Dolphin roundups and slaughter happen for five months of every year in Japan. Many of the animals captured are taken into captivity, while others are used for their meat. There are lots of ways you can help end Taiji's dolphin slaughter:
Visit Sea Shepherd's site to learn more about the problem. The organization has many ways you can help -- from volunteering to simply spreading the word on Facebook and Twitter.
You can also call and write to the authorities in Taiji as well as the Japanese Embassy in your country, the U.S. Embassy to Japan, U.S. and Japanese Ambassadors to the UN and the U.S. Senate members of the Committee on Foreign Relations, among others -- all of their contact information is on this page.
Another organization that you can voice concern to is the International Whaling Commission (IWC), which upholds a moratorium on killing whales that does not include small cetaceans like dolphins or porpoises. Their information is below:
The Red House, 135 Station Road, Impington, Cambridge, Cambridgeshire CB24 9NP, UK.