Estimating field metabolic rates of pinnipeds: doubly labelled water gets the seal of approval

1. Measures of the field metabolic rate of marine mammals are extremely difficult to make but they are essential for assessing the impacts of marine mammals on prey populations, and for assessing dive performance in relation to aerobic limits.

2. The doubly labelled water (DLW) method is an isotope-based technique for the estimation of the CO2 production, and hence energy expenditure, of free-living animals. Estimates of field metabolic rate (FMR) from DLW in pinnipeds to date are extremely high and at the upper range for most mammals. DLW has previously been validated in pinnipeds but logistical difficulties meant for these validations were less than ideal, and it has been hypothesised that DLW may overestimate FMR in these animals.

3. To test this hypothesis, we used DLW and simultaneous open-flow respirometry to determine the daily energy expenditures (DEE) of wild grey seals (Halichoerus grypus) held temporarily in a captive facility, during 4-5 days of simulated foraging at sea. Comparing DEE from DLW and respirometry, we found that DLW predicted DEE accurately (average difference between the two estimates was 0.7% SD = 17% n = 31), but as with validations of other species there was a large range of individual errors (from -39% to +44%).

4. The results dispel the doubts surrounding the use of DLW as a field method for estimating energy expenditure in grey seals, and by implication other pinnipeds, and simultaneously open a series of questions about their ability to maintain surprisingly high metabolic rates for protracted periods.

5. We make a number of recommendations for future studies of pinniped FMR using DLW. We suggest use of the Speakman two-pool calculation will be most appropriate. Studies should aim for enrichment levels as high as economically feasible but to at least 150 p.p.m. above background for the O-2 isotope. Measurement periods should be extended between one and two half-lives (5-10 days for a typical foraging seal). We also encourage the calculation and presentation of estimates of precision in estimates of FMR.