viigfish -
great question!

Pisco hit part of the answer dead on. Pink and chums are much less depenedent on the freshwater habitat. But equally important they have difference ocean migration patterns and conditions preferences (temeratures etc) in the ocean. Recent years have seen well above average survival for pinks and chums and below average for the steelhead.

The situation with the coho is much the same as with steelhead. Some of the data I have seen is that for more than a decade they have experience well below average survivals. We are seeing more coho (at least most years) in the terminal areas because of major reductions of catches of Puget Sound coho off the west coast of the Vancouver Island. That reduction (overall catches reduced from 1 to 1.7 million/year to less than 100,000) has masked the declined in survival as measured by what we see locally.

Bottom line the salmon/steelhead world is extremely complex with a large number of variables in play and each species having different needs and preferences. The net result is that it is common over a serveral decade period to see survivals of individual species to flucate more than 10 fold and for different species to respond differently to the same conditions.

Tight lines
Curt