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Sprats With Polonium

July 19, 2002

There are many radioactive elements in the world. For example, natural element polonium has 33 radioactive isotopes. Fortunately, only one isotope, 210Po, has a relatively long (138.4 days) half-life period. This isotope appears in the atmosphere as a result of radon decay, sinks to oceans and seas, and accumulates in organs of some animals, in particular, of those which are served people for dinner. A man receives more than a half of the internal radiation doze from 210Po. Research into the abilities of animals and plants to concentrate this radionuclid from seawater is very extensive in recent times. Polonium accumulation by the Black Sea fishes has been studied by G.E. Lazorenko and G.G. Polikarpov from the Kovalevskii Institute of the Biology of Southern Seas of the National Academy of Sciences of Ukraine under the project of the International Atomic Energy Agency "Marine Environmental Assessment in the Black Sea Region".

In the coastal area of Crimea, the scientists studied 17 fish species living at a mean depth (pelagic), in low water layers (demersal), and in the bottom of sea (benthonic). It was found that Black Sea fish species contain 210Po in similar quantities as their relatives from other parts of the World Ocean. But the Po concentration in the body of fish depends on its habitat. Predators living in the bottom, e.g., gobies are "cleanest". Most demersal fishes, such as wrasse, usually contain more 210Po. But merlang (Odontogadus merlangus, species of cod) contains polonium in a concentration of 15 becquerels per kilogram of flesh (becquerel is a unit of radioactivity, equal to the amount of a radioactive isotope that decays at the rate of one disintegration per second). However, merlang spends the first year of life within the pelagic zone. In general, the shallower is the habitat, the higher is the Po concentration in fish. Pelagic fish usually contain highest concentrations of polonium. But the 210Po concentration significantly depends also on the menu of a particular fish and on its spawning ground. For example, pelagic scad eats small fish and concentrates less 210Po than merlang does. The champion concentrators of polonium are plankton eaters: Black Sea anchovy and sprat (40 and 30 becquerels per kilogram of flesh, respectively). They spawn and spend all their life in medium-deep layers of water. The distribution of 210Po within the fish body is not uniform: highest concentrations are found in liver and other internal organs.
        
The concentration of 210Poin Black Sea fish is not dangerous, if one does not consume several kilograms of fish daily. Normally, a man never eats the sprat`s liver by kilograms. However, the percent of fish in daily ration of people living near sea is very high. And it is better to eat fewer sprats and to remove the guts from all kinds of fish. This is the way to reduce the radioactivity of the eaten by 2 or 3 times.

People concerned about the state of environment focus usually only on the anthropogenic pollution and forget about the natural pollution. Radioactive elements normally occur in air and water. Some creatures have an ability of accumulating these elements. For example, polonium (Po) is concentrated in fish inhabiting the Black Sea. The present research is performed within a framework of the regional technical project of the International Atomic Energy Agency "Marine Environmental Assessment in the Black Sea Region".

Informnauka (Informscience) Agency