Berkman et al.: Effects on survival of Oncorhynchus tshawytscha in rivers of Southeast Alaska 213 
ne 
habitat that was safe for drift foraging and increased risk- 
taking behaviors induced by foraging (Neuswanger et al., 
2014). High spring flows have been determined to improve 
survival during smolt outmigration because higher dis- 
charge is associated with increased turbidity, which could 
protect fish visually from predators (Lawson et al., 2004). 
In our study, increased smolt production associated with 
low discharge in fall could indicate that these conditions 
enable increased fall foraging opportunities without lim- 
iting pre-winter migration corridors or habitat. Higher 
flows in early spring could increase habitat and drift- 
feeding opportunities while reducing competitive interac- 
tions between juvenile salmonids after the less productive 
winter period (Lawson et al., 2004). 
Acclimation costs linked to changing water temperatures 
that occur in the fall and spring can result in the depletion 
of lipid reserves and lower body condition, possibly reduc- 
ing salmonid survival during long winters (Cunjak et al., 
1987). In our study, increased smolt production of Chinook 
salmon in the Chilkat River was correlated with warmer 
temperatures (>4°C) in early spring, potentially reflecting 
a survival advantage when winter periods are shorter and 
followed by warmer spring water temperatures. In contrast, 
Lawson et al. (2004) related air temperature to freshwa- 
ter production of coho salmon in Oregon, near the southern 
end of their range, and found lower smolt production during 
years with higher annual temperatures. The variance in 
these results may be attributed to the lower range of tem- 
peratures in glacially fed streams, temperatures that do not 
reach the higher than optimal temperatures for streams, as 
they do in the streams in southern Oregon. 
Body size of juveniles has also been proven to influence 
freshwater survival and production of salmonids. Larger 
body size has been linked to increased overwinter sur- 
vival and production because higher lipid reserves enable 
larger individuals to tolerate harsh winter conditions, 
increase size-based predator avoidance abilities, and 
allow competitive advantages over smaller individuals 
for quality habitats (Quinn and Peterson, 1996; Meyer 
and Griffith, 1997; Zabel and Achord, 2004). In our study, 
higher smolt production was related to smaller parr body 
size for Chinook salmon, indicating density dependence in 
the freshwater environment because parr length was also 
negatively correlated with parr abundance. Armstrong 
and Griffiths (2001) found that the proportion of Atlantic 
salmon sheltering during the freshwater overwinter 
period decreased with abundance. In the Chilkat River, 
higher densities could have reduced the scope for growth 
because of competition for food or habitat refugia. Small 
body size may also be beneficial. For example, Carlson 
and Letcher (2003) observed that in 2 trout species, the 
brown trout (Salmo trutta) and brook trout (Salvelinus 
fontinalis), older, larger individuals had lower survival 
rates than younger, smaller individuals and suggested 
that this relationship may be a result of larger fish being 
more habitat limited in winter and, therefore, more sus- 
ceptible to predation. Density-dependent effects during 
the freshwater phase that limit parr body size may also 
affect marine survival, given that smolt size was an 
important indicator of BY success for Chinook salmon 
from the Chilkat River. 
Conclusions 
The results of our study indicate that environmental 
mechanisms influencing survival of Chinook salmon in 
SEAK were stock specific, as has been observed for other 
stocks of Pacific salmon in this region (Malick et al., 2009). 
River conditions in spring appear to have influenced 
marine survival of Chinook salmon in the Chilkat River 
but not in the Stikine River, possibly as a result of the 
differences between stocks in offshore foraging locations. 
Smolt length may influence survival in both rivers, and 
results from this study reinforced the importance of envi- 
ronmental and biological factors on freshwater production 
and early marine survival for determining brood strength 
of stocks of Pacific salmon. Warmer water temperatures 
and increased river discharge projected to occur as a result 
of climate change may affect these critical survival peri- 
ods if mismatches between feeding and growth conditions 
are exaggerated in both freshwater and marine systems. 
Because reductions in the productivity and abundance of 
stocks of Chinook salmon throughout SEAK have led to 
increased effort for stock assessments and to reductions in 
allowable catch, understanding critical periods in the life 
history of these stocks was key (ADFG CSRT?). 
Acknowledgments 
We thank T. Jaecks and P. Richards for providing ADFG 
data for Chinook salmon in the Chilkat and Stikine Rivers 
and the ADFG field crews for spending their summers 
collecting and reporting the data used in this study. We 
also thank M. Adkison of the University of Alaska for his 
input, guidance, and review throughout the project. This 
research would not have been possible without funding by 
the Alaska Sustainable Salmon Fund, the University of 
Alaska Fairbanks, and the ADFG. 
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