Excluding all other sources of adult mortality (i.e. none for the orcas, sea lions or bears) in order to support a 75% exploitation rate, the stock must be able to produce a pre-harvest adult recruit ratio of at least FOUR adult recruits per spawner. Three of those recruits would be expended as harvest, leaving one recruit for the gravel to sustain the original spawning population.

So let's throw the predators a bone…

Now you need production of FIVE pre-harvest recruits per spawner to support that level of exploitation. One for all the predators, three for all the nets and hooks, leaving one lonely fish for the gravel.