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First published online August 25, 2003
Blubber and buoyancy: monitoring the body condition of free-ranging seals using simple dive characteristics
1 Sea Mammal Research Unit, Gatty Marine Laboratory, University of St
Andrews, St Andrews, Fife, KY16 8LB, Scotland
2 Antarctic Wildlife Research Unit, School of Zoology, University of
Tasmania, GPO Box 252-05, Hobart, Tasmania 7001, Australia
3 Australian Antarctic Division, Channel Highway, Kingston, Tasmania 7050,
Australia
* Author for correspondence (e-mail: emb7{at}st-and.ac.uk)
Accepted 2 July 2003
Elephant seals regularly perform dives during which they spend a large
proportion of time drifting passively through the water column. The rate of
vertical change in depth during these `drift' dives is largely a result of the
proportion of lipid tissue in the body, with fatter seals having higher (more
positive or less negative) drift rates compared with leaner seals. We examined
the temporal changes in drift rates of 24 newly weaned southern elephant seal
(Mirounga leonina) pups during their first trip to sea to determine
if this easily recorded dive characteristic can be used to continuously
monitor changes in body composition of seals throughout their foraging trips.
All seals demonstrated a similar trend over time: drift rates were initially
positive but decreased steadily over the first 30-50 days after departure
(Phase 1), corresponding to seals becoming gradually less buoyant. Over the
following 
100 days (Phase 2), drift rates again increased gradually,
while during the last 20-45 days (Phase 3) drift rates either remained
constant or decreased slightly. The daily rate of change in drift rate was
negatively related to the daily rate of horizontal displacement (daily travel
rate), and daily travel rates of more than 80 km were almost exclusively
associated with negative changes in drift rate. We developed a mechanistic
model based on body compositions and morphometrics measured in the field,
published values for the density of seawater and various body components, and
values of drag coefficients for objects of different shapes. We used this
model to examine the theoretical relationships between drift rate and body
composition and carried out a sensitivity analysis to quantify errors and
biases caused by varying model parameters. While variations in seawater
density and uncertainties in estimated body surface area and volume are
unlikely to result in errors in estimated lipid content of more than
±2.5%, variations in drag coefficient can lead to errors of 
10%.
Finally, we compared the lipid contents predicted by our model with the lipid
contents measured using isotopically labelled water and found a strong
positive correlation. The best-fitting model suggests that the drag
coefficient of seals while drifting passively is between 0.49 (roughly
corresponding to a sphere-shaped object) and 0.69 (a prolate spheroid), and we
were able to estimate relative lipid content to within approximately
±2% lipid. Our results suggest that this simple method can be used to
estimate the changes in lipid content of free-ranging seals while at sea and
may help improve our understanding of the foraging strategies of these
important marine predators.
Key words: buoyancy, marine mammal, elephant seal, body composition, foraging ecology, satellite telemetry
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