718 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



Potential Impact of Ionizing Radiation on Aquatic Resources 



The available data on the effects of ionizing radiation on individual organisms indicate 

 tliat, when experimental and field exposure dose rates are less than 1 rad/day, it is 

 difficult to demonstrate effects that are not already within the inherent variation already 

 present. It is pertinent, however, to discuss some of the concerns and potential problems 

 to provide an improved basis for the assessment, understanding, and acceptance of the 

 degree of risk tliat aquatic populations are exposed to as a result of the introduction of 

 radionuclides into the aquatic environment. A valid perspective can be developed by 

 comparing potential effects with losses caused by natural mortahty and fishing. This 

 aspect has been discussed in some depth by an international panel (International Atomic 

 Energy Agency, 1976). 



The available evidence suggests that (1) fish are the most radiosensitive component; 

 (2) developing gametes, fertilized eggs, and larvae are the sensifive stages; and (3) any 

 damage that might occur to a fishery resource would most Hkely arise from the direct 

 effects of radiation on the fish rather than from effects from disturbances or changes in 

 the food web. Some of the earlier literature from laboratory experimentation observed 

 that effects, seen as the number of abnormal larvae hatched, resulted from exposure to 

 radionuclides at extremely low concentrations. These findings were extrapolated to 

 suggest that, if fish were exposed to these concentrations, the yield from commercial 

 fisheries would be adversely affected. However, consideration needs to be given to (1) the 

 nature of the stock and recruitment relationships for highly fecund fish where large 

 numbers of eggs are produced at each spawning (10^ to 10^ per female); (2) the small 

 number of eggs required to survive to maintain stock at equilibrium; (3) the high 

 mortality that occurs during the larval stages such that, say, only 1 in 10,000 survives; 

 and (4) the fact that a direct relationship does not necessarily exist between spawning 

 stock size and recruitment to the reproductive stock. These factors suggest that, even if 

 mortality at this stage of development is being enhanced by the low dose rates presently 

 existing in the aquatic environment, recruitment in highly fecund species of marine fish 

 would unlikely be adversely affected unless these stocks were already at risk because of 

 severe overexploitation. The mechanisms controlling recruitment in invertebrates appear 

 similar, except that environmental factors probably play a more important role. 



For species with low fecundity (rays, sharks, dogfish, and marine mammals), most of 

 whom produce live young, recruitment is related to adult stock size, and, for the marine 

 mammals, the relationship may be almost direct. Although it is reported that the 

 fecundity of baleen whales (Laws, 1962) and elasmobranch stocks (Holden, 1973) has 

 increased as a result of exploitafion, there is an obvious upper limit; available data would 

 suggest that this upper limit lias been reached by the exploited stocks. Further stress 

 would cleariy increase the chances of extinguishing the populations. However, although 

 there are no data on the radiosensitivity of these low-fecund species, the estimates of the 

 dose rates received by aquatic organisms in the natural waters of the world are rarely of 

 the same order as, and generally less than, the limits recommended for humans. Hence 

 they are unhkely to provide any additional stress in comparison to man's continued 

 exploitation of some of these stocks. 



The possible increase in mortality rates due to radiafion in exploited fish stocks can 

 also be compared with mortality rates experienced by those fisheries presently exploited 

 by man. Rates of exploitation as high as 50% a year on all classes recruited to a fishery 

 are common. In addition, mortality due to natural causes occurs; thus a heavily exploited 



