A huge factor that is often ignored is the impact that ocean survival has on various population parameters. During periods of poor ocean survival measures such as carry capacity and MSY levels for a given basin/population are reduced - marginal habitats no longer producing enough smolts to replace spawners. This of course increases the likely of over harvest.

Freespool -
the answer to your question -

"My question is why do stocks rebound quickly when ocean conditions improve, but respond slowly, or not at all when harvest is curtailed?"

is that in those situations the quality of the freshwater is such a dominate factor that it often over shadows the harvest reduction. The two big factors at play in your example is the carry capacity and production parameters of the population.

If historically the carry capacity of system could be symbolized as a gallon jug the population productivity would be how quickly that jug would refill from low spawner abundance - historically that could visualized as the water tap wide open. That gallon jug would refill quickly; in the real world depressed populations would respond within a generation to a condition near full seeding -what ever spawners reach the gravel produce large number of smolts.

Contrast to the situation we now see on many of degraded system. Instead of a gallon carrying capacity they often have only a pint or is some cases a cup for a capacity. Even more important is that their population has been also been greatly reduced. To condition this example the productivity wouold be just drips from the facet; meaning that even though the capacity is a pint it will take a longer to refill. The result is the it takes a long time (generations) for a population to respond from a depressed conditions -those spawners reaching the gravel are producing very few smolts.

When marine survival increases; let's say it doubles those limited numbers of smolts being produced in the degraded system do twice as well leading to larger runs. Of course the next generation doesn't benefit as much as we would think due to the reduced productivity of the population/habitat. All of this of course is why so many of us continually harp on the freshwater habitat conditions as key even though harvest and other factors can be important.

Tight lines
Curt