Interesting analysis RG. As far as outward spring smolt migration goes, I have always tried to note the spring weather conditions for future reference of return survival. Whether it be rain, or early spring melt runnoff, I alway hoped for one of the 2 to dirty up the rivers to help the smolts out.
My home river, the Puyallup, has a large cormorant problem. They show up around the 1st of April and stack the lower river, anticipating the smolt migration. They leave when the river gets turbid. The lower Puy. is like freeway. Man made dikes, no or little cover, and staight as an arrow to Puget Sound. The cormorants capitalize on this, are smart visual hunters, and smolts have little chance in low clear water. I have watched them in action. I don't know for sure, but logic tells me if the water gets dirty it is more difficult for them to see to hunt, aiding smolt survival.
I remember reading the early pit tag tracking results for Winter Steelhead on the Puyallup and other South PS rivers. Results displayed missing fish at the 1st PS pit tag reading station. I think that station is located in Seattle? A large majority of tagged smolts went missing before this 1st station, and were all missing by the time they got to the last station in the Straits. Obviously, and it makes sense, is they got eaten or died before then. Researchers probably could find those pit tags in cormorant and pinniped feces.
Anyway, this spring the Puyallup got turbid early from rain and now is puked from glacial melt, cutting short the slaughter.
I never got a correlation between early spring turbidity and greater fishing success years later. Currently, the South PS rivers are closed for Winter Steelhead and it's been years since I've fished them.
I know lots of factors are at play for smolt survival, but I think the cormorants and pinnipeds are guilty more than researchers think.