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

Abstract

With the development of time depth recorders (Kooyman, 1965) and satellite, radio and acoustic telemetry (e.g. Fancy et al., 1988;Fedak, 1992;Fedak et al., 1983;McConnell, 1986), it is now possible to study the behaviour of many free-ranging marine organisms. Although these data have provided new insights into the foraging ecology of marine species, there are still limitations in the types of information that can be collected. Specifically, direct observation or recording of feeding events is often impossible (particularly for animals ranging over large areas during extended trips), and it is consequently difficult to determine where and when individuals encounter and ingest food and how much of this is assimilated into the body energy stores. In some cases, instrumented individuals can be recaptured when they return to land, and their net growth, energy expenditure and change in body condition can be estimated and correlated with at-sea behaviour (Bost et al., 1997;Boyd et al., 1993;Boyd and Arnbom, 1991;Chappell et al., 1993;Kooyman et al., 1992;Le Boeuf et al., 2000). However, this only provides information on the relationship between body condition and the behaviour and movements integrated over months and thousands of kilometres, while we are frequently interested in 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 v...

Hyperbaric computed tomographic measurement of lung compression in seals and dolphins

“…Indirect measurements have suggested that the depth where gas exchange ceases is between 30m (Falke et al, 1985) and 70m (Ridgway and Howard, 1979), but direct measurements of animals diving with a lung volume of only 18% of TLC indicate that complete shunt may not occur until a depth of 170m (Kooyman and Sinnett, 1982). Studies of maximal depth of neutral buoyancy, which has to be at a depth less than atelectasis (Biuw et al, 2003;Skrovan et al, 1999;Williams et al, 2000), agree with the pulmonary shunt data published by Kooyman and Sinnett showing that the collapse occurs at depths below 100m (Kooyman and Sinnett, 1982). Our results using the hyperbaric chamber suggest these last estimates have worth.…”

The foraging benefits of being fat in a highly migratory marine mammal

“…the seal is passively drifting through the water column), (iii) be longer than 20% of the total duration of the dive, and (iv) have little variance in depth change rate during the entire drift phase (i.e. mean-squared residual should be less than 5 m 2 as per Biuw et al [27]). After running the automated algorithm, we excluded 5.3 + 5.8% (mean + s.d.)…”

“…The total buoyancy of breath-hold divers changes with depth owing to residual air in the lungs, but this effect is reduced at greater depths because air volume decreases with depth following Boyle's law [27]. To minimize the effect of gases on buoyancy, we only used data from depths 100 m or more, as per Aoki et al [23], because in this study we focused not on the effect of residual air but on the effect of body density on swimming costs.…”

“…The buoyancy of seals at depth is related to their relative amounts of adipose and lean tissue [42], making it possible to estimate changes in body density from their buoyancy, measured by drift rate [27]. Drift rate has strong positive correlations with body density obtained from isotope dilution analysis of body composition [23].…”

“…This prediction has not been fully tested under natural conditions owing to the technological difficulties of measuring the swimming costs of free-ranging animals in relation to significant changes in their buoyancy that can occur as fat stores increase while foraging at sea (e.g. [26,27]). …”