Answer:
The doctrine of respondeat superior is generally applied to torts and is used by civil courts, but can also include criminal activities. In this case, the main issue is the fine imposed on Oil Dawg, so yes, this doctrine applies.
Respondeat superior basically makes the principal (the employer in this case) legally responsible for unlawful or negligent acts committed by its agents (employees in this case). This doctrine applies as long as the illegal acts were committed within the scope of the normal employment, e.g. crashing a van while making a delivery. In this case, the illegal polluting was carried out while transporting oil to the US.
The only possible defense that Oil Dawg might have is that the people that committed the illegal polluting were independent contractors and they weren't actual employees of the company. But according to the text given, that is not the case.
This doesn't mean that only the employer will be go to trial, the employees that committed the illegal polluting will also go to trial since they are both liable, and maybe face the same or even different charges.