Fish stocks recover by fishing for their prey. Works for Cod and Salmon Trout

Work done in Norway with salmon trout gives a proof.  When a predator fish stock plunges, there is a population explosion amongst their prey.  The prey compete for food resources and face so much competition from their peers that few of them reach full size and sexual maturity.  This means the number of minnow-sized prey fish for the predator to eat declines markedly.  When we deliberately fish to thin the ranks of the prey, the remaining individuals have better food supplies and reach sexual maturity at a large size.  These larger fish are successful in reproducing themselves.  Their off-spring, the stocks of minnow-sized prey fish bounce back and the predator fish start feeding and recovering in number.

By harvesting char in the lake in Norway,
Lennart Persson and his colleagues have managed
to increase the number of small char individuals that predator fish prefer.

See also this story about Canada's codfish recovery.  There's a twist.  The prey fish which had a population explosion, dined upon baby cod, further shrinking the adult cod populations. Canadian cod recovering, but it's tricky.