Blubber and buoyancy: monitoring the body condition of free-ranging seals using simple dive characteristics

• Published in: Journal of Experimental Biology
• Main Authors: Biuw, Martin, McConnell, Bernie, Bradshaw, Corey J. A., Burton, Harry, Fedak, Mike
• Format: Text
• Language: English
• Published: Company of Biologists 2003
• Subjects: Research Article; Elephant Seal; Elephant Seals; Mirounga leonina; Southern Elephant Seal
• Online Access: http://jeb.biologists.org/cgi/content/short/206/19/3405; https://doi.org/10.1242/jeb.00583

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 ...