Their populations would probably remain down until the krill or some other extremely abundant prey returned. The ecosystem is very fragile and complex. Each part of the system serves an important role. The removal of a part, or change of a part in one direction or another, effects the entire system. After the removal or change, the system will find a new equilibrium, but the resulting ecosystem WILL be different than it was before.
Humans are very effective in in causing changes to the ecosystems of the world.
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