Not being a biologist, I cant give you a 100% accurate answer, but I think I
can offer a couple of suggestions. Steelhead, like coho and sea-run
cutthroat, spend at least a year and sometimes as many as three in the
stream environment where they were born, and are subject to predation
throughout that period as well as the seasonal vicissitudes of floods and
low, warm water periods in our degraded stream habitats. Sockeye, on the
other hand, swim down (or in some cases up) to the lakes which seem to be
necessary for good populations of sockeye to exist. Typically a Lake
Washington sockeye will spawn in the Cedar River or Issaquah Creek (a few
spawn on the lakeshore where water percolates up through the gravel)
sometime from September through early December and the hatched fry migrate
to the lake in March. Lake Washington provides a much more friendly
environment for these little fish and they usually remain there for twelve
to fifteen months feeding on zooplankton before becoming smolts and
migrating out to Puget Sound and the ocean. I think that this particular
survival strtategy is the primary reason for their large numbers (sockeye
are the second most abundant species of salmon, behind the humpy, on the
Pacific coast). Many rivers (notably the Stillaguamish) have small
populations of so-called "creek sockeye" who have no lakes to rear in and
these runs never amount to more than enough fish to barely maintain
themselves. Perhaps this is nature's way of providing for any geological
accident that might form a lake in the drainage. The fry of humpies and
chums move downstream almost as soon as they are hatched. The fact that
they have been able to maintain their numbers pretty well despite of the
loss of substantial amounts of suitable spawning areas would indicate that
the time spent in stream and river is the most hazardous period that a young
salmonid has to deal with.
