Cesium found in Japans Fish

Nearly 18 Months after the Fukushima leak and there are still problems with many things. Including Japans fishing industry. The Japanese government has taken a precaution to stop fish with high levels of cesium into the market. But what is cesium? Highly reactive, soft, metallic element of the alkali metal group, used chiefly in photoelectric cells. Symbol:  Cs; atomic weight:  132.905; atomic number:  55; specific gravity:  1.9 at 20°C; melts at 28.5°C. (dictionary.com). Scientists think that there is still an undetected leak somewhere. But is is something we should worry about? The are said to have 250 times the limit for cesium. But is it really toxic? Cesium-137 is one of the most mild kinds of radiation. Comparing to radioiodine which 10% is lost through urine ans sweat, but the other 90% stays in the body for 50-150 days. But still certain doses can cause cancer, and really high doses can be lethal. So thats why the government is trying to make a wall that reaches 100 feet underground and is set to finish in mid 2014. But will that be soon enough?