indeed, but only considering one factor (ie the ocean) means you are getting an incomplete understanding. Particularly in freshwater rearing species like coho and steelhead if habtiat is degraded then long term the population is in trouble. Yes ocean survival can fluctuate dramatically (eg. 1-25% for steelhead) that means from 1,000 smolts you might get anywhere from 10 adults to 250 depending on ocean conditions, but long term one year of good or bad adult returns don't make or break a population of salmon.