Goetz and Quinn: Behavioral thermoregulation by adult Oncorhynchus tshawytscha prior to spawning 
265 
Figure 4 
Mean depth of tagged Chinook salmon (Oncorhynchus tshawytscha) 
(bars) and mean bottom depth (solid line) near acoustic receivers 
along the migration route of Chinook salmon in 2005 and 2006. Loca¬ 
tions in Washington where depth data were recorded include Puget 
Sound, the lower and upper estuary, the large lock, and the ship 
canal of the Lake Washington watershed, central and south Lake 
Washington (LWA), Sammamish River delta, Cedar River delta, and 
Lake Sammamish outlet. Error bars indicate standard deviations. 
were assigned to the lower estuary, 31.3% to the upper 
estuary, 1.4% to the ship canal, 21.0% to Lake Washing¬ 
ton, 19.7% to the Sammamish River, and 11.6% to Lake 
Sammamish. Most individuals experienced a wide range 
of water temperatures with typical minima of 9-12°C 
and maxima of 20-22°C (examples in Figure 3). The fish 
experienced diel temperature fluctuations (warm in the 
afternoon and cool in early morning) in all areas, but 
daily ranges were greater (up to 4-6°C) in Lakes Wash¬ 
ington and Sammamish, moderate in the estuary and 
river (2-3°C), and smallest in the ship canal (1-2°C) 
where the water was more uniformly warm. The 
highest temperatures and greatest change in 
temperatures fish experienced were from entry 
into the lower estuary (mean: 15.1°C) to the fish 
ladder’s exit (mean: 21.6°C) for an average 2-h 
change of 6.5°C. 
Cool, saline water near the bottom was more 
available during the day than at night because 
this water intruded during locking events, which 
took place primarily during the day. The typical 
behavior of Chinook salmon included holding 
for several days below the thermo-halocline in 
the lower estuary in the large lock (mean depth: 
10 m; salinity: 12-27) and upper estuary (mean 
depth: 13 m; salinity: 1-18). Temperatures at the 
depths most often used varied markedly between 
the large lock (14-17°C) and the upper estuary 
above the locks (19-21°C). In the upper estuary, 
fish experienced warmer temperatures in the 
morning (mean: 19.9°C) as their depth shallowed 
after dawn (~11 m), through late morning, and 
then the temperatures decreased (18.6°C) with 
the increased depth (~13 m) into the evening. 
Therefore, despite the afternoon rise in tem¬ 
peratures at the surface, the temperatures occu¬ 
pied by the fish decreased. Cool, saline water at 
depth was increasingly available during the day 
because this water entered during locking events 
that occurred primarily during the day. Within 
Lake Washington, Chinook salmon occupied 
depths from near the surface to >20 m, above 
and below the thermocline (Figs. 3 and 4). Most 
(73%) recorded temperatures were 15-20°C, cor¬ 
responding to depths near and above the thermo¬ 
cline (10-15 m; Fig. 5). In 2006, the salmon spent 
more time within 10 m of the surface than in 
2005, the warmer year (21% versus 4%). 
The highest temperatures experienced by 
individual Chinook salmon were 22.6°C in 2005 
and 22.5°C in 2006. The average maxima among 
fish were the same (21.4°C) in both years, with 
the warmest temperatures in the upper estuary 
(22.6°C), Lake Washington (22.6°C), the ship 
canal (22.5°C), and Lake Sammamish (21.5°C) 
(Fig. 6). All Chinook salmon experienced one or 
more 12-24-h periods of temperatures >18°C, 
and 85% of salmon had one or more such periods 
>20°C. Overall, 40.1% of the records were >18°C, 
and 2.1% were >20°C. Chinook salmon were not exposed 
to water >18°C in Puget Sound but had extended exposure 
in the upper estuary (98% of daily mean temperatures 
>18°C in August) and in Lake Washington (45% of daily 
means >18°C in September). 
Chinook salmon that arrived and were tagged earlier in 
the summer tended to spend longer in transit prior to enter¬ 
ing a tributary for spawning than did those arriving later. 
Specifically, fish tagged in the first week spent an average 
of 50.7 d in 2005 and 42.7 d in 2006, decreasing to 31.8 d 
in 2005 and 26.2 d in 2006 for the last group (Table 1). 
