I've got that book, also. Just can't lay my hands on it, and forget the author's name. The title is something like Pacific migrations of steelhead (and salmon?). I just hate this old timer's disease. So I quick looked again at Trey Comb's Steelhead Trout (1971). It's got an old and short description of what was known about steelhead ocean migrations at that time. Funny, doesn't seem we know a lot more now.

They do migrate up the coast sort of like salmon, but don't hug the shore or continental shelf like coho and chinook. Neither do they school up like salmon. High seas research and drift net ships (remember them?) pick up the odd steelhead mixed with various assortments of sockeye and chum and very occasionally pink salmon. They migrate far to the west, many crossing the international date line. It isn't unusual for U.S. marked steelhead to be collected by Japanese research vessels. There southern ocean distribution depends on time of year and surface sea temperature. It looks like the 50 degree isotherm sets their southern limit as it swings north and south from summer to winter.

The upshot is that they exhibit greater wanderlust than salmon, and seem to often be solo travelers.

Sincerely,

Salmo g.