Geoscience Reference
In-Depth Information
In his topic
King of Fish: h e h ousand-Year Run of Salmon,
geomorphol-
ogist David Montgomery at the University of Washington describes how
salmon migration patterns and speciation co-evolved with the geomorphol-
ogy of western North America over the past twenty million years. During
this time, the plateau between the Rocky Mountains and the Pacii c frag-
mented into the fault-blocked basin and range topography in Nevada, while
the mountain ranges along the Pacii c coast, from California to A laska, were
uplit ed. h ese topographic changes inland and along the coast formed many
disconnected watersheds with distinct stream conditions. h e isolation of
these streams eventually led to the evolution of dif erent salmon species.
For many millennia, the streams and rivers of these coastal watersheds
have provided breeding grounds for the salmon. Hatching in the cold, rap-
idly running mountain streams, the young salmon fry develop into juvenile
salmon and then make their way downstream and eventually out to sea.
h ere, they spend their adult years in the open waters of the Pacii c before
beginning their arduous and improbable return-migration to their ancestral
streams, where they lay their eggs in the cold water and gravel beds before
they die. How they accomplish this remains to some extent a mystery, but
these faithful i sh are emblematic of the interconnectedness of conditions in
the oceans, in the climate, and on land.
h
e Climate Connection
Salmon populations have l uctuated in response to climate change over the
northern Pacii c Ocean operating on a variety of timescales. When sea surface
temperatures are unusually cold of the coast of the Pacii c Northwest, the
upwelling of nutrient-rich waters enhances the growth of phytoplankton and
krill, allowing salmon populations to do well. During periods when reduced
upwelling leads to higher surface water temperatures, krill and phytoplankton
decrease, leaving less food for salmon and causing a decline in their populations.
Nate Mantua, a climatologist at the University of Washington in Seattle,
has shown, with his colleagues, that salmon population size changed in
Alaska and the Pacii c Northwest in response to ocean conditions over the
past century. h ey demonstrated that North Pacii c sea surface tempera-
tures were generally warmer from 1925 to 1947 and again from 1977 to 2000;
between 1947 and 1977 and again at er about 2000, sea surface temperatures
were generally cooler. h ese shit s in ocean conditions are part of the Pacii c
Decadal Oscillation discussed in previous chapters. Here, we note simply
