Keller et al. : Variations in eastern North Pacific demersal fish biomass, 2003-10 
209 
other than the best model was plausible, the difference 
in AIC values for each model was calculated as 
A=A 1C r A IC m m . 
Models with A t < 2 are considered equivalent to the 
best model ( AIC min ) and candidate models with A t >10 
are highly unlikely to be plausible alternatives for the 
best model. Candidate models with A l between 3 and 7 
have less support than the best model (Burnham and 
Anderson, 2002). 
Results 
Biomass 
Between 2003 and 2010, 5271 trawls were successfully 
conducted as part of the groundfish survey with an 
annual average of 659 trawls yr -1 (range: 505 to 722 
trawls; Fig. 1). Although an average of 265 individual 
fish taxa were identified each year (range: 252- 310), 
the 60 demersal fishes and two benthic invertebrates 
included in this analysis comprised greater than 99% 
of the total catch. Annual biomass indices (t) for the 62 
individual species revealed variable trends over time 
(2003-10) (Figs. 2 and 3). Six species exhibited signifi- 
cant (P<0.001) increases in biomass indices over time 
(Fig. 2A), 20 species displayed significant (P<0.05) or 
near significant (P<0.10) negative trends (Fig. 3, Califor- 
nia skate [Raja inornata] and pygmy rockfish [Sebastes 
wilsoni] not shown), and 36 species exhibited nonsig- 
nificant trends over time. Representative examples for 
species with no significant trends are shown for the 
most abundant species within each group (Fig. 2B). 
Regardless of trends, both target and nontarget species 
occurred in each group. 
Mean, minimum, and maximum depth (m), and total 
numbers of positive hauls over the eight year study 
are shown for the 62 individual species included in 
the analyses (Table 1). Catch for these 62 species was 
initially partitioned into seven groups based on tax- 
onomy and depth (in order of decreasing biomass): flat- 
fish (30%), other shallow to mid-depth species (20%), 
shelf rockfish (15%), sharks, skates, and ratfish (13%), 
other deep water species (9%), thornyheads (8%), and 
slope rockfish (5%), to examine trends over time. The 
weighted mean depths for shallow to mid-depth spe- 
cies was <500 m, and the weighted average for deep 
water species was >650 m. Shelf rockfish occurred at 
average depths ranging from 101 to 209 m, whereas 
slope rockfish were somewhat deeper (226-456 m). In 
general, rockfish were encountered in fewer hauls than 
other subgroups. 
Despite variations in biomass indices at the species 
level, four of the seven groups initially examined here 
(with the exception of slope rockfish, thornyheads, and 
other deep water species) and overall biomass indi- 
ces decreased significantly (P<0.05) over time (Fig. 4). 
Overall aggregate coastwide biomass indices for all 62 
species decreased approximately 60% from 2,308,207 
t in 2003 to 1,384,391 t in 2010. However the lowest 
biomass (1,373,473 t) was recorded in 2008 (the year of 
the lowest PDO and also possibly a good recruitment 
year) followed by slight increases in 2009 and 2010. 
Recruitment versus PDO indices 
Our initial analyses based on biomass indices for spe- 
cies grouped taxonomically or by depth indicated that 
deepwater species, such as thornyheads and slope rock- 
fish, did not significantly decrease over time (Fig. 4). 
However because some species within these groups 
either decreased significantly or displayed decreasing 
trends (Figs. 2 and 3), we included all 62 species when 
we separated demersal catch into categories based on 
recruitment. Our examination of 24 stock assessment 
models revealed that 13 species had large recruitment 
events occurring primarily in 1999 (arrowtooth flounder 
[Atheresthes stomias], English sole, Pacific hake, sable- 
fish [ Anoplopoma fimbria], bocaccio [Sebastes paueispi- 
nis ], chilipepper rockfish [Sebastes goodie ], splitnose 
rockfish [Sebastes diploproa], but occasionally some- 
what earlier (petrale sole [Eopsetta jordani ], longspine 
thornyhead [Sebastolobus altivelis]), or later (Dover sole 
[Microstomas pacificus], greenstriped rockfish [Sebastes 
elongates ], darkblotched rockfish [Sebastes crameri ], 
Pacific ocean perch [Sebastes alutus]). Eleven additional 
species assessed since 2005 did not display significantly 
larger individual recruitment levels during the period 
examined (spiny dogfish [Squalus acanthias], longnose 
skate [Raja rhina], lingcod [Ophiodon elongatus], black- 
gill rockfish [Sebastes melanostomus], canary rockfish 
[Sebastes pinniger], greenspotted rockfish [Sebastes 
chlorostictus], shortbelly rockfish [Sebastes jordani[, 
widow rockfish [Sebastes entomelas] yelloweye rockfish 
[Sebastes ruberrimus], yellowtail rockfish [Sebastes flavi- 
dus], shortspine thornyheads [Sebastolobus alascanus]). 
Stock assessments have not yet been conducted on the 
remaining 38 species included in our analyses. 
During the 1999-2010 period, when depletion of a 
strong year class by fisheries was expected to result in 
declining biomass trends, we observed variable trends 
in the PDO index (Fig. 5). Changes in the PDO index 
from 1999 to 2010 indicate that average climate in the 
California Current system gradually shifted from cool 
(1999-2001) to warm (2003-06) and back to cool ( 2007— 
10) conditions (Fig. 5). We summed biomass for each of 
the three groups previously described and overall (all 
species combined) and regressed aggregate biomass 
(natural log transformed) versus year and annual PDO 
indices (Fig. 6). We noted significant (P<0.001) or near- 
significant (P=0.06) inverse relationships for all groups 
and overall versus year (Fig. 6). For species with strong 
recruitment events in 1999, the trend in biomass over 
time was increasingly downward, whereas for those 
with no or unknown recruitment there was a tendency 
for biomass to increase in recent years (Fig. 6). All 
groups also demonstrated significant relationships with 
the annual PDO indices (Fig. 6); however, the greatest 
