Japanese fishermen have finished killing about 40 dolphins targeted for their meat as part of a larger group trapped recently in what activists say was the biggest roundup they have witnessed in the last four annual hunts.

Sea Shepherd, best known for its anti-whaling activities, said that of roughly 250 captured dolphins, the fishermen first selected 52 to keep alive for sale to aquariums and other customers. They included a rare albino calf and its mother.

Of the rest, about 40 were killed, one became stuck in a net and drowned, and the others were released, it said.

A video released Tuesday by Sea Shepherd shows dozens of fishermen on boats surveying the dolphins after they were confined to a cove with nets. Divers can be seen holding the dolphins selected for sale and guiding them to nets hanging off the boats.

While other dolphins have been killed since the hunting season began in September, Sea Shepherd said the 250 herded into the cove last Friday was the largest group it has seen since it began monitoring the hunt.

A rare albino calf was among 52 dolphins selected for sale according to the Sea Shepherd, who took this photo on Friday, in Taiji. (Sea Shepherd Conservation Society/AP)

The annual hunt in the village of Taiji received high-profile criticism when U.S. Ambassador to Japan Caroline Kennedy tweeted last weekend that she was deeply concerned about the practice.

The fishermen say the hunt is part of their tradition and call foreign critics who eat other kinds of meat hypocritical.

A Japanese government spokesman defended the annual dolphin hunt on Monday, saying it is carried out in accordance with the law.

The hunt was the subject of the Academy Award-winning 2009 film The Cove.