-7- upon other radioactive organisms, like land plants or animals, fish as a food of man, can be a pathway by which radioisotopes are transferred from the environment to man. The presentation will include a discussion of the nature of radioactivity, radiations, the biological effects of ionizing the evaluation of hazard from internal emitters, the distribution of radioisotopes in the sea, and the uptake of radioisotopes by fish. The Nature of Radioactivity The biological effects to be expected from exposure to radioactivity can be explained, in part, by the nature of the energies that are released from the nucleus of an atom of a radioisotope. a nucleus, All atoms consist principally of in which there are protons and neutrons, electrons that orbit about the nucleus. and of Each chemical element is characterized by having a specific number of protons in the nucleus; however, the number of neutrons associated with any given number of protons may vary. Atoms with the same number of protons but with various numbers of neutrons are known as isotopes of the element. For example, in the nucleus of the iron atom there are 26 protons, but