Yasumiishi et al.: Effect of population abundance and climate on 2 populations of Oncorhynchus keta 
215 
nutritious prey (Salo, 1991), and switch from eating 
crustacean zooplankton to eating gelatinous zooplank- 
ton (Tadokoro et al., 1996). From these studies it ap- 
pears that the influence of pink salmon abundance may 
be greater than that of chum salmon because of their 
high population levels, fast growth rates, high degree 
of diet overlap with other salmon species, as well as 
their preference for lower trophic level prey, and their 
early migration to sea (Ruggerone and Nielsen, 2004). 
Differences in the influence of pink salmon popula- 
tion abundances on growth of the l st immature stage of 
chum salmon from the 2 populations investigated may 
be the result of differences in the migration and distri- 
bution of ageO.l chum salmon. The SW2 of Fish Creek 
chum salmon was inversely related to the abundance of 
maturing pink salmon and age-0.1 chum salmon from 
SE to AP, whereas Quilcene River chum salmon were 
more significantly correlated with the abundance of im- 
mature age-0.1 chum salmon from Asia. Chum salmon 
from WA and SE reside primarily in the eastern North 
Pacific and GOA (Urawa et al., 2009). However, 70% of 
the age-0.1 chum salmon along the 145°W longitude 
from 48°N to 53°N during February were of SE origin 
and less than 1% were of WA origin (Beacham et al., 
2009). Pink salmon are more abundant farther north in 
Prince William Sound than in SE or WA. That is, the 
southern-origin chum salmon were not likely distrib- 
uted as far west or north as their northern conspecifics 
at age-0.1. 
For both stocks, growth during the 2 nd immature 
stage was inversely related to chum and pink salmon 
abundance from British Columbia to Southeast Alaska, 
and growth during the maturing life stage was inverse- 
ly related to maturing chum salmon from Asia. These 
results indicate possible co-occurrence of these 2 North 
American chum salmon populations in the ocean dur- 
ing their 2 nd immature life stage. To address similari- 
ties and differences in the migration routes of summer 
and fall chum salmon, a comparison of marine growth 
could be made between summer and fall chum salmon 
from the same river or area. 
Several direct and indirect mechanisms exist that 
cause salmon to compete for resources in the ocean, 
particularly because the different species of Pacific 
salmon have a high degree of overlap in prey and habi- 
tat (Myers et al., 2007). Physical and biological condi- 
tions in the marine environment have direct and in- 
direct influences on density-dependent growth through 
their influences on feeding and metabolic rates (Davis 
et al. 4 ). In salmon, behavioral responses to competition 
or interference include reduced feeding, prey switch- 
ing, and migration (Azumaya and Ishida, 2000; Davis 
et al., 2009). In addition to direct interaction, there 
may also be indirect density-dependent interactions 
among pink and chum salmon that could affect growth 
of chum salmon. For example, while although the den- 
sity of pink salmon was higher in odd-numbered years 
in the Bering Sea, the growth of chum salmon was 
also higher (Azumaya and Ishida, 2000). This may be 
explained by the observation that in odd years, chum 
salmon are more likely to move from the Bering Sea to 
the eastern North Pacific Ocean (Azumaya and Ishida, 
2000) — a behavior that we interpret as a response to 
avoid interaction with Asian-origin pink salmon. In the 
eastern North Pacific Ocean, chum salmon were more 
abundant and consumed lower quality prey (gelatinous 
zooplankton) in odd-numbered years, whereas in even- 
numbered years chum salmon abundance was lower 
and they consumed higher quality prey (squid and fish; 
Tadokoro et al., 1996), indicating an indirect effect of 
pink salmon abundance in the Bering Sea on the in- 
tra-specific competition of chum salmon in the eastern 
North Pacific Ocean. The ability of chum salmon to dis- 
play plasticity in migration and feeding patterns may 
also be an adaptive response to more effectively reduce 
the impact of density on growth. 
Growth and climate 
Contrary to our hypotheses, a shallower mixed layer 
depth in the previous winter at the inner continental 
shelf of the northern GOA was not associated with an 
increase in growth. An early and stronger stratification 
of depth in the spring favors primary production in the 
middle shelf of the GOA (Henson, 2007). Therefore, wa- 
ter column stability during spring rather than deeper 
mixing during winter may influence prey densities in 
the surface layers where juvenile chum salmon feed 
and may favor their growth. 
As anticipated, a fall phytoplankton bloom, which 
was generally followed by a peak in secondary pro- 
ductivity, as indexed by lower wind speed in Septem- 
ber and October, was associated with an increase in 
the late juvenile growth of Fish Creek chum salmon. 
However, the index was not correlated with the late 
juvenile growth of Quilcene River chum salmon. In 
the GOA, a bloom of phytoplankton can occur during 
September and October but does not occur every year 
(Cooney, 2005). The fall bloom is initiated when fall 
winds deepen the mixed layer and resupply nutrients 
to the photic zone. Once nutrients are added to surface 
waters, a stratification of the water column is required 
before a fall bloom can occur (Cooney, 2005). Thus, a 
fall bloom is only possible in those years when wind 
speeds in September and October are not excessive. 
Cooler late summer and fall SSTs were associated 
with an increase in growth during the 2 nd immature 
stage for the Fish Creek chum salmon, but growth was 
more strongly correlated with population abundance. 
Immature growth of western Alaska chum salmon 
was negatively correlated with GOA SSTs from 1965 
to 2006 (Agler et al., 2013). This pattern might be ex- 
plained by the fact that salmon in the eastern North 
Pacific Ocean and eastern Bering Sea are consuming 
higher quality prey in cool years (Aydin et al., 2000; 
Coyle et al., 2011). Specifically, in the eastern Bering 
Sea, chum salmon consumed primarily euphausiids in 
cold years and walleye pollock ( Gadus chalcogrammus ) 
in warm years (Coyle et al., 2011). Similarly, during 
the cold years 1996 and 1998 chum salmon were larger, 
