Paul Watson, the conservation group's founder, said Japanese military technology, increased costs and "hostile governments" made it too hard to save whales again this season.
"For the first time ever, they have stated they may send their military to defend their illegal".
Tokyo claims to conduct vital scientific research using a loophole in an global whaling ban, but makes no secret that the mammals end up on dinner plates.
Japan defies worldwide protests to carry out what it calls scientific research whaling, having repeatedly said its ultimate goal is to whale commercially again.
Watson said his volunteer organisation could not compete with Japanese military satellite technology, which tracked Sea Shepherd in the ocean.
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"New Zealand has always been at the forefront of the worldwide movement to ban whaling, and Japan is testing our leadership on this issue once again". "We are going to have to find an alternative way to deal with them, and we will", Watson says, not elaborating on what alternatives may be, the ABC notes.
Mr Watson said if countries were against whaling they should monitor and deter whaling in southern waters.
In January, Australia said it was "deeply disappointed" by Japan's decision to keep whaling and reiterated its opposition to "all forms of commercial and so-called "scientific" whaling". "This has been extremely hostile".
Sea Shepherd is headquartered in Amsterdam, with offices in more than 20 countries around the world.
The Sea Shepherd achieved fame for its attempts to ward off whales from Japanese boats.
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Security experts have criticised Sea Shepherd's tactics at sea, saying they endanger lives.
For instance, a US court delivered an injunction previous year against Sea Shepherd to keep its American branch of ships from coming within 500 yards of Japanese whalers.
Watson says in 2012/2013 the Japanese whalers went home with only about 10 percent of their intended kills because of Sea Shepherd's efforts. But Watson recalled that they Japanese did so for a year and then returned with a new program, which in his opinion is illegal and reduces their bogus self-allocated kill quota.
The ocean-focused conservationist group has sent ships to harass and hinder Japanese whalers for the past 12 years, but in an announcement on its website says its resources can't compete with those of its opponents, who are backed by a "major economic super-power".
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The Kim family has ruled North Korea since the country's founding in 1948, but very little is known about its members . It was only in 2012, three years after the pair's reported marriage, that she was first seen in public.