Let’s not get too concerned: The first article (below) quotes the maximum activity in the crabs found near Fukushima as 9 Bq/kg – by way of comparison, according to Wikipedia, the activity in a banana (admittedly a different isotope, K-40) is typically 15 Bq (see further below). The average banana weighs about 0.1 kg, so this means bananas are about 150 Bq/kg.
The fact that you can detect as little as 9 Bq (assuming a 1 kg crab) is amazing, but just because you can measure something doesn’t necessarily mean it is hazardous!
"According to Tepco, radioactive silver Ag-110m (half-life 250days) was detected from 8 of 8 crab samples. The crab is Ovalipes unctatus. The highest reading was 9.0 Bq/Kg. The sampling place was 20km radius area around Fukushima nuclear plant port. The sampling dates were from this April to May. Ag-110m is spreading to the sea and taken by marine creatures.” http://www.tepco.co.jp/en/nu/fukushima-np/f1/smp/2013/images/fish01_130828-e.pdf
The above was taken from ‘Fukushima Diary’: http://fukushima-diary.com/2013/08/ag-110m-detected-from-8-of-8-crab-samples-from-20km-radius-area-of-fukushima-nuclear-plant-port/ (5th November 2013)
The major natural source of radioactivity in plant tissue is potassium, which in nature contains 0.0117% of the unstable isotope potassium-40 (40K). This isotope decays with a half-life of about 1.25 billion years (4×1016 seconds), and therefore the activity of natural potassium is about 31 Bq/g – meaning that, in one gram of the element, about 31 atoms will decay per second. Plants naturally contain other radioactive isotopes, such as carbon-14 (14C), but their contribution to the total activity is much smaller. Since a typical banana contains about half a gram of potassium, it will have an activity of roughly 15 Bq. Although the amount in a single banana is small in environmental and medical terms, the radioactivity from a truckload of bananas is capable of causing a false alarm when passed through a Radiation Portal Monitor used to detect possible smuggling of nuclear material at U.S. ports.
The above was taken from wikipedia: http://en.wikipedia.org/wiki/Banana_equivalent_dose (5th November 2013)