Research Handling Effects on Stress Hormones, Blood Parameters, and Heart Rate in Juvenile Northern Elephant Seals (Mirounga angustirostris)

Abstract

Wildlife researchers must balance the need to safely capture and handle their study animals while simultaneously ensuring study results are not confounded by stress artifacts caused by handling. To determine the physiological effects of research activities including chemical immobilization, transport, instrumentation with biologgers, and overnight holding on a model marine mammal species, we collected hormone, blood parameter, and heart rate data from 19 juvenile northern elephant seals (Mirounga angustirostris) throughout a translocation experiment. Blood samples were collected during the initial capture (baseline), the instrumentation procedure conducted under isoflurane gas anesthesia, the post‐procedure recovery period, and the recapture (2‐10 days post‐release). Continuous heart rate data was collected using electrocardiogram biologgers during the procedure, recovery, transport, and release. Serum cortisol levels increased from 6.6 ± 5.3 μg/dL at capture to a maximum of 29.2 ± 23.7 μg/dL during recovery. Similarly, serum aldosterone increased from baseline levels of 69.7 ± 63.1 pg/mL to 217.8 ± 101.2 pg/mL during the procedure. Cortisol was positively associated with aldosterone, which supports growing evidence that aldosterone is a component of the acute stress response in marine mammals. Blood glucose and hematocrit both varied throughout the experiment, while there were no significant changes in β‐hydroxybutyrate blood ketones or erythrocyte sedimentation rate. Seals exhibited bradycardia (mean heart rate = 49.7 ± 3.7 bpm) while anesthetized, followed by a gradual increase in mean heart rate during the recovery period to 73.5 ± 7.9 bpm. Despite these short‐term changes, by the time seals were recaptured after several days at sea, all hormonal and hematological parameters had returned to baseline levels and 95% of study animals were resighted in the wild up to two years post‐translocation. Together these findings suggest that while northern elephant seals exhibit mild physiological stress responses to capture and handling activities, they recover rapidly and show no long‐term deleterious effects, making them a robust species for ecological and physiological research.