Biology Reference
In-Depth Information
Once salmon arrive at a suitable spawning ground, they sweep their
powerful tails across the bottom to dig a redd (rocky nest), deposit and
fertilize their eggs, cover them up, guard them for a few weeks, and then
die. For those who make it far up into tributary streams, the journey is
arduous. They must swim against downstream flows, battle up natural rap-
ids, climb fish ladders, and endure unhealthy warm-water reaches.
Adult Chinook travel inland through the bay to spawn in four different
seasons. Scientists thus refer to them by the season of their spawning run—
winter, spring, fall, or late fall—as they pass through the Golden Gate.
To reach distant spawning grounds, salmon likely rely on a medley of
environmental cues. Biologists think day length, the sun's position, the
earth's magnetic field, and water salinity and temperature gradients are
among the signs salmon read to find their way from the open ocean to the
coast. From there, these remarkable fish literally follow their noses, navi-
gating upstream toward the distinctive scent of their natal waters. Salmon
undertake their spawning journey at three to six years of age.
Scientists are tracking where juvenile salmon go and what they do in
the upper reaches of the estuary. They saddle 3- to 5-inch-long juvenile
salmon with one-gram radio transmitters and release them above river
and canal junctions or pumping intakes. “The most dramatic result of our
telemetry on smolts is the sheer magnitude of distances they move each
day, twice a day, up and downstream with the tides,” said Dave Vogel, a
consulting scientist, in a 2004 publication. Vogel observed french fry-
sized smolts traveling more than 9 miles with the tides in a single day. At
delta channel junctions where flows split, the smolt didn't always go with
the biggest flow or the straightest migration route. “What determines
whether a fish turns left or right are site-specific conditions—velocity
changes in the water over minutes and seconds related to tides and chan-
nel geometry and time of day.”
Scientists outfitted this fingerling
salmon with a transmitter in order to
track its movements and habitat prefer-
ences within the estuary. (Dave Vogel)
Vogel's work has often been coordinated with research on flows from
Jon Burau of the U.S. Geological Survey, so data on where the salmon went
could be correlated with data on where the water was headed. In a more
recent study, Burau and Columbia Basin biologists tagged and released
6,000 juvenile salmon smolts throughout the delta, each with its own iden-
tifying acoustic ping. Receivers placed at 70 delta channel junctions
