220 
Fishery Bulletin 1 10(2) 
contributed to recruitment success as well. Brodeur 
and Ware (1992) and Sugimoto and Tadokoro (1997) 
both demonstrated changes in zooplankton biomass and 
distribution coupled with changes in ocean conditions, 
and Lenarz et al. (1995) reported reduced primary pro- 
ductivity and zooplankton biomass, coupled with poor 
rockfish recruitment off the west coast during El Nino 
events. Recruitment success of salmon within the north- 
ern California Current system has previously been tied 
to fluctuations in the PDO index (Mantua et ah, 1997). 
Hollowed et al. (2001) examined the timing of the PDO 
and ENSO and correlated changes in both with recruit- 
ment success in groups such as flatfish and gadoids, 
but further study is needed to confirm their findings. 
Like others, they caution that additional factors could 
also affect biomass, species richness, and distributional 
changes such as density-dependent habitat selection, 
timing of migrations, changes in local currents, catch- 
ability, and shifts between nursery and feeding grounds 
(Swain et al., 1994; Delworth et al., 1997; Attrill and 
Power, 2002; Magill and Sayer, 2002). The interactions 
among climate variability, currents, and the seasonal 
strength of upwelling and downwelling is particularly 
interesting given our prior research where we directly 
linked changes in demersal biomass and species diver- 
sity to depressed oxygen levels along the Oregon shelf 
(Keller et al., 2010). 
At the species level, changes appear driven by cli- 
mate-induced variation in primary and secondary 
productivity and recruitment (Beaugrand et al., 2003; 
Steingrund and Gaard, 2005), although the nature of 
the relationship has not been deciphered. We recognize 
that the relatively short time series examined may in- 
crease the likelihood that the results are spurious. The 
observed tight correlations between total and grouped 
groundfish biomass indices and the PDO are expected if 
the underlying relationship results from reduced popu- 
lation growth due to poor environmental conditions or 
if environmental conditions, such as phase shifts in 
regional climate (Mantua et al., 1997; Hollowed et ah, 
2001; Castonguay et al., 2008) are also coupled with pe- 
riodic strong recruitment events, such as the emergence 
of the 1999 year class. 
Natural mortality for species with relatively high 
natural mortality rates could play an additional role 
in declining biomass indices. However, for rockfish with 
low natural mortality rates, it is likely that growth 
would be more important than mortality during most of 
the time period. The relationship between the PDO and 
biomass indices may also be due to changes in catch- 
ability or selectivity, rather than to actual population 
changes. A decrease could be due to higher selection 
of younger fish (i.e., to peak selection around age 4 or 
5 and a decline afterwards). Although understanding 
the mechanisms supporting the relationships observed 
here remains problematic, our results demonstrate the 
importance of incorporating environmental conditions 
in management decisions. Despite enactment of highly 
effective management measures and the occurrence of 
periodic strong recruitment, biomass indices declined 
as oceanographic conditions changed throughout much 
of the survey period. 
Acknowledgments 
The authors are indebted to the captains and the crew 
of the chartered fishing vessels Excalibur, Ms. Julie , 
Noah’s Ark, Raven , B. J. Thomas, and Sea Eagle for 
providing at-sea support. We especially thank the West 
Coast Groundfish Bottom Trawl Survey team (V. Simon, 
M. Bradburn, K. Bosley, D. Kamikawa, V. Tuttle, J. 
Buchanan, and J. Harms) for their skill and dedication 
in collecting high quality data for the groundfish survey 
and C. Whitmire for preparing GIS charts. 
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