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predators, and ability to respond to shifting ice conditions 
(Brown et al., 2011). Stressors from environmental condi- 
tions can cause metabolic deficits that lead to starvation 
or increased predation due to forage-based, risk-taking 
behaviors (Cunjak, 1988; Biro et al., 2003). Reductions in 
river discharge during the fall, when fish are moving to 
overwintering sites, and during winter can limit habitat 
accessibility and availability, and higher river discharge 
generally increases suitable habitat and carrying capacity 
of fish, reducing competitive interactions among juveniles 
(Lawson et al., 2004; Huusko et al., 2007). However, events 
of extremely high winter discharge can flush parr from 
winter habitats, restrict areas for effective drift foraging, 
and cause ice-related events of fish mortality (Lawson 
et al., 2004; Huusko et al., 2007; Neuswanger et al., 2014). 
Increases in turbidity that result from high discharge can 
further reduce the foraging ability of parr by limiting vis- 
ibility but can protect fish from visual predators as well 
(Lawson et al., 2004; Neuswanger et al., 2014). 
Following winter, parr undergo behavioral and physi- 
cal changes, known as smoltification, and become adapted 
to living in a marine environment (Healey, 1991). Early 
marine entry is a critical period for juvenile salmon that 
can determine strength of brood years (BYs) (Beamish and 
Mahnken, 2001; Mueter et al., 2002a). Movements of salmon 
smolts from fresh to marine waters follows the evolutionary 
concept of risk and reward, the notion that it is energeti- 
cally advantageous yet riskier to migrate to and feed in the 
ocean than it is to remain in resource-limited fresh waters 
(Jonsson and Jonsson, 1993; Quinn, 2005). Results of previ- 
ous research on smolt migration indicate that large variabil- 
ity in marine survival may be due to migration timing and a 
match or mismatch between smolt arrival in the ocean and 
nearshore marine conditions, including food availability 
(Rikardsen et al., 2004; Hvidsten et al., 2009). Smolt body 
size and changes in river conditions, such as increases in 
temperature and river discharge, have been linked to the 
timing of smolt migration (Bohlin et al., 1993; Vega et al., 
2017). Environmental changes, such as increases in water 
temperature, trigger migratory movements, but river dis- 
charge may augment migration as higher flows allow some 
passive transportation downstream (Bohlin et al., 1993; 
McCormick et al., 1998). 
Correlations between body size, early marine growth, 
and survival have been observed in most species of salmon 
in the Pacific and Atlantic Oceans (Holtby et al., 1990; 
Koenings et al., 1993; Mortensen et al., 2000; Antonsson 
et al., 2010; Murphy et al., 2013). The critical size and 
critical period hypothesis is that, for Pacific salmon (Onco- 
rhynchus spp.), the majority of natural mortality in marine 
environments occurs during 2 periods: 1) mortality of fish 
due to predation in the early marine period and 2) mortality 
of fish smaller than a critical size due to their inability to 
meet minimum metabolic requirements during their first 
year at sea (Beamish and Mahnken, 2001). Larger, faster- 
growing individuals are able to capture larger prey at an 
earlier age, resulting in improved metabolic efficiency and 
greater growth (Quinn, 2005). Fish typically have an opti- 
mal temperature range, within which metabolic rate is 
most efficient and growth is maximized (Wootton, 1998). 
Regional ocean conditions, including upwelling and advec- 
tion, affect water temperatures and prey abundances and 
distributions; in turn, these conditions may benefit or hin- 
der growth and survival (Cole, 2000; Mortensen et al., 2000; 
Mueter et al., 2002b). Body size of juveniles during the early 
marine period is more influential during years when mor- 
tality is high and marine survival is below average, indicat- 
ing that having a large body size is more important when 
marine conditions are suboptimal for growth and survival 
(Holtby et al., 1990; Graham et al., 2019). 
Although results of previous research indicate the impor- 
tance of physical and environmental conditions to the sur- 
vival of stocks of Pacific salmon in fresh water during the 
early marine period, the factors influencing variability in 
recruitment and survival can be unique to each stock and 
remain unknown for many stocks (Malick et al., 2009). 
Chinook salmon (O. tshawytscha) in Alaska support import- 
ant and diverse subsistence, commercial, recreational, and 
personal use fisheries. In Southeast Alaska (SEAK), declines 
in abundance and catch of stocks of Chinook salmon, 
increased variability in productivity, and overfishing over 
the past 60 years have led to fishing restrictions, which 
have created cultural, social, and economic hardships for 
communities in this region (ADFG CSRT’). With impending 
threats, including climate change and mine development, it 
is imperative to understand the basic mechanisms affecting 
survival and recruitment of this species. 
The objectives of this study were 1) to examine influ- 
ences of biological (parr length) and local environmental 
(temperature and discharge) factors on freshwater over- 
winter survival and smolt production of Chinook salmon 
in the Chilkat River, in SEAK near Juneau; 2) to deter- 
mine how biological factors, such as smolt body size and 
migration timing, and local environmental factors, includ- 
ing regional sea-surface temperatures (SSTs) and river 
discharge and temperatures, influenced marine survival 
of Chinook salmon in the Chilkat River; and 3) to deter- 
mine how a biological factor, smolt body size, and local 
environmental factors, including regional SSTs and river 
discharge, influenced marine survival of Chinook salmon 
in the Stikine River, further southeast in SEAK. This 
research helps clarify the factors that influence freshwa- 
ter overwinter and marine survival of Chinook salmon 
and will allow managers to develop more accurate and 
reliable catch forecasts. 
Materials and methods 
Study sites 
The Chilkat and Stikine Rivers were selected for this 
study because they support important regional stocks 
! ADFG CSRT (Alaska Department of Fish and Game Chinook 
Salmon Research Team). 2013. Chinook salmon stock assess- 
ment and research plan, 2013. Alaska Dep. Fish Game, Special 
Publ. 13-01, 56 p. [Available from website.] 
