Barnacles and whales have a commensalistic relationship in which barnacles benefit. What would most likely happen to the whale population if the barnacle population decreased?

The whale population would grow at the same rate it used to. It would remain unaffected.

Explanation:

In the event of a decrease in the number of barnacles, the whale population shall remain unaffected.

This is because the relationship between the two is a commensalistic relationship, which means that the barnacle population that depends on the whale population does not in any way affect the whale population.

Thereby, the whales shall continue life as usual.