sockeye has been October 19. During the prior cooler years (1947 to
1985), the average peak spawn date was October 11.
This delay means “you’re starting to pushthe ecological envelope
that these guys have in terms of available energy resources,” Hyatt
says. Even for those that make it, this delay bodes ill for their success-
ful reproduction. By spawning later, they leave their eggs more prone
to being washed away when the spring snow thaws start and river-
scouring floods rip through the gravel where the eggs are laid. Eggs
now hatch 15 days later—on average, on March 3. Compare that to
February 16, the average peak hatching date back in the cooler years of
1947 to 1985. The longer it takes for eggs to hatch, the likelier it is they
will be swept away in springtime floods.
And if that were not enough, juvenile salmon—the next life history
stage of the fish—find the going tougher as temperatures warm. They,
too, live by strict rules, Hyatt found while studying them at British
Columbia’s Osoyoos Lake. Rule no. 1: When the fish come up toward
the surface to feed at night, they rise only so far. Once they hit the invis-
ible line where the water temperature starts to surpass 63° or 64°F,
they stop and will not go beyond it. Rule no. 2: The juvenile fish will
swim only in water that contains at least 4 milligrams of oxygen per
liter of water. It is “the four-milligram rule,” as Hyatt calls it. Colder
water retains more oxygen.
“So in the central basin,” Hyatt says, the salmon are “squeezed by
super-optimal temperatures and sub-optimal oxygens. In the north
basin, they still have some latitude to move, but this has profound
influences on... areas they can use in the lake to feed and grow. The
south basin of Osoyoos Lake is unsuitable for rearing through the
summer and fall. Sockeye avoid this basin or die.”
Warming temperatures have dealt a blow. If the lake’s surface
warms to 63° early in the season and lingers late, the oxygen progres-
sively depletes. This temperature/oxygen squeeze varies from being
severe in years like 2001, to moderate as in 1996, to low as in 1997.
Still, on average, the squeeze now occurs more than 90 days per year.
Back in the cooler era of 1945 to 1985, the squeeze occurred fewer than
60 days. The lake can become a watery death chamber. In hot 1998, the
152 Sally Deneen