Biomagnification of radiocesium in a marine piscivorous fish

Radiocesium is the only trace element apart from Hg that may be potentially biomagnified at the top of the marine planktonic food chain. We quantified the assimilation efficiency from ingested prey, uptake rate from the aqueo us phase, and efflux rate of radiocesium in a marine piscivorus fish (the m angrove snapper Lutjanus argentimaculatus), Aqueous Cs-137 exhibited an app roximately linear uptake pattern over a 4 d exposure period, and was immedi ately transported to the muscles. The calculated uptake rate constant (0.00 145 l g(-1) d(-1)) was independent of the ambient Cs concentration. Salinit y variation appeared to have no influence on the Cs-137 influx within the r ange of 20 to 30 psu, but the influx rate increased when the salinity was f urther reduced to 15 psu. The assimilation efficiency in fish ingesting dif ferent prey (copepods, Artemia, clam tissues, and herbivorous fish), measur ed by a pulse-chase feeding technique, ranged between 78 and 95 %. The effl ux rate constant of Cs-137 in fishes following uptake from the dissolved an d dietary phases ranged between 0.020 and 0.023 d(-1). The higher efflux ra te in marine fishes compared to those in freshwater fishes may have been du e to the ionic regulation in marine teleosts (e.g., high excretion rate to counteract the high ambient K+ concentration). Using a simple kinetic model , we show that the dietary uptake of Cs-137 plays a dominant role when the concentration factors of Cs-137 in prey range between 50 and 100. At a lowe r value for the concentration factor (10), Cs-137 bioaccumulation in fish i s dominated by uptake from the aqueous phase. The predicted trophic transfe r factor (concentration in the predator to concentration in the prey) in th e predatory fish ranges between 1 and 4.4 (with a median value of 2), and i s consistent with the field measurements of trophic transfer factor of Cs-1 37 in the piscivorous fishes in both marine and freshwater systems. Thus, t he biomagnification Of Cs-137 in marine predatory fishes is largely caused by the extremely high Cs-137 assimilation from ingested prey, despite the r elatively high ef flux rate of Cs-137 compared to those measured in freshwa ter fishes.