The so-called Antarctic Ocean, or the Southern Ocean, refers collectively to the vast expanse of sea surrounding the continent of Antarctica, composed of parts of the South Pacific Ocean, South Atlantic Ocean, and the South Indian Ocean, as well as the surrounding seas like the Weddell Sea, Ross Sea, Amundsen Sea, and Bellingshausen Sea.
The South Pacific Ocean from south of 60 degrees latitude up to the continent of Antarctica is the Southern Ocean. Although now we can roughly determine the active area of the Nisshin Maru, the whaling fleet keeps moving, and the Southern Ocean covers several million square kilometers. Finding a few ships in such a vast area is truly like finding a needle in a haystack.
With a ship and torpedoes on hand, the only thing left is to find the Nisshin Maru. The area where Yelena and others encountered trouble was near the Amundsen Sea, but by the time the Avenger set sail, the Nisshin Maru was long gone to an unknown location.