Fishermen often perceive marine mammals as posing a threat to their business.  Seals, sea lions, dolphins and whales can be viewed  as pests that cause damage to nets and deplete fish stocks that are necessary for commercial fisheries to survive.  Moreover, seals can also act as hosts for various parasites whose larval stage occurs in commercial fish.  An example of this is the cod worm, which lives as an adult in the stomach of grey seals but as larval form in cod and cod-like fish.  If too many commercial fish are effected by parasites in an area, the fish become harmful to human consumption and the fisheries may even need to relocate to an alternative stock.  To avoid such problems, at times in the past  seal populations have been culled to reduce their impact on the fish populations.

From the marine mammal side, the relationship is no less negative.  For one thing, marine mammals frequently get caught in commercial fish nets and these animals, being air-breathing mammals, drown when trapped under the water by nets.  This includes both nets currently used for fishing as well as discarded nets and debris left by fishermen in the water.

Most significantly, even when marine mammals and fishery operations do not directly interact, the enormous scale of the growing human population is having a significant negative impact on marine mammals.  Over fishing of the oceans by commercial fleets has significantly reduced fish populations that are main sources of food for marine mammals.  It is plain to see that if the amount of fish in the ocean continues to be reduced due to human consumption, it will lead to severe consequences for marine mammals.  

It is important to remember that there are only so many fish in the ocean.  If humans take too many, the marine mammals and other creatures of the sea will have nothing left for them.