sought to verify the true sex composition of commercial 
landings. 
Acknowledgments 
The authors thank those International Pacific Halibut 
Commission staff who provided the support, guidance, 
and expertise that enabled this project to be a success, 
especially: L. L. Sadorus, E. D. Soderlund, E. J. Ander- 
son-Chau, T. M. Kong, T. O. Geernaert, sea samplers 
B. Biffard and E. White, and the crew of the FV Kema 
Sue. W. G. Clark provided valuable comments on early 
drafts of the manuscript, and three anonymous review- 
ers helped improve the quality of the final draft. 
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