Keller et al : Variations in eastern North Pacific demersal fish biomass, 2003-10 
207 
Newport 
Cape Flattery 
f>art Franciscc 
Morro Bay 
0 25 50 
Pt Conception 
Los Angeles 
Newport Bead - 
NWFSC West Coast 
Groundfish Survey 
(2003-2010) 
| 1-5 samples 
7“W 
Figure 1 
Geographical extent of the Northwest Fisheries Science Center’s West Coast Groundfish Bottom Trawl Survey and the 
location of stations (shaded) trawled one to five times from 2003 through 2010. (A) Stations off Washington and Oregon. 
(B) Stations off northern California. (C) Southern California stations. The Cowcod ( Sebastes levis ) Conservation Area 
in Southern California was excluded from the experimental design because it is closed to fishing for groundfish species. 
fishery management plan (PFMC, 2008b), as well as 
two important benthic invertebrates and 15 non-fished 
species. The break point also represented a greater 
than 10% difference in catch relative to the next most 
abundant species. Species-specific catch per unit of 
effort (CPUE, kg ha -1 ) was calculated for each tow 
on the basis of area swept. Area swept was computed 
from the mean net width for each tow multiplied by the 
distance fished. Mean CPUE was calculated for each 
stratum (depth and geographic region, including those 
with zero catch), by species. Species-specific biomass 
indices (6, in metric tons [t] ) were computed by multi- 
plying the mean CPUE by the appropriate stratum area 
and then summing the strata biomasses. The estimate 
of the variance of the biomass was the sum of the vari- 
ances of the strata involved: 
Var(b) = X ^(VariCPUE , ) x A?, 
with n equal to the number of strata, and A equal 
to the area of each stratum (km 2 ). Standard errors 
(SE) for the annual species-specific biomass indices 
were calculated using standard statistical techniques 
(Cochran, 1977). 
To examine variation in biomass over time, species 
groups were initially designated on the basis of either 
taxonomy or depth (e.g., shelf and slope rockfishes). 
Subsequently we examined relationships between bio- 
mass and both year and the PDO index using groups 
designated by the presence, absence, or unknown oc- 
currence of a strong recruitment during the mid to 
late 1990s and early 2000s. In all cases biomass was 
summed over the appropriate group and variance was 
calculated as previously described. For those species 
included in the 2003-10 biomass analyses, we also cal- 
culated mean depth (m) by averaging station values 
weighted by catch for each species in each year (Hsieh 
et al., 2008). 
