MARINE MAMMAL RESEARCH NEWSLETTER | Sept. 2014 (Issue 11)
Diving dynamics: validating a new method to estimate the energy expenditure of Steller sea lions
According to scientific theory, animals should adopt foraging strategies that maximize the amount of energy they acquire relative to the energy they expend to obtain prey. The ways in which an animal optimizes its “rate of return” has direct bearing on its growth and reproductive success.
Energetically, locomotion is the biggest cost of finding food, so it is important to be able to accurately measure this cost to assess the amount of food marine mammals require.
Quantifying acceleration of a swimming animal is one means of measuring locomotion. Recent technological advances allow acceleration to be easily measured with miniature data logging tags attached to animals that incorporate three-axis accelerometers. Mathematical methods can then be used to convert acceleration data into a single measure of movement.
One widely used method for estimating animal acceleration generates a metric called ODBA (Overall Dynamic Body Acceleration). This measure has been applied to many studies of marine mammals, including Steller sea lions.
Unfortunately, OBDA assumes that the swimming animal is not rotating, which is something that aquatic animals tend to do a lot. Overcoming this shortcoming means finding a way to differentiate the active movement of a diving animal from its own spinning during a dive.
Hassen Allegue, an NSERC Undergraduate Student Research Award recipient, spent his summer at UBC working with Dr. Andrew Trites, Dr. Colin Ware (University of New Hampshire) and the 4 trained female Steller sea lions housed at the UBC Open Water Research Station in Port Moody. His goal was to work on the spinning problem using a new tag called the OpenTag that incorporates an accelerometer, a magnetometer and a gyroscope.
Initial studies with the sea lions and OpenTag suggested that a new measure of movement called ELBA (Estimated Linear Body Acceleration) that uses the gyroscope in addition to the accelerometer yields more accurate estimates of animal speed in swimming sea lions than does ODBA. As a continuation of that project, Hassen investigated the relationship between ELBA and energy expenditure in diving sea lions.
This summer, Hassen and the team at the Open Water Research Station attached OpenTags to the harnesses of the trained sea lions as they performed dives to the bottom of fish delivery tubes at set depths. The animals were trained to only surface into a respirometry dome floating at the surface, which allowed researchers to measure the animal’s oxygen consumption after each dive, and obtain an accurate estimate of “true” energy expenditure.
Back at UBC, Hassen developed complex computer programs that allow him to sort through the tag data to quantify the sea lion’s true locomotion over a dive. Preliminary results show a significant positive relationship between the animal’s known energy expenditure and different measures of physical movement and acceleration during the dives. Additional analysis is being undertaken to determine whether the inclusion of gyroscopes provides a better picture of a sea lion’s true underwater activity.
The results of this research will allow scientists to more accurately measure the costs of diving in wild Steller sea lions, and ultimately provide a clearer understanding of their foraging strategies and the potential negative effects of changes in their ocean environment on their individual energy budgets.
Hassen Allegue, is now an MSc student at UBC’s Marine Mammal Research Unit.