The Uncertain Future of Fraser River Sockeye - Publications du ...

Cohen Commission of Inquiry into the Decline of Sockeye Salmon in the Fraser River • Volume 2
zooplankton such as Daphnia. They tend to remain
near the surface at dawn and dusk while they are
feeding, and migrate deeper during the bright
daylight hours to avoid predators. 88
Several Fraser River sockeye populations,
including the Harrison River population, do not
spend a year in a nursery lake and are thought to
have a different outmigration pattern. They migrate
downstream almost immediately after emerging
from the gravel and, after spending a few months in
sloughs and estuaries of the Lower Fraser River, enter
the Strait of Georgia before they are one year old. 89
Mr. Lapointe testified that, in the case of a
spawning female that lays 3,000 eggs, only about
420 eggs survive to become fry. 90 He identified
the following naturally occurring stressors that
contribute to fry mortality: lack of food, predation,
diseases, and environmental stresses such as
water temperature. 91
Several of the Commission’s technical reports
examined, and witnesses testified about, the
stressors at these two life history stages that may
have caused or contributed to the recent decline.
I summarize these discussions below.
Predation
In Technical Report 8, Predation, Dr. Christensen
and Dr. Trites reported that, as both coho and
chinook salmon age, they increasingly prey on other
fish. Because they tend to have a longer residence
in freshwater, these species typically reach a size
where they potentially can prey on small sockeye fry.
However, Dr. Christensen and Dr. Trites concluded
that the recent decline in population estimates for
chinook and coho in the Strait of Georgia indicates
that these species are not likely responsible for the
decline in survival of Fraser River sockeye. 92
The researchers stated that, although a 1996
study in Lake Washington found that cutthroat
trout was the only important predator on sockeye
fry, little information is available on the abundance
and trend of coastal cutthroat trout in the
Fraser River system. Dr. Christensen and Dr. Trites
concluded that cutthroat trout are unlikely to be
abundant enough to constitute a major factor
in the decline. 93 They reported that, although
steelhead can consume a significant amount of
emergent sockeye smolt salmon, steelhead are
unlikely to be a major factor in the Fraser River
sockeye decline, given that the steelhead population
is reduced in the Fraser River. They reached a
similar conclusion about bull trout. 94
Several studies in Washington and Oregon
found that a significant proportion of the annual
diet of large northern pikeminnows consists of
sockeye salmon. A pikeminnow eradication program
has been in place in Cultus Lake since 2005. 95
The researchers concluded that pikeminnow “may
also be important predators of Fraser River sockeye
salmon, but there does not seem to be abundance
or trend estimates for the pikeminnow, and, hence,
their importance cannot be quantified.” 96
During the evidentiary hearings, Jeremy
Hume, research biologist, Lakes Research
Program, Science Branch, testified that, in 2005,
DFO removed about 45,000 northern pikeminnows
from Cultus Lake, a practice that has led to
increased survival of sockeye salmon. 97
Dr. Christensen and Dr. Trites also considered
several introduced fish species and made the following
observations:
• A previous study had identified smallmouth
and largemouth bass as predators on juvenile
salmon, and considered the risk very high in
small lakes. However, notwithstanding the
absence of abundance and trend estimates in
the Fraser River system, the researchers found
it unlikely that these species were abundant
enough to have had major influence on the
recent decline. 98
• Yellow perch, which were introduced illegally
as live bait, have been confirmed in 59 lakes
or ponds and 19 streams, including within
the Lower and middle Fraser River and South
Thompson River watersheds. Although yellow
perch are potentially an important predator
on and competitor with sockeye salmon in the
Fraser River system, the researchers concluded
that “the available information provides little
support for the hypothesis that yellow perch
were a major factor for sockeye survival trends
over the last three decades.” 99
Climate change
The authors of Technical Report 9, Climate
Change, Dr. Hinch and Dr. Martins, testified that
scientists have identified water temperature as the
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