my understanding is that ‘half-life’ applies to the time for the radioactivity in a substance to decrease by half, and ‘biological half-life’ is the time required for a body to expel half of the substance. You seem to be talking about the former, but for exposure I think the latter is the vital factor – for if Cesium isotopes don’t reside in the body for long then we’d have to consider them less hazardous to humans than if they resided for a long time.

"The long-distance migrations of commercial fish in the arctic seas may lead to the biological transfer of artificial radionuclides from highly contaminated local zones in the arctic seas to remote non-contaminated areas. Based on experimental data the dynamics of 137Cs bioaccumulation in fish from the Barents Sea (arcto-norwegian cod) was analysed. It was demonstrated that the 137Cs concentration factors for arctic fish were not constant, but gradually changed from 28 ± 5 in 1979 up to 182 ± 48 in 1992 with a slow decrease in subsequent years. The conclusion reached was that the radionuclide 'concentration factor' approach to the prediction of arctic fish contamination was not very applicable. An original calculation method is proposed for modelling the dynamics of radioactive contamination of fish, with consideration of fish feeding behaviour, growth and seasonal migrations. Results of the computer simulation of the fish contamination in the Barents Sea after a hypothetical accidental release of 1 TBq of 137Cs from the radioactive waste dumping site are presented."

Skwarzeca et al 2001

And I know nobody mentioned Plutonium, but just for fun (and it has info on different tissues, which is intersting):

In this paper, the results of 238Pu and 239+240Pu determinations in four representative species of Baltic fish collected in Gdansk Bay: flounder, herring, cod and sprat, are presented and discussed. The plutonium isotopes are amongst the more radiotoxic nuclides. In the marine environment, the highest concentrations of plutonium are found in the sediments, but the complex biogeochemical cycle of the element means that it is also found in all other compartments. The activities of the fish samples were measured using alpha spectrometry and the concentrations of plutonium 238Pu and 239+240Pu were estimated for particular organs and tissues and the whole body. The 239+240Pu concentrations for fish species were: flounder 0.94 mBq kgâ??1 w.w., herring 2.22 mBq kgâ??1 w.w., cod 2.35 mBq kgâ??1 w.w. and sprat 0.33 mBq kgâ??1 w.w. On the basis of the 238Pu/239+240Pu activity ratio in the organs and tissues, the proportion of Chernobyl-derived plutonium in the Baltic Sea was calculated. The lowest values of Chernobyl plutonium were accumulated in flounder stomach, herring skin, cod intestine, the highest in cod gills and skin.

Skwarzeca et al 2001

All three of those are cited in the above linked list of journal articles.

The other dozens of papers: http://goo.gl/9xrxi

http://toxics.usgs.gov/definitions/bioaccumulation.html

It is pretty obvious that the fish have become contaminated with radioscesium, but for there to be bioaccumulation the radiocesium has to be selectively concentrated over non-radioactive cesium which I don’t think happens.

For radioactive contaminates, what is important is the concentration of radioactive atoms relative to non-radioactive atoms. Here is some data on Japanese fish from 1996.

http://www.ncbi.nlm.nih.gov/pubmed/8840718

This is important because if organisms high in the food chain become highly contaminated is it because there is a large amount of contamination in the environment, or is it because the radiocesium got concentrated as it moved up the food chain.

Fish probably mostly get cesium through their gills and not through their food. They regulate their ion balance, they don’t accumulate sodium because the sodium concentration in fish tissues is lower than that of sea water. Cs acts like potassium. Radiocesium can be mobilized by addition of non-radioactive cesium.

http://www.sciencedirect.com/science/article/pii/S0265931X04002917

This is in fresh water which doesn’t have the cesium level of sea water.

I assume cesium in general does bio accumulate simply because we KNOW the radioactive cesium does, because it has been measured and documented. This is a different system than pcb acting as lipids, but it does appear to happen. Were not arguing about terminology here.

Do the non-radioactive cesium levels in fish increase over their lifetime? That is what would have to happen if cesium was not excreted.

Cesium is very different than iodine. Iodine is concentrated and bioincorporated into thyroid hormones and is held extremely strongly. But sea water has 50 ppb iodine. Radioiodine exposure on land is very different than exposure in sea water because the background level of iodine on land and in the land food chain is very low.

I found this which in chapter 8 shows a modest ~10 to 20x increase in Cs concentration when fish were held in constant concentrations in sea level. I think these were growing fish. At some point the Cs concentration has to reach steady state and that steady state represents the normal accumulation of CS and the ratio of radioactive to non-radioactive Cs in the water and food.

http://books.google.com/books?id=JDsrAAAAYAAJ&lpg=PA80&ots=gY0yvWjmfj&dq=cesium%20accumulation%20fish&lr&pg=PA83#v=onepage&q=cesium%20accumulation%20fish&f=false

Just as iodine loading can be used to block radioiodine takeup, Cs loading could be used to block radiocesium takeup. There are also some drugs that cause Cs to be excreted. They were used in Brazil when people found a Cs radiotherapy unit, opened it up, and used the glowing powder as makeup.

Fresh water animals are very different because the background of non-radioactive minerals in fresh water is very low.

I think the bioaccumulation of Cs is very different than things which actually do bioaccumulate (like PCBs, methyl mercury). There concentration per trophic level is appropriate because the excretion of PCBs is much smaller than absorption.

Cold blooded organisms also don’t eat as much, so bioaccumulation is not as much of a problem as with top mammalian predators.

Maybe we are not disagreeing so much about what happens as disagreeing about terminology.