46 
Abstract — Field experiments were 
conducted to test the hypotheses that 
Pacific halibut (Hippoglossus stenol- 
epis) display small-scale spatial struc- 
ture within longline catches, relative 
to other species and empty hooks, or 
within-species based on sex or length. 
Sequential hook-by-hook inventories, 
along with length and sex data, were 
taken at thirty-one survey stations. 
Two-dimensional spatial statistics 
were used to test for 1) aggregation, 
defined as the clustering of individu- 
als within a given demographic of size 
or sex over small intervals of distance; 
and 2) segregation, defined as the 
sequential occurrence of individuals 
within a given demographic of size 
or sex, uninterrupted by other obser- 
vations, irrespective of the distance 
between individuals. Statistically 
significant structure was detected 
within catches that is more commonly 
associated with fish length than 
sex. Significant spatial structuring 
occurred at 60% of all stations tested. 
Significant aggregation of halibut of 
legal length for commercial retention 
(>82 cm) was detected at 44% of sta- 
tions and aggregation of sublegal-size 
halibut was detected at 11%. Male- 
and female-based aggregations were 
observed at 22% and 11% of stations, 
respectively. Significant segregation 
of females was observed at 20% of 
stations, male segregation occurred 
at 8% of stations, and segregation by 
size at 16% of stations. Understanding 
small-scale spatial structure within 
longline catches may help us interpret 
changes in survey and commercial 
catch data. If structure is generated 
by behavior, then observed size-at-age 
or relative sex-ratios may be biased 
relative to underlying distributions. 
Although physical processes such as 
gape limitation should remain stable 
over the time, dynamic processes may 
be spatially and temporally variable. 
Manuscript submitted 28 April 2011. 
Manuscript accepted 21 September 2011. 
Fish. Bull. 110:46-51(2012). 
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. 
Length and sex effects on the 
spatial structure of catches of Pacific halibut 
( Hippoglossus stenolepis ) on longline gear 
Timothy Loher (contact author ) 1 
Jessica C. Hobden 2 
Email address for contact author: tim@iphc.int 
1 International Pacific Halibut Commission 
2320 West Commodore Way 
Seattle, Washington 98199-1287 
2 University of Victoria 
School of Earth and Ocean Sciences 
Victoria, British Columbia V8W 2Y2 Canada 
Pacific halibut (Hippoglossus stenol- 
epis) support a valuable fishery in 
the eastern Pacific Ocean that has 
been monitored and managed by the 
International Pacific Halibut Commis- 
sion (IPHC) since 1923. The IPHC’s 
numerical stock assessment model 
(Clark and Hare, 2006) relies upon 
commercial catch-per-unit-of-effort 
(CPUE) data, and length-at-age, sex 
ratio, and maturity data collected 
during summer (May- August) long- 
line surveys conducted from southern 
Oregon through the western Aleutian 
Islands, and northward along the con- 
tinental shelf edge from the eastern 
Aleutian Islands to the U.S. -Russian 
border. Estimation of female spawning 
stock biomass (SSB f ) and sex ratios in 
the commercial catch and within the 
total biomass are important aspects 
of the assessment. These ratios are 
expected to be biased at any given 
size or age because halibut display 
sexually dimorphic growth (Clark 
et ah, 1999) and the current mini- 
mum legal commercial size is above 
the male size at 100% maturity and 
below female size at 50% maturity (T. 
Loher, unpubl. data). However, sex 
ratios in the commercial catch cannot 
be obtained through catch sampling 
because halibut are eviscerated 
before landing. Instead, estimation is 
accomplished by applying sex ratios 
observed in surveys to the harvest 
data (Clark, 2004). This should pro- 
duce accurate stock metrics as long 
as commercial catch demographics are 
the same as in the survey. However, 
there are numerous aspects of stock 
structure that may be exploited in 
order to maximize CPUE and could 
cause decoupling between commercial 
and survey sex ratios. 
In general, any process influenced 
by fish size or that results in the 
spatial partitioning of individuals 
within cohorts can influence catch 
demographics. Lpkkeborg and Bjordal 
(1992) suggest that local spatial dis- 
tribution, the size of individuals, 
effective feeding range, the mix of 
species present, relative competi- 
tive abilities, and the configuration 
of sampling gear, all contribute to 
longline catch-composition. Demo- 
graphic partitioning is common in 
species that redistribute for repro- 
duction (Moyer et al., 1985; Hannah 
et al., 2002), and seasonal migration 
between summer feeding grounds 
and winter spawning grounds is 
well documented for Pacific halibut 
(St. Pierre, 1984; Loher and Blood, 
2009; Seitz et al., 2011). Size-specific 
dispersal capabilities (Dorazio et al., 
1994) and sex-specific use of spawn- 
ing grounds (Robichaud and Rose, 
2003) have been demonstrated for 
other teleosts, and analyses indicate 
that commercial fishing periods for 
Pacific halibut may allow for inter- 
ception fisheries that take advantage 
of seasonal migration (Loher, 2011). 
Given that commercial fishermen 
have long asserted the existence of 
size-structured aggregations of Pa- 
cific halibut, and that density-de- 
pendent social facilitation and size- 
