National Marine 
Fisheries Service 
NO AA 
Abstract— The movements of matur¬ 
ing Pacific salmon (Oncorhynchus spp.) 
from marine waters into rivers pose 
challenges for their physiology, espe¬ 
cially in basins affected by human 
structures and increasing tempera¬ 
tures. This study determined the ther¬ 
mal regimes experienced by maturing 
Chinook salmon (O. tshawytscha ) enter¬ 
ing the Lake Washington basin in west¬ 
ern Washington through navigational 
locks and a ship canal and migrating 
through the lake to spawning grounds. 
Chinook salmon entered in mid-summer, 
when temperatures were warmest, but 
often moved back into the cool, marine 
waters of Puget Sound before travelling 
quickly through the ship canal and into 
Lake Washington. In Lake Washing¬ 
ton, they swam above and below the 
thermocline and made little use of the 
available thermal refuge in deep water 
before ascending rivers to spawn. The 
migration timing and tactics of Chinook 
salmon indicate a greater tolerance for 
warm water than that of sympatric sock- 
eye salmon (O. nerka, documented in an 
independent study), but in each species 
the ability to exploit thermal refuges 
may be essential for their survival in the 
face of the increasing temperatures seen 
in this basin in the past decades and 
those that are likely to continue rising 
in the future. 
Manuscript submitted 24 October 2018. 
Manuscript accepted 22 August 2019. 
Fish. Bull. 117:258-271 (2019). 
Online publication date: 6 September 2019. 
doi: 10.7755/FB.117.3.12 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Fishery Bulletin 
established in 1881 
Spencer F. Baird 
First U.S. Commissioner 
of Fisheries and founder 
of Fishery Bulletin 
Behavioral thermoregulation by adult Chinook 
salmon {Oncorhynchus tshawytscha) in estuary 
and freshwater habitats prior to spawning 
Frederick A. Goetz (contact author } 1 
Thomas P. Quinn 2 
Email address for contact author: fred.goetz@usace.army.mil 
1 Seattle District 
U.S. Army Corps of Engineers 
P.Q. Box 3755 
Seattle, Washington 98124-3755 
2 School of Aquatic and Fishery Sciences 
University of Washington 
Box 355020 
Seattle, Washington 98195 
Most fish species live entirely in fresh 
or salt water, and only a small fraction, 
from many different evolutionary lin¬ 
eages, migrates regularly between these 
environments (McDowall, 1988). These 
diadromous fish species always experi¬ 
ence a decrease in salinity and tempera¬ 
ture changes when entering fresh water. 
In the summer and early fall at higher 
latitudes (>40°N), the marine waters 
are typically cooler than the rivers, but 
the reverse may be the case in winter. 
Estuaries are the transition zone for this 
shift between marine and riverine hab¬ 
itats, affording intermediate conditions 
of salinity and temperature. However, 
estuaries and their associated water¬ 
ways are often altered by human devel¬ 
opment (Nichols et al., 1986; Emmett 
et al., 2000; Lotze et al., 2006). 
The thermal transition that diadro¬ 
mous fish species experience in estuar¬ 
ies is important because temperature 
affects so many aspects of their biology 
(Magnuson, 2010) and has been called 
the abiotic “ecological master factor” 
(Brett, 1971). For example, temperatures 
that adult Pacific salmon (Oncorhyn¬ 
chus spp.) experience in coastal marine 
waters and during upriver migration 
can affect survival (Gilhousen, 1990; 
Hinch et al., 2012), maturation (Hoseec 
et al., 2010), and reproductive success 
(Minke-Martin et al., 2018). Conse¬ 
quently, adult Pacific salmon and Atlan¬ 
tic salmon (Salmo salar) show many 
behavioral responses to temperature, 
including changes in vertical distribu¬ 
tion at sea (Quinn et al., 1989; Tanaka 
et al., 2000; Walker et al., 2000), holding 
in river plumes outside estuaries (Cooke 
et al., 2008; Strange, 2013), changing 
vertical distribution in estuaries (Olson 
and Quinn, 1993), delay in upriver 
migration (Kristinsson et al., 2015), use 
of cool tributaries or deep pools (Berman 
and Quinn, 1991; Goniea et al., 2006; 
Strange, 2010; Moore et al., 2012), or 
spending the summer below the thermo¬ 
cline in lakes (Newell and Quinn, 2005; 
Mathes et al., 2010). 
Many Pacific salmon populations 
experiencing high water temperatures 
are at low levels of abundance, and 
salmon experience en route or prespawn¬ 
ing mortality in some years (Gilhousen, 
1990; Richter and Kolmes, 2005; Hinch 
et al., 2012). Temperatures are predicted 
to continue to increase in the future, 
exacerbating the problem (Yates et al., 
2008; Mantua et al, 2010). The chang¬ 
ing thermal regimes may be especially 
