Geoscience Reference
In-Depth Information
that this climatic shit , or oscillation, has had direct ef ects on salmon pro-
ductivity throughout this vast region.
Salmon populations have also responded to broader climatic shit s on
longer timescales—like those described in chapters 9 and 10. For instance,
a study looking at the chemistry in sediments cored from a Kodiak Island
lake in Alaska has documented shit s in salmon abundance over the past
two thousand years. During the three to i ve years that adult salmon spend
in the ocean, they assimilate an isotope of nitrogen (nitrogen-15) in their
tissues that is unique to the marine environment. At er the salmon migrate
back into the watershed (in this case, they migrate up the rivers into a lake),
the nitrogen-15 from their tissues ends up in the lake sediments at er the i sh
die and decompose. Analyses of nitrogen-15 in the lake sediments collected
in layers provide a measure of the amount of salmon entering the lake from
the ocean over time. h e lake sediment cores reveal a pattern of l uctuating
populations, with a low abundance of Alaskan salmon prior to AD 800, then
increasing from AD 800 to 1200, and decreasing again at er AD 1200. h is
pattern correlates with climate changes over the American West. During the
Medieval Climate Anomaly (discussed in chapters 9 and 11), ocean surface
temperatures of the coast were more ot en cooler than usual; these data
correlate with a period of greater abundance of salmon as seen in the lake
sediments. Subsequently, the isotope data in the lake sediments indicate that
salmon populations dropped during the Little Ice Age, a time when ocean
temperatures were ot en unusually warm, with frequent and intense El Niño
events (see chapters 10 and 11).
Despite the natural l uctuations in salmon abundance associated with
changing climatic conditions, salmon have been a mainstay for Native
American populations along the Pacii c coast for at least 5,000 years. Many
tribes today still consider salmon critical for their survival as a culture and a
people. h e relationship between these tribes and salmon along the Pacii c
coast evolved as salmon habitats and annual runs became more stable and
abundant during the mid-Holocene, when sea levels stabilized following the
last ice age (as described in chapter 7). h e native people understood the
salmon's natural migration patterns and managed for a sustainable harvest,
using barbed spears, harpoons, dip nets, and seine nets. Prior to European
contact, Native Americans in California each year caught and consumed
an estimated 15 million pounds of salmon. Farther north, on the Columbia
River, a population of 50,000 Native Americans harvested between 20 and
40 million pounds of salmon annually—representing about 5-20 percent of
