EFFICACY OF THE TRITIATED-WATER AND NA-22 TURNOVER METHODS IN ESTIMATING FOOD AND ENERGY-INTAKE BY EMPEROR PENGUINS APTENODYTES-FORSTERI

The accuracy of the tritiated water (HTO) and sodium-22 (Na-22) turnover methods as estimators of dietary water and sodium intake was evaluated in emperor penguins fed separate diets of squid and fish. Emperor penguins assimilated 76.2% and 81.8% of available energy in the squid and fish diets, respectively. Both isotopes bad equilibrated with body water and exchangeable sodium pools by 2 b after intramuscular injection. The tritium method yielded reliable results after blood isotope levels bad declined by 35%. On average the tritium method underestimated water intake by 2.9%, with a range of -10.3% to +11.1%. The Na-22 method underestimated Na intake on average by 15.9% with the errors among individuals ranging from -37.2% to -1.8%. Discrepancies with Na-22 turnover were significantly greater with the squid diet than the fish diet. The results confirm the reliability of the tritium method as an estimator of food consumption by free-living emperor penguins (provided seawater and freshwater ingestion is known) and support the adoption of the Na-22 method to derive an approximation of seawater intake by tritiated emperor penguin chicks and by tritiated adults on foraging trips of short duration.