Human activities can harm dolphins.

In some parts of the world, humans hunt certain types of dolphins for food. Yet even where they're not hunted, dolphins are threatened by some human activities.
Toxic chemicals that pollute nearshore waters may contaminate the fish on which dolphins feed. Scientists believe these chemicals might affect the health of dolphins and cause tumors. Pollution may have contributed to the deaths of dolphins that have washed up on beaches in recent years.
In the eastern tropical Pacific ocean, tuna travel under dolphin pods. When tuna fishermen set their nets around the dolphins to catch the tuna, the dolphins are trapped too. To help save dolphins, many tuna fishermen now use special nets and techniques to release the dolphins. From 1972 to 1994, dolphin deaths from purse seine fishing in the eastern tropical Pacific dropped more than 99%, from 423,678 to 4,095 individuals.
A more deadly type of fishing is done with gill nets. These nets stretch for miles across the ocean and extend deep under water. Once the fishermen have set the nets, they leave and return a few days later to haul them in. They remove the fish they were hoping to catch and discard all other animals that have died in the nets. Thousands of dolphins and other marine creatures drown in these huge nets each year.