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Fishery Bulletin 1 13(3) 
the ecosystem-based management of this commercially 
harvested fish. 
The primary objective of this study was to describe 
the distribution and abundance of Pacific ocean perch 
in relation to physical and biological oceanographic 
factors across this species’ range, on the basis of data 
from bottom trawl surveys conducted periodically in 
the Aleutian Islands during the summer. We postulate 
that biogenic structures (sponges, corals, and bryozo- 
ans) can modify Pacific ocean perch distribution and 
abundance across gradients of environmental and 
physical conditions by providing additional structural 
heterogeneity in trawlable habitats where we sample. 
We used generalized additive modeling (GAM) to iden- 
tify the physical and biological oceanographic predictor 
variables that influence the relationships between Pa- 
cific ocean perch distribution and abundance. We used 
field observations and out-group comparisons to vali- 
date the resulting models. 
Materials and methods 
Trawling procedures 
A stratified-random sampling design was used for the 
AFSC RACE Division Aleutian Islands bottom trawl 
survey of trawlable areas shallower than the depth 
of 500 m across the Aleutian archipelago (Fig. 1). The 
survey area extends on the north side of the Aleutian 
island chain from Unimak Pass in the east (165°W) to 
Stalemate Bank in the west (170°E); on the south side 
of this archipelago, the survey extends from Samalga 
Pass (170°E) to Stalemate Bank in the west. Strata are 
based on 4 depth intervals (1-100 m, 101-200 m, 201- 
300 m, and 301-500 m) over the continental shelf and 
upper slope. The depth strata are further segregated 
by the North Pacific Fisheries Management Council’s 
(NPFMC) Bering Sea Aleutian Islands regulatory area 
(NPFMC 3 ) into survey districts that correspond to the 
NPFMC subdivisions of western, central, and eastern 
Aleutian districts and an additional southern Bering 
Sea survey district that roughly corresponds to the 
Bogoslof district. Trawl sample allocation in each stra- 
tum was achieved with a modified Neyman optimum 
allocation sampling strategy (Cochran, 1977) to provide 
representative samples of fishes and invertebrates oc- 
curring at each sampling location within each stratum. 
Bottom trawl surveys were conducted according to 
standard protocols established in Stauffer (2004). Our 
goal was to land each trawl net quickly on the bot- 
tom in fishing configuration at a towing velocity of 
1.5 m/s (3 kn) and to maintain vessel speed, with the 
net retaining fishing configuration and proper bottom 
contact for 15 min (an area of approximately 2.25 ha 
3 NPFMC (North Pacific Fishery Management Council). 2014. 
Fishery management plan for groundfish of the Bering Sea 
and Aleutian Islands Management Area, 144 p. NPFMC, 
Anchorage, AK [Available at website.] 
was swept on average during each tow). Tables of stan- 
dard scope ratios of trawl warp in relation to bottom 
depth were used to reduce potential fishing power dif- 
ferences between the vessels used in different surveys. 
Date, time, and GPS-generated position were recorded 
throughout each tow; depth, water temperature, and 
time of collection also were recorded during each tow 
with an SBE 39 4 microbathythermograph (Sea-Bird 
Electronics, Inc., Bellevue, WA). During each tow, the 
vertical and horizontal trawl openings were measured 
with Scanmar acoustic net mensuration sensors (Scan- 
mar International, Point Richmond, CA). A bottom con- 
tact sensor was attached to the midpoint of the roller 
gear and was used to measure the degree of contact 
between the ground gear and the bottom. Trawl hauls 
were performed during daylight hours (i.e., between 
0.5 h after sunrise and 0.5 h before sunset), and all 
trawl performance data were judged after completion 
through the use of computer-generated graphics and 
data summaries. For our analyses, we included only 
catches obtained with satisfactory trawl net perfor- 
mance and bottom contact and only those for which 
distance fished, net width, bottom depth, and water 
temperature were recorded. 
Data used to parameterize and select the best-fitting 
GAMs were collected during the periodic bottom trawl 
surveys of the Aleutian Islands used to assess Pacific 
ocean perch distribution and abundance. The survey 
completed in 2000 was the last in a series of triennial 
surveys. Since 2000, Aleutian Islands bottom trawl sur- 
veys have been conducted biennially with the exception 
of 2008, when insufficient funds led to the cancellation 
of that year’s survey effort. For the surveys undertak- 
en between 1997 and 2010, an average of 394 stations 
that met our criteria to be included in this study were 
sampled per year. Total station counts ranged from 355 
in 2006 to 413 in 2002. 
Physical variables 
An array of physical, environmental, and spatial pre- 
dictor variables was used to parameterize the GAMs. 
At each bottom trawl survey station, start and end po- 
sitions of the trawl were recorded and bottom depth 
and temperature were measured. Kriging, a geostatisti- 
cal procedure that estimates a spatial surface from an 
array of point values, was used to calculate an index of 
local bottom slope at each survey station from 100-m 
bathymetric contour increments between 0 and 2000 
m derived from ETOP02 gridded elevation data by fol- 
lowing the method of Rooper and Martin (2009). Tide 
velocity (Vt) was predicted at each sampling station 
for the date and time of the bottom trawl survey by us- 
ing tidal prediction software developed at Oregon State 
University (Tidal Prediction Software [OTPS], website 
accessed January 2010) (Egbert et ah, 1994; Egbert 
4 Mention of trade names or commercial companies is for iden- 
tification purposes only and does not imply endorsement by 
the National Marine Fisheries Service, NOAA. 
