One of the problems in the ocean is that steelhead apparently eat a lot of plastic, which ends up killing them. Fits what Smalma said, the longer they are out there eating plastics.
We could, of course, clean up our act and not dispose of plastics as we do. nah, that's a non-starter.
The plastic ocean, it is a problem and steelhead are at risk because they basically feed in the top of the water column where the most plastics is found. It probably kills some of them, nobody is out in the ocean looking for dead steelhead. The bigger problem is exposure to pollutants and the resulting loss of reproductive potential. Plastic particles concentrate all of the persistent organic pollutants on their surface by a factor of 1 to 10 million times over the sea water concentrations. The other part of the story is that as the larger pieces of plastic break up the total volume remains constant but the surface area is always increasing. Styrofoam coolers are one of the worst offenders. Small particles of styrofoam are common in ocean caught steelhead stomachs along with large pieces of other plastics and wads of mono filament line.
There have been a few studies addressing the absorption of pollutants from plastics in fish digestive systems. I don't think anyone has measured the actual pollutant concentration in adult steelhead. I suspect it is similar to that of chinnok salmon if not higher.
The link is to an extended abstract discussing the problem.
Plastic in Steelhead
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