Bioaccumulation factor for 32P measured in bluegill, Lepomis macrochirus, and catfish, Ictalurus punctatus.

The ratio of the bioaccumulation factors for 32P and phosphorus was determined for edible tissue in two species of freshwater fish by measuring the specific activity (32P activity per milligram phosphorus) in muscle relative to feed. The 32P tracer was added to the feed at a uniform level throughout the study. Feeding was at two levels: ad libitum and at a lower but constant intake per body weight. In the main experiment, bluegill were maintained in a large flow-through tank and sacrificed at approximately weekly intervals for 51 d of 32P accumulation and 28 d of depuration to compare the specific activity with values predicted with a calculational model. In experiments performed in smaller aquaria, the specific activity in bluegill and catfish muscle was compared at two feeding levels and two temperatures. In addition, unfed fish were exposed to 32P in water at a known specific activity to determine the extent of phosphorus uptake directly from water. The pattern of specific activity increase and decrease in fish muscle during the accumulation/depuration experiment was consistent with a one-compartment model, so that specific activity ratios at steady state could be predicted from measurements during relatively brief exposures. On this basis, the ratio of the bioaccumulation factors of 32P and phosphorus in fish feeding ad libitum was 0.081 for bluegill and 0.17 for catfish. Hence, at a mean phosphorus bioaccumulation factor of 70,000, the factors for 32P are 6,000 and 12,000, respectively. The ratios were less at lower phosphorus intakes associated with lower feeding rates; moreover, the lesser value for bluegill occurred at a much lower phosphorus intake than by catfish. The bioaccumulation factor ratio was lower by an order of magnitude at a water temperature of 11 degrees C than at 16-27 degrees C, and was lower by two orders of magnitude when phosphorus uptake was from water by unfed fish.

The bioaccumulation factor for phosphorus-32 in edible fish tissue.

Information used to derive the bioaccumulation factor for 32P in edible portions of fish was reviewed to evaluate the currently recommended values of 100,000 in fresh water and 29,000 in sea water that are applied in calculating radiation doses to persons from nuclear-power reactor effluents. A generic phosphorus bioaccumulation factor of 70,000 was obtained for larger rivers and estuarine waters on the basis of geometric mean phosphorus concentrations of 2 mg/g wet weight in fish muscle and 0.03 mg/l dissolved in water. A 20-fold lower bioaccumulation factor was inferred for 32P because radioactive decay is much faster than phosphorus turnover. A phosphorus turnover rate in muscle of 0.2% per day was estimated as a long-term average for edible-size fish, although more rapid turnover has been observed for brief periods. Large deviations from the generic bioaccumulation factor will occur for different phosphorus concentrations in water and turnover rates in fish. Site-specific determinations are also needed because 32P is bioaccumulated at lower trophic levels in the food web, not in the fish. Hence, the availability of concentrating organisms determines the bioaccumulation factor. Several other conditions that affect the 32P bioaccumulation factor have not been quantified and are suggested for study.