Effects of chronic gamma radiation on the growth and survival of juvenile clams (Mercenaria mercenaria) and scallops (Argopecten irradians).

Juvenile clams (Mercenuria mercenaria) and scallops (Argopecten irradians) from North Carolina were exposed to five intensities of chronic gamma radiation for 3-14 months during 1973-1974. Exposure rates ranged downward from 37 radslhr to levels slightly higher than background radiation. Deleterious effects on growth and survival of clams occurred only at the highest exposure rate (37-16 rads/hr). Clams exposed to low radiation levels (0.0070.008 rads/hr) increased in weight, but not in length, faster than the controls. One-half of the clams exposed to the highest radiation level survived to 159 days, after a cumulative dose of 132 krads, and only 10% survived to 14 months after a cumulative dose of 150-346 krads. No deleterious effects on growth and survival of scallops were apparent during 84 days of exposure to a cumulative radiation dose of 71.7 krads. DATA on the effects of chronic low level radiation on aquatic organisms have been quite limited, despite the need for such research brought about by the rapid increase of nuclear power in the past decade. O n the other hand, considerably more data are available on the effects of acute radiation exposure on aquatic organisms, but these data cannot realistically be related to environmental situations. The proposed Federal guidelines for the annual discharge rate of radioactive liquid effluents from nuclear power plants to unrestricted areas have been set at 5 Ci per reactor, exclusive of tritium (USAEC, 1971). Radiation exposures to aquatic animals living in effluents released according to the above guidelines were calculated at less than 100 mradslyr (USAEC, 1973). To quantify the threshold doses for radiation mortality and growth retardation in juvenile clams (Mercenaria mercenaria) and scallops (Argopecten irradians) we examined experimentally the effects of several chronic gamma radiation exposure rates higher than the above calculated values. We exposed 150 juvenile clams (10.8mm, mean length) and 150 juvenile scallops (7.5 mni, * This research was supported through a cooperative agreement between the National Marine Fisheries Service and the US. Atomic Energy Commission. mean length) of the same broods to five different intensities of gamma radiation. Thirty clams and 30 scallops were exposed a t each of the five radiation levels. Each group was divided into three subgroups and held in glass containers, 10 clams and 10 scallops per container (Fig. 1). Water flow to each container ranged from 1 to 2 l/min, and temperature varied from 9 to 28°C with the season. Natural food was available in the sea water circulating system. Gamma radiation was provided by a retractable cobalt-60 source. Different exposures were achieved by distance and concrete shielding (Fig. 1). Dose rates to clams and scallops of groups A, B and C were measured by glass rod and thermoluminescent dosimeters in order to measure total dose, including that from backscatter. Within each of the above three groups, dosimeters were placed in the subgroup containers nearest and farthest from the radiation source, but none was placed in the intermediate subgroups. Excessive backscatter from the concrete shielding near the source prevented dose calculations for intermediate subgroups of group A by use of the inverse squares ratio for distance. Dose rates to groups D and E (control), however, were calculated by the inverse squares ratio for distance, I , = Il (d,/d,)2 and a factor for shielding, dcrived from dosimeter readings on both sides of the first two shields