The tank leak prompted the nuclear authority to upgrade its rating Wednesday to a level-3 "serious incident," from a level 1 on the International Atomic Energy Agency radiological event scale.
"We cannot fully stop contaminated water leaks right away. That's the reality. The water is still leaking in to the sea, and we should better assess its environmental impact," Tanaka said in a speech in Tokyo.
Tanaka said his agency recently set up a team to collect data more systematically and comprehensively to assess the extent of contamination and evaluate the impact on the ocean.
Scientists have said contamination tends to be carried by a southward current and gets largely diluted as it spreads into the sea.
Fisheries officials are not convinced. The recent leaks aggravated the image of Japanese seafood in and outside the country, and consumers are even shunning fish proven to not be tainted, said Japan Fisheries Cooperatives Chairman Hiroshi Kishi.
"We think that contaminated water management by your company has completely fallen apart," Kishi said, as he confronted TEPCO President Naomi Hirose at the company's headquarters in Tokyo. "This deals an unmeasurable blow to the future of Japan's fishing industry and we are extremely concerned."
Commercial fishing off the Fukushima coast has been mostly banned since the accident, except for limited catch of selected fish and deep sea catch.
In a nation highly sensitive to food safety, there is no market for the fish caught near the stricken plant.