As an extreme overpopulation of fish is provided and a variety are plenty of visible fish, this attracts predators since predators are most likely to eat easy prey and are not willing to search for "hard to catch" food.
In order to survive, the breed of fish must adapt to new genetic variety which will allow them to survive, thus leading to less "easy to catch" prey, which forces some predators to leave since there is a low amount of food.
After the fish have adapted into a breed that is harder to catch due to its camouflage, all predators have no food source and leave the place in search of more food, while the new breed are now able to increase their overall fitness.
In this scenario, their is a large amount of mice, and most of those mice visible mice, meaning they are easily spotted by the owl (predator), decreasing the fitness of that species. However, the lighter colored breed of mice are not visible, making a contribution to the survival of the breed of mice.
In search of survival, the mice will adapt to a genetic variation since that will allow them to increase fitness. Since the colored mice are easily being seen as prey, there is no point of passing that trait. Therefore, their is a high probability there will be no new dark colored mice and the light colored mice will increase in the population.
With the camouflaged mice adapting as the main breed, the predator decides to leave the environment in search of new source of food since there are no more visible targets, whereas the mice population are able to survive due to he adaptions it managed to convey.