Salmo, it was Meigs. Think I have a couple of their papers.

To echo the variability that Salmo talks about, I have not seen any data that shows recruit per spawner to first return even averages 1; it is generally .6-.9. Of course, this means repeat spawners are critical. But, the R/S data seem to show some cycles with high years (still only occasionally >1) every 3 or 4. The low years are then buoyed up by the repeats. So steelhead evolved to be highly successful spawners once every few years. Unlike salmon, which have to get it right the one time they spawn.

Which also may explain why steelhead have such a long return and spawn time; the evolutionary expectation is that only a part or two of the annual spawn will be successful. One year the tributaries, one year the mainstem, one year the early spawners, and one year the ones that spawn after the March deluge.