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
generated by rainfall. However, increased rainfall
may also be associated with increased freshwater
survival, presumably because it increases the area
available for spawning and, hence, reduces mortality
caused by superimposition of eggs. 14
The researchers found that adjacent to
freshwater habitats, air temperature in the Pacific
Northwest has increased an average of 0.08°C per
decade over the past century. Precipitation has
increased by 14 percent per century, with more of
it now occurring as rainfall. In British Columbia,
minimum temperatures have shown the highest
rate of increase at 0.17°C per decade, and precipitation
has increased by 22 percent per century.
The highest increases in precipitation have occurred
in the interior area. 15
Warm winters and springs since the 1950s
have caused earlier snowmelt and, hence, an
advance in the spring freshet by one to four weeks
across a large number of rivers in the Pacific
Northwest. In the Fraser River, the dates that mark
one-third and one-half of the cumulative annual
flow have been occurring approximately one day
earlier per decade since the 1950s. 16
There has also been a noticeable increase
in water temperatures of rivers and streams.
In the Fraser River, water temperatures in the
summer have increased at the rate of 0.33°C
per decade since the 1950s, and the river is now
2.0°C warmer than 60 years ago. No long-term
records of water temperatures in the winter and
spring, when sockeye salmon eggs are incubated,
are available, but the Fraser River watershed has
likely warmed at the highest rates during that
season, since that is when the province’s climate
has warmed the most. 17
The researchers made a qualitative assessment
that life stage–specific survival has changed in the
past 20 years owing to the recent climate patterns,
particularly in temperature. Assuming that average
stream temperatures through winter and spring
have not become warmer than 8°C throughout the
Fraser River watershed, survival of sockeye salmon
eggs has possibly increased, though that of alevins
has unlikely changed. However, climate warming
may not have affected the early life stages of all
Fraser River stocks equally – the survival of interiorspawning
stocks may actually have been negatively
affected because their eggs and alevins seem better
adapted to colder temperatures. 18
A confounding factor in this assessment
relates to increased precipitation. Higher precipitation
in the form of rain may have led to an
increase in the mortality of eggs (due to scouring)
in recent decades. Interior-spawning stocks
would have been more affected because changes
in precipitation have been greater in the interior.
Also, returning adult Fraser River sockeye are
now smaller than in the past and, since smaller
females bury their eggs at shallower depths than
larger fish, the chances of scouring is increased.
On the other hand, as noted above, increased
precipitation means an increased area for spawning,
which may result in lower levels of mortality
due to egg superimposition. 19
Habitat impacts
I heard from DFO witnesses that habitat degradation
and loss pose risks to Fraser River sockeye
and that, if trends persist, there is going to be a
significant decline in the productive capacity of
Fraser River sockeye habitat. These trends will affect
sockeye in an incremental way over time because
habitat productivity has some direct bearing on the
ability of an ecosystem to produce fish (although
one DFO witness said habitat is not believed to be
implicated in the dramatic fluctuations in sockeye
returns). 20 I also heard that spawning, rearing,
and all the migration routes, including the Fraser
estuary, are critically important for maintaining the
productivity of these stocks. 21
Other freshwater stressors
This section considers evidence of the impact on
incubating Fraser River sockeye of forest harvesting,
the mountain pine beetle, surface water withdrawals
and groundwater extraction, small hydroelectric
projects, agriculture, and linear development. Some
of the stressors discussed here could also affect the
emergence and freshwater-rearing phases, which
are discussed later in the chapter.
Technical Report 3: Freshwater Ecology
The authors of Technical Report 3, Marc Nelitz and
others, sought to understand the potential role of
freshwater stressors in recent Fraser River sockeye
declines by compiling and analyzing the best
24