Measurement of 02 Saturation in Freely Deep Diving Northern Elephant Seals (Mirounga angustirostris) Using a Novel Data Logging Tag

Marine mammals live a life of dual constraints and must balance energetic demands (oxygen consumption) and limited O2 availability while breath-holding. In order to overcome these dual constraints, marine mammals have developed unique physiological traits to cope with their aquatic environment, thereby taking advantage of a unique foraging niche. Extending breath-hold dive time increases the opportunity to obtain nutrients, but it can be energetically costly if marine mammals exceed the threshold for aerobic metabolism. We know relatively little about the physiological mechanisms employed by these animals that allow them to flourish in such an extreme environment. Due to the difficulty obtaining direct physiological measurements, there are gaps in knowledge for the field of diving physiology, particularly in the understanding of muscle perfusion during a dive. In order to improve our understanding of how marine mammals adjust muscle blood flow (oxygen and nutrients) during diving to increase dive time, we developed and tested a tag with oxygen sensors that attempted to measure O2 saturation in the muscle of freely deep-diving elephant seals. The purpose of this part of the study aimed at using an archival tag and implanted sensor system to measure physiological variables in freely deep-diving Northern elephant seals. Following the sensor and data logger development, we set out to test a long-held assumption that oximeter use in marine mammals could be calibrated with terrestrial mammal blood. This was of particular importance to this study as calibrations of the oximeter data loggers in the Northern elephant seal were needed. As part of the ability of marine mammals to prolong apnea for diving, some have modified hemoglobin to change its affinity for oxygen. These polymorphisms alter the affinity for O2, possibly by altering the folding of the protein. Alteration in the quaternary structure may also alter the optical properties of Hb. Oximeters use the change in the optical properties of oxygenated (HbO2) and reduced Hb (HbR) to determine arterial blood O2 saturation. Given the differences in Hb isoformss, the optical properties need to be assessed before conventional veterinary oximeters can be used on marine species. Therefore, the objective of this part of the study was to determine the absorbance spectra of HbO2 and HbR in several species of marine mammals (killer whale, short-finned pilot whale, beluga whale, and northern elephant seals) and compare these against humans. Whole blood samples were opportunistically obtained during routine health assessment, and the Hb was isolated via a series of centrifugation, dialysis, and filtration steps. The isolated Hb was oxygenated or deoxygenated using 5% CO2 in 95% O2 or N2, respectively. Under gas-tight conditions, the HbO2 and HbR samples were placed in a UV-visible light spectrophotometer and the absorption spectra measured from 600 nm to 1000 nm. The absorption spectra were overlaid and compared between species. The results indicate the absorption spectra are similar between humans and the species investigated and the point where absorbance is equal for HbO2 and HbR in all species at ~800 nm. The results of the Hb study allowed the possibility of properly calibrating the oximeter we surgically implanted in the Northern elephant seal. The oximeter sensor contained 3 LEDs in the visible red, near infrared (NIR), and infrared (IR) spectral regions (emitting at red:660, NIR:810, and IR:940 nm, respectively) and a photo detector. The sensor was attached to the external data logger with a thin (2-3 mm diameter) flexible silver (Ag) cable. The external data logger was attached with epoxy to the skin and fur of the seal following implantation of the sensor. In April 2013, 5 northern elephant seals were captured from the Año Nuevo and were instrumented with the oxygen sensor and data logger via aseptic surgical technique. Following implantation of the muscle O2 sensor and attachment of logger and satellite and radio transmitters, the animals were allowed to recover. Every 1-2 hours signs of inflammation, infection, or changes in swimming behavior were documented and none were observed in any seal. After translocation, all of the seals returned to the Año Nuevo rookery. All 5 seals implanted returned to Año Nuevo, had the instrumentation easily and successfully removed, and showed no signs of infection, inflammation or trauma concluding that we were able to develop a surgical technique that minimized the invasiveness of implanting a small sensor into the major swimming muscle of the Northern elephant seal. Free-diving ocean data was collected from the oximeter data logger only from the last two seals implanted, however only 1 out of 5 had oximetry data. Additionally, there was difficulty in calibrating the oximeter/sensor data logger. However, GPS tracks and dive patterns were recorded and analyzed from all 5 seals. All seals were equipped with GPS tags with ARGOS satellite uplink and had similar dive patterns to one another and to previous studies. Furthermore, we were able to obtain data by the end of the field season from one seal out of five indicating that with a few adjustments to the data logger, a second field season may yield a larger sample size and more reliable data