Specific activity measurements to determine phosphorus turnover in bluegill, Lepomis macrochirus

Abstract

The biological turnover constant for phosphorus was determined in muscle and five other sections of bluegill. Bluegill of average weight 121 g were maintained in a large flowthrough system at 22°–16°C and fed worms, Eisenia foetida, at two feeding levels, ad lib. (which averaged 2.6 g/d per 100-g fish, wet weight), and 1.5 g/d per 100-g fish. The daily phosphorus intakes at the two levels per 100-g fish were 3.1 and 1.8 mg. The average phosphorus concentration was 2.4 mg/g in muscle and 15.4 mg/g in the whole fish. Worm food was spiked with P-32 at increasing daily increments to balance radioactive decay. The radioactive worms were fed daily to the bluegill during the P-32 accumulation period of 51 days. For the next 28 days of depuration, nonradioactive worms were fed. Sets of three bluegill were collected at approximately weekly intervals, sectioned and analyzed for P-32 and phosphorus. All data were reported as specific activity in tissue relative to specific activity in feed, with P-32 count rates corrected for its 14.3-day half life. Phosphorus turnover constants were obtained by three approaches: (1) from the relative specific activity measured near steady state; (2) by fitting an equation for a 1-compartment model to the accumulation and depuration data; and (3) by fitting an equation for a simplified 2-compartment model to the depuration data. The biological turnover constant calculated with all three approaches was 0.004 d−1 for phosphorus in muscle of bluegill fed ad lib.