Fishy-
You comments seem to indicate that you consider anadromous O. mikiss to be a different “species” than the resident form - interesting.

However more to the point regarding the chromosome numbers found in steelhead -

From - http://www.nwfsc.noaa.gov/publications/techmemos/tm19/hatch.html

“Chromosome karyotypes in steelhead and rainbow trout have also been extensively studied (see review in Thorgaard 1983). In a survey of steelhead from Alaska to central California, Thorgaard (1983) found that although chromosome numbers ranging from 58 to 64 were observed, a 58-chromosome karyotype was the most common in most samples. In contrast to results for studies of morphological and allozyme characters, Thorgaard did not find chromosomal differences between interior and coastal O. mykiss populations. All interior/redband trout populations had predominately 58 chromosomes, as did most coastal rainbow trout and steelhead populations.

The exceptions to the 58-chromosome pattern, however, provide insight into population genetic structuring in O. mykiss. Two geographic regions were characterized by steelhead with 59 or 60 chromosomes: the Puget Sound/Strait of Georgia region and the Rogue River/northern California region. However, the karyotypes of fish from these two regions were different; northern fish with 59 or 60 chromosomes had a different number of subtelocentric and acrocentric chromosomes than did southern fish (Thorgaard 1977). Farther south, winter steelhead in the Mad and Gualala Rivers from northern California and resident trout from the San Luis Rey River in southern California had 61-64 chromosomes (Thorgaard 1983).”

Tight lines
S malma