The answer would be survival of the fittest.
This metaphor, "survival of the fittest," was used by Herbert Spencer to explain natural selection. It is sometimes also referred to as the survival for existence. This phrase came from the Dawinian theory of evolution where he explains that all species are capable of producing many offsprings. However, the survival of those offsprings depends on their ability to adapt to the risks and opportunities of the external environment.
This relates to your scenario in the sense that fish lay a lot of eggs in the beginning but not all of them would live to achieve maturity. This is because even as eggs, they have many predators and are exposed to other elements. The threat pushes on even if they are able to hatch and grow up.
