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.