Members of the Sea Shepherd Conservation Society are chasing the fleet in the hopes of interrupting Japan's annual whale hunt, which kills up to 1,000 whales a year.
The two sides have clashed violently in the past, including last year, when a Sea Shepherd boat was sunk after a collision with a whaler. Japan is accused by Sea Shepherd of abusing a ruling by the International Whaling Commission which permits it to harvest a quota of certain species of whales for research, rather than commercial, purposes.