Answer is: when amount of available plankton is greatly reduced.
Competition is rivalry between two or more organisms for territory, a niche, for scarce resources (in this example available plankon).
Competition occurs naturally between living organisms which co-exist in the same environment (in this example some water area).
Competition between species (in this example fishes) for resources plays a significant role in natural selection.
Genetic variation is important to the population's ability to survive in different situations that affect natural selection. New habitat and increased food supply can increase genetic variation.
It is under the condition when amount of available plankton is greatly reduced that competition among fish most likely to occur. Hope this answers the question.
