FISH AND RADIOACTIVITY Introduction With the advent of the atomic era there has been an increase in the number and in the quantity of radioisotopes that occur on the surface of the earth, and in the sea. both on land Naturally occurring radioisotopes always have been present in our environment; however, the detona- tion of nuclear devices and the operation of atomic reactors have added more and new radioisotopes to the environment. In this chapter, the consequences of adding radioisotopes to the aquatic environment will be discussed, especially in regard to the effects upon man of the uptake of radioisotopes by fish. Radioisotopes in the sea can affect fish in two ways, first, as an external source of ionizing radiation as the fish swims about in radioactive water; and second, as an internal source of fionizing radiation from radioisotopes that have been taken into the body of the fish either directly from the water or indirectly through the food chain. Radioisotopes as an external source of radia- tion are called external emitters, but once within the