The relationship between marine mammals and fishermen is one of conflict. Both parties are after the fish. Often the same fish.
Marine mammals get themselves caught in the fishermen's nets and are drowned.
Marine mammals are the host of parasites that end up contaminating the fish, causing substantial losses for the fishing industry. It is the task of management authorities to decide between these conflicting demands. The first problem they are met with is a lack of information. Research efforts have only recently begun to come up with good answers about what, and how much, marine mammals eat. These answers vary considerably between the various areas of the oceans.