Keller et at. Variations in eastern North Pacific demersal fish biomass, 2003-10 
219 
unknown recruitment and overall, the best fit incorpo- 
rated both year and PDO indices combined. We think it 
is important to note that for all subgroups, particularly 
the group without species with known strong recruit- 
ment events, there is still a significant decrease in 
biomass indices over time. This decrease indicates that 
the depletion of an exceptionally strong 1999 year class 
for multiple species is not the only factor contributing 
to decreasing biomass of west coast demersal fish spe- 
cies. Despite the overall decline in biomass observed 
in recent years, very few of the species examined here 
are considered overfished (NMFS, 2009) and therefore a 
loss of yield cannot be inferred. However, the continued 
decline in overall observed biomass from 2003 to 2010, 
despite enactment of multiple management measures 
and reduced catch, emphasizes the need for the state 
of the ecosystem to be considered when setting catch 
limits. During periods of low ocean productivity, a pre- 
cautionary approach is advised. 
During the NWFSC bottom trawl survey, random, 
rare, and very large tows of schooling rockfish are oc- 
casionally taken. If a tow occurs near rocky but still 
trawlable habitat when schooling rockfishes are pres- 
ent, then very large catches of rockfish are possible 
(Stewart, 2007). Occurrence of these large catches may 
prevent detection of underlying biomass trends when 
only a small number of years are available for analysis, 
as with the 2003-10 survey. Although this phenomenon 
appears clear for canary and widow rockfish, for spe- 
cies like sharpchin and redbanded rockfish, it is harder 
to clearly separate large rare tows from an increase 
or decrease in biomass, and certainly random large 
catches and biomass trends may be occurring at the 
same time (Fig. 3). 
The higher productivity associated with the cool PDO 
phase in the California Current system may have re- 
sulted in another strong recruitment event in 2008 
(Ralston 3 ) and if cool conditions continue, the associated 
higher productivity could promote enhanced growth 
and survival of groundfish. Our data indicate that the 
return to cool conditions in 2007-10 was followed by a 
slight increase in overall biomass in 2010 — an increase 
that suggests a time lag between cool conditions and 
increased demersal biomass within the California Cur- 
rent. Although many rockfish species are long lived and 
exhibit highly variable recruitment, previous studies 
have additionally indicated that both rockfish recruit- 
ment and juvenile growth respond to broad indicators 
of productivity, and that juvenile abundance is corre- 
lated with large-scale oceanographic events such as El 
Niho-Southern Oscillation and the PDO (Ainley et ah, 
1993; Laidig et ah, 2007). For now we find our results 
interesting and recognize that the value of the ground- 
fish survey increases with each annual increment and 
over time will provide additional information to unravel 
these relationships. 
3 Ralston, S. 2011. Personal commun. National Marine 
Fisheries Service, Southwest Fisheries Science Center, Fish- 
eries Ecology Division. Santa Cruz, CA 95060. 
The best models to describe the variation in species 
richness (restricted to fish only) included the PDO indi- 
ces (shallow, mid-depth, and overall) or a combination 
of the PDO indices and year (deep depths) but were not 
based on year alone. Species richness was positively 
correlated with the annual PDO index indicating that 
more species were present within the survey area and, 
in particular, at shallow and mid-depth strata during 
the warm phase of the PDO. Tolimieri and Levin (2006) 
and Tolimieri (2007) examined patterns of diversity in 
groundfish assemblages in relation to depth (200-1200 
m) and latitude (33-37° N) along the U.S. Pacific Coast. 
They found, as we did, that species richness declined 
with depth (Fig. 7), but they did not examine changes 
over time or in relation to climate indices. However, 
their observation that patterns of diversity were cor- 
related with temperature may partially support our ob- 
servation of elevated species richness during the warm 
phase of the PDO. Tolimieri (2007) points out that lati- 
tude and depth are factors well known to correlate with 
diversity and assemblage structure. Both species rich- 
ness and biomass decreased along the U.S. west coast 
for demersal groundfish as the PDO index shifted from 
a warm to a cool phase. Given that the swept area per 
haul remains constant, fewer species per haul are ex- 
pected if the underlying population densities decrease; 
however, as demonstrated above, the densities for the 62 
most abundant species exhibited variable trends from 
2003 through 2010. 
Mueter and Litzow (2008) provided convincing evi- 
dence of climate-linked changes in the distribution of 
demersal fishes in the Bering Sea (1982-2006), cou- 
pled with reorganization in community composition by 
latitude. They observed increases in both biomass and 
species richness in an area characterized by warming 
temperatures. Community-wide patterns indicated that 
taxa shifted northward and also were captured with 
increasing frequency at shallower stations from 1982 to 
2006. Interestingly, they noted that mean species rich- 
ness significantly increased within the portion of the 
survey area termed the “cold-pool” as it warmed over 
time, in a similar relationship to that observed here. 
Species richness along the U.S. west coast was elevated 
during the warmer phase of the PDO and lower during 
the cool phase. Changes in species richness are most 
likely caused by movement of species in response to 
environmental conditions (Trenkel and Cotter, 2009). 
Understanding the mechanisms underlying the ob- 
served relationships between biomass indices, species 
diversity, depletion, and climate remains a challenge. 
However, Brodeur et al. (2008) recently related varia- 
tions in the abundance of dominant ichthyoplankton in 
the northern California Current to oceanic and climatic 
indices, thus providing a link between climate and 
recruitment success. Both larval concentrations and 
diversity varied on a semidecadal basis in conjunction 
with fluctuations in the PDO. Zheng and Kruse (2006) 
found some evidence that recruitment variation in east- 
ern Bering Sea crabs may be related to climate forcing, 
although interaction with groundfish predators likely 
