Makes me wonder if their decision to become anadromous is based more on avoiding bad river conditions (extremely hot and dry summers, lack of food,) or more on genuinely taking the “high risk, high reward” opportunity that the ocean provides.
These fish with a brain the size of a pea aren't really making a decision to be resident or anadromous. Their body decides for them. According to some recent research, the age, size, and body lipid values they achieve affects their behavior to remain in the river or to migrate. It is the behavioral "choice" that is acted upon by the environment. Environmental conditions influence subsequent reproductive success. What we were used to seeing were environmental conditions that favored anadromy, meaning that the mykiss that adopted a migratory life history ended up experiencing higher reproductive success than their resident counterparts.
We have seen in some rivers, most particularly the Cedar (Lk WA), where the anadromous life history has resulted in very poor SAR and therefore, very low reproductive success. During the same time period, resident, and particularly adfluvial, rainbow trout have experienced higher reproductive success. So for the past 20 years resident rainbow trout have been the dominant mykiss type present in the Cedar River.
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