Berkman et al.: Effects on survival of Oncorhynchus tshawytscha in rivers of Southeast Alaska 211 
their life history can enhance the accuracy of run fore- 
casts of management agencies that are critical to efforts 
to sustain healthy populations well into the future. In this 
study, available data from 2 rivers in SEAK were used. 
Results of this research highlight variation in biological 
and environmental factors linked to marine survival of 
Chinook salmon from different BYs and stocks. Marine 
survival of Chinook salmon from the Chilkat River was 
correlated with the timing of the end of the smolt outmi- 
gration period, smolt length, and spring river conditions, 
and marine survival for Chinook salmon from the Stikine 
River was correlated to smolt body size. Previous research 
on marine survival of Pacific salmon has emphasized the 
importance of the early marine period in determining 
BY strength (Holtby et al., 1990; Graham et al., 2019). 
Although different factors influenced marine survival of 
Table 4 
Loadings of the first, second, and third principal compo- 
nents from a principal component analysis of environmen- 
tal and biological variables used to explain variation in 
log-transformed marine survival of Chinook salmon (Onco- 
rhynchus tshawytscha) in the Chilkat River in Southeast 
Alaska. Environmental variables include sea-surface tem- 
perature (SST), river discharge in spring, and river tem- 
perature in spring. Biological variables include migration 
timing and smolt length. Data used in this analysis were 
for Chinook salmon from the brood years 1999-2009. 
Explanatory variable PC1 PC2 #£PC3 
Migration timing -0.54 0.33 -0.10 
SST 0.25 0.85 <0.01 
Smolt length -0.49 0.31 -0.29 
Spring discharge 0.53 0.27 0.24 
Spring river temperature 0.06 0.92 
Proportion of variance explained 0.22 0.15 
Cumulative proportion 
Chinook salmon in the Stikine and Chilkat Rivers during 
the study period, these results reinforce the importance of 
the early marine period. 
Marine survival was higher in the BYs when mean 
smolt length was longer, indicating that size-selective pro- 
cesses influence marine survival of Chinook salmon from 
both the Stikine and Chilkat Rivers. The movements of 
juvenile Chinook salmon from these rivers upon ocean 
entry are not well understood. However, the data on CWT 
recoveries in studies conducted by the ADFG indicate that 
adult fish from the Stikine River move offshore into the 
Gulf of Alaska and the Bering Sea and that fish from the 
Chilkat River inhabit the inside waters of northern SEAK 
(Pahlke et al.°; Elliott and Peterson). Similarities in the 
relationship between smolt body size and marine survival 
in these 2 stocks further indicate the importance of fresh- 
water growth and the early marine period, regardless of 
stock size or behavior. 
Results of research on Chinook salmon in the Yukon 
River, sockeye salmon (O. nerka) across Alaska, and wild 
and hatchery-born chum salmon (O. keta) in SEAK also 
indicate that fish encounter increased size-selective pres- 
sures upon marine entry (Koenings et al., 1993; Murphy 
et al., 2013; Duncan and Beaudreau, 2019). Upon marine 
entry, smaller fish may encounter higher rates of preda- 
tion from common nearshore species in SEAK, such as the 
Pacific staghorn sculpin (Leptocottus armatus) and Dolly 
Varden (Salvelinus malma), and larger fish may be more 
capable of emigrating farther from shore where pelagic 
zooplankton are more abundant (Koenings et al., 1993; 
Duncan and Beaudreau, 2019). Although the mechanisms 
that enhance the survival of larger (>74 mm in mid-eye 
fork length) juvenile fish in the Stikine and Chilkat Riv- 
ers are unclear, the results of our study and these other 
studies may indicate that size-selective pressures during 
the marine entry period are similar across stocks of Pacific 
salmon in the region. 
The relationship between smolt size and marine survival 
also highlights the importance of the freshwater period 
Table 5 
Results from the final principal component regression and linear regression models 
used to examine the relationships between log-transformed marine survival and 
principal component 1 (PC1) and smolt length for Chinook salmon (Oncorhynchus 
tshawytscha) in the Chilkat and Stikine Rivers, respectively. The statistics used to 
evaluate the models include standardized regression coefficient (), standard error 
of the coefficient (SE), coefficient divided by its standard error (¢), and the coefficient 
of multiple determination (R”). Data used in the models were for Chinook salmon 
from the brood years 1999-2009 for those from the Chilkat River and from the 
brood years 1998-2009 for those from the Stikine River. 
Explanatory variable: 
marine survival 
Response 
variable 
Chilkat River PC1 
Stikine River 
Smolt length 
Model statistic 
B SE t P R? 
-0.21 0.07 -3.03 0.01 0.5 
0.25 0.11 2.18 0.05 0.26 
