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Abstract

The potential use of heart rate to monitor energy expenditure in free-ranging Steller sea lions (Eumetopias jubatus) was investigated by establishing whether a relationship exists between heart rate (fa) and oxygen consumption ( V 0 2 ) in captive sea lions while swimming and resting. Four trained Steller sea lions (2 males and 2 females; mass 87.4 -194.4 kg; ages 16 months-3 years) were equipped with a datalogger and two dorsal electrodes to record ECG (from which f[sub H] was calculated). Four styles of electrodes were developed and tested before selecting a final subcutaneous design. VO₂ (measured with open-circuit respirometry) was simultaneously recorded while the previously-fasted animal was at rest within an enclosed dry metabolic chamber or while it swam in an enclosed swim mill against water currents of various speeds (0-1.5 ms⁻¹). The mean regression equation describing the relationship between fa (beats min⁻¹) and VO₂ (ml min⁻¹ kg⁻⁰.⁷³) for all four animals was VO₂ = (0.68f[sub H] ± 0.07 s.e.) - (15.07 ± 6.20) (r²=0.72, p<0.01). The possibility that the fa/ VO₂ relationship could be affected by digestion was investigated by feeding one of the male Steller sea lions either 6 or 12 kg of herring prior to entering the swim mill. VO₂ increased over time after ingestion, while heart rate usually remained stable or decreased. The resulting relationship, VO₂ = (0.24f[sub H] ± 0.03) - (18.49 ± 02.68) (r²=0.19,p<0.01), differed significantly from the relationship derived while the animal was fasted, indicating that digestion may alter the relationship between f[sub H] and VO₂. Fasting and feeding intervals must therefore be taken into account when considering the use f[sub H]/VO₂ relationships to estimate energy expenditure from heart rate of free-ranging sea lions. The relationship demonstrated between f[sub H] and VO₂ while fasting suggests that heart rate can potentially be used to monitor energy consumption in free-ranging Steller sea lions. However, additional research should be conducted to further elucidate how the relationship is affected by such factors as digestion, sex, stress, and development.