The statement is not really true that the radiation level inside the harbor right at Fukushima I Nuclear Power Plant is no different than those outside, further away from the plant, say 10, 20 kilometer radius, as far as the contamination in fish goes.
The data that TEPCO released on January 18, 2013 shows the fish inside 20 kilometer radius to have radioactive cesium in one to three digits, and the fish inside the harbor in three to six digits.
It is not just the "murasoi" spotbelly rockfish with 254,000 Bq/kg of radioactive cesium but also jacopever with 110,000 Bq/kg, greenling with 40,000 Bq/kg of cesium. There was another spotbelly rockfish with 140,000 Bq/kg cesium.
The highest outside the harbor is 480 Bq/kg, and the lowest inside the harbor was 770 Bq/kg of radioactive cesium.
From TEPCO's Handout for the Press, January 18, 2013 (click to enlarge):
(Page 1, with highest number outside the harbor)
(Inside the F-I harbor, pages 8, 9, 10)