Answer: stay the same
Explanation:
Commensalism can be define as relationship between two organisms which lives in close association with each other. In this relationship one organism is benifited and the other is neither harm or benifited.
The barnecal is the lice which attach themselves to the whale skin which sucks the blood. But whale having a large body remain unaffected.
On the basis of the above description, if the barnecle population decrease than the whale population will remain the same.