Why Did the Walleye Move to a Different River?

A large population of Yellow perch lives in the East river. The Walleye is a larger fish that eats the Yellow Perch. Despite lots of prey all the Walleye have moved from the East river to a new river. What most likely caused the Walleye to move to a different river?

(A)There is a lack of food in the East river which caused the Walleye to move to a different area.
(B) The Walleye are no longer able to catch the Yellow Perch and need to move to a different area.
(C)There is climate change which has caused the resources to decline in the East river.
(D) There is pollution in the East river that the Yellow Perch are adapted to but the Walleye are not.

(D) There is pollution in the East river that the Yellow Perch are adapted to but the Walleye are not.

To ensure survival and continued sustenance in an environment, there is a need for an animal to possess certain features which would enable her to cope with the conditions of the environment. The ability of animals to successfully live and survive in an environment is made possible through adaptation as the animal's adaptive features ensure the animal is able to live through and cope. If the adaptive features aren't suitable for such an environment, then migration has to occur, leading the animal to find a more suitable environment for survival. In the scenario above, the East river was presumed to be a good habitat for the Walleye and Yellow Perch, in spite of the available and abundance of food, the Walleye migrated, which is most likely due to the fact that the Walleye lack the adaptive features to survive in the East river probably due to pollution which the Yellow Perch is well adapted to.