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Fishery Bulletin 110(2) 
10 to 30 years. During a “warm” or “positive” phase, 
part of the eastern Pacific Ocean warms and productiv- 
ity of waters off the U.S. west coast declines; during a 
“cool” or “negative” phase, the opposite pattern occurs 
(Schwing et ah, 2009). 
The aim of this study was to evaluate the importance 
of depletion after strong recruitment versus environ- 
mental effects on declining biomass observed during 
groundfish surveys in the western U.S. shelf system. 
We used data from 24 stock assessments conducted 
since 2005 ( http://www.pcouncil.org/ . accessed Septem- 
ber 2011). With information contained in the assess- 
ments we separated 62 dominant species into three 
groups: those with strong recruitment during the late 
1990s-early 2000s, those without a strong recruitment 
during this period, and those with unknown year-class 
strength. For each group and the overall biomass indi- 
ces for all groups we evaluated regression models be- 
tween demersal fish biomass (2003 through 2010) along 
the U.S. west coast versus year (as a proxy for gradual 
depletion after recruitment of exceptional year classes 
to the fishery) and the PDO index, an ecosystem-level 
indicator of climate variability. For each comparison, 
the most appropriate model for describing the relation- 
ship with biomass was determined. A similar analysis 
was undertaken for species richness. We additionally 
present information on frequency of occurrence (number 
of positive hauls) and depth distribution by species. 
Materials and methods 
Survey design and methods 
The NWFSC conducted annual bottom trawl surveys of 
groundfish resources off the U.S. West Coast using stan- 
dardized procedures from 2003 through 2010 (Keller et 
ah, 2008). Surveys occurred May through October from 
the area off Cape Flattery, Washington (lat. 48°10'N), 
to the U.S. -Mexico border (lat. 32°30'N) at depths of 
55-1280 m (Fig. 1). The entire geographic extent of 
the survey was covered twice each year by two west 
coast commercial fishing vessels (20 to 28 m in length) 
per pass. Each year sampling extended from late May 
through late July for the first period and mid-August 
through late October for the second. A stratified random 
sampling design was used, in which the surveyed region 
was subdivided into -13,000 cells of equal area (1.5 nmi 
longitude by 2.0 nmi latitude) (Fig. 1). An average of 700 
primary cells was randomly selected each year, strati- 
fied by geographic location and depth. The geographic 
allocation was based on a simple north-south division 
at 34°30'N lat. (Point Conception, California) with 80% 
of the effort in the northern portion of the survey and 
20% in the southern range. The survey area was further 
stratified into depth zones as follows: north of Point 
Conception, 40% of the cells were in the shallow depth 
zone (55-183 m), 30% at mid-depths (184-549 m), and 
30% in the deep stratum (550-1280 m); and south of 
Point Conception, 25% were in the shallow depth zone, 
45% at mid-depth, and 30% in the deep stratum. Four 
chartered west coast fishing vessels were assigned an 
equal portion of stations to sample per year except in 
2004 when only three vessels were used. 
Vessels were equipped with customized Aberdeen- 
style nets with a small mesh (3.8 cm stretched measure) 
liner in the codend, a 25.9-m headrope, and a 31.7-m 
foot rope. All fishing operations were conducted in strict 
compliance with national and regional protocols de- 
tailed in Stauffer (2004). Simrad Integrated Trawl In- 
strumentation (ITI , Kongsberg Simrad Mesotech Ltd., 
Port Coquitlam, B.C., Canada 1 ) was used to monitor 
and record net performance and position for each haul. 
A differential global positioning system (DGPS) naviga- 
tion unit (Northstar 500, Northstar Technologies, Ac- 
ton, MA) was used to monitor towing speed during each 
haul. Standard survey haul positions were estimated 
from DGPS data — generally the mid-point between the 
net touchdown and net liftoff positions. Average net 
speed over ground and distance fished were calculated 
from the position data for the trawl and actual bottom 
time (Keller et ah, 2008). 
Samples were collected by trawling within the ran- 
domly selected cells (Fig. 1) for a target fishing time of 
15 minutes at a target speed of 1.13 m sec 1 (2.2 knots). 
All fish and invertebrates were sorted to species (or the 
lowest possible taxon), and then weighed by using an 
electronic, motion-compensated scale (Marel, Reykjavik, 
Iceland). Abundance was not analyzed in this study be- 
cause not all individuals were counted. Total abundance 
is estimated from biomass and the two cannot be con- 
sidered independent without analysis of the variability 
of mean weights. That analysis is beyond the scope of 
the present study. Near bottom temperature (°C) and 
depth (m) were measured during each trawl with an 
SBE 39 temperature and pressure recorder (Sea-Bird 
Electronics, Inc., Bellevue, WA) attached to the head 
rope. Mean tow depths were computed as the average 
of all depth recordings from the center 80% of the trawl 
duration (net touch down to lift off). Only tows judged 
to be acceptable (based on postcollection analysis of 
bottom contact, net performance, and other metrics; 
Stauffer, 2004) were included in the data analyses. 
Analyses of catch 
To limit this analysis to the most reliably sampled 
species, we initially examined catch for 310 individual 
fish species summed over the 2003-10 period. When 
graphed by species in order of descending catch, no 
obvious break was apparent and therefore we included 
all demersal species with an overall catch greater 
than 450 kg. This break point included the 62 most 
abundant species in the survey and incorporated 45 of 
the demersal groundfish species present in the Pacific 
Fishery Management Council Pacific Coast groundfish 
1 Mention of trade names or commercial companies is for 
identification purposes only and does not imply endorsement 
by the National Marine Fisheries Service, NOAA. 
