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A few important points:
* The above anadromous behavior is common in the northern latitudes where the ocean environment is often more productive than stream environments. Anadromous salmonids take advantage of both environments while in their different life stages.
* Smoltification only facilitates the transition that young salmonids make between fresh and salt water, it doesn't cause the movement.
* To this day, steelhead and
salmon in the Great Lakes have retained this anadromous migratory behavior
of their ancestral populations, but use the lakes to mature. It is
theorized that upon reaching the Great Lakes, steelhead will reverse
their smoltification process to maintain themselves in freshwater, due
to the increased forage available to them which satisfy their increased
This illustration was taken from
the following source: