QUICK SEARCH:   [advanced] Author: Keyword(s):
Home     Help     Feedback     Subscriptions     Archive     Search     Table of Contents

First published online August 25, 2003

This Article Summary Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Related articles in JEB Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Biuw, M. Articles by Fedak, M. Search for Related Content PubMed PubMed Citation Articles by Biuw, M. Articles by Fedak, M. Social Bookmarking                         What's this?

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

Martin Biuw^1,*, Bernie McConnell¹, Corey J. A. Bradshaw², Harry Burton³ and Mike Fedak¹

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

View larger version (15K): [in a new window]   Fig. 1. Schematic representation of a drift dive, showing the original sampling points, the inflection points stored and transmitted by the Satellite Relay Data Logger (SRDL) and the reconstructed time-depth profile. The drift rate is calculated as the slope coefficient (d/t) of the best-fitting regression line. Here, only the regression line for the entire segment between d1 and d4 is shown, but in the actual selection process three different regression lines were evaluated (see text for further details).

View larger version (21K): [in a new window]   Fig. 2. Histograms showing the frequency distribution of start (red) and end (black) depths for the drift segments of (A) negative and (B) positive drift dives.

View larger version (33K): [in a new window]   Fig. 3. Time traces of drift rates for each individual seal over the course of the first trip to sea. Each point represents a drift dive, while the solid red lines represent smoothed spline functions fitted by the GCV algorithm, constrained to an initial interval between spline knots of 14 days (see text).

View larger version (36K): [in a new window]   Fig. 4. Daily change in drift rate (grey bars) and daily travel rate (daily horizontal displacement; black bars) for individual seals over the course of the first trip. The vertical dotted and dashed lines indicate the switch between phase 1 and 2, respectively, based on the first and last day with a five-day running average of daily travel rate below 20 km (McConnell et al., 2002).

View larger version (42K): [in a new window]   Fig. 5. Daily change in drift rate plotted against daily horizontal displacement (i.e. the distance between average daily locations). Each red point represents a drift dive. The thin dashed lines represent the 90th quantile regressions fitted for each individual seal, while the thick solid line represents the line obtained using the mean of the slopes and intercepts from each individual 90th quantile regression.

View larger version (21K): [in a new window]   Fig. 6. Vertical frequency distribution of observed drift rates (left), and curves representing the predicted drift rates (assuming no residual air) of a seal with a total body volume of 100 litres (right). The three curves correspond to the predicted drift rates using published drag coefficients (CD) for a cylinder (red dashed line) and a sphere (solid black line) from Vogel, 1981, as well as for a harbour seal (blue dotted line) from Williams and Kooyman, 1985. The horizontal dashed lines represent the maximum kernel density for all observed positive and negative drift rates separately, while the dotted lines represent the range of the 95% probability of occurrence (kernel density function, bandwidth = 2 cm s-1).

View larger version (26K): [in a new window]   Fig. 7. Relationships based on theoretical calculations for a seal pup with a total volume of 100 litres. (A) Buoyant force attributed to different body components. The broken blue line represents the buoyant force (negative) of the total non-lipid body, while the red line represents the buoyant force of the whole body, including lipid but assuming no residual air. The broken black lines represent the buoyant force of the total body, including residual air left in the lungs at 10 m-depth intervals from 10 m to 50 m. (B) Drift rates predicted from equation 9 for the 100-litre seal, assuming a total surface area of 1 m2. The red line represents the drift rate assuming no residual air in the lungs, while the broken black lines represent drift rate accounting for residual air at 10 m intervals from 10 m to 100 m. (C) Drift rate calculated from equation 9, assuming no residual air in the lungs. The red line represents drift rate assuming a surface area of 1 m2 (i.e. identical to the red line in B), while the broken black lines represent the drift rate resulting from variations in surface area. The two extreme values of surface area, 0.5 m2 and 1.5 m2, are indicated by the blue and green lines, respectively. (D) Drift rates calculated from equation 9 using the minimum and maximum seawater densities (1.027 g cm-3 and 1.030 g cm-3; blue and green curve, respectively) likely to be encountered by southern elephant seal pups from Macquarie Island. See text for further details.

View larger version (17K): [in a new window]   Fig. 8. Regression slope coefficient and sum of squared residuals between predicted and measured lipid contents expressed as functions of drag coefficients (CD; ranging from 0.09 to 1.20) used in the predictive model of lipid content. The red line represents the sum of squared residuals (SSR) for each CD value used, while the blue line represents the slope coefficient. The horizontal broken line represents a slope coefficient of 1, and the left-hand vertical broken line represents the corresponding CD value (0.49). The right-hand vertical broken line represents the CD value (0.69) that corresponded to the minimum SSR of predicted and measured lipid contents (see also Fig. 9).

View larger version (18K): [in a new window]   Fig. 9. Correlation between lipid content at the start of the trip, predicted from the fitted drift rates at departure (using the fitted spline function), for each individual seal and the lipid content measured just prior to departure using the labelled water method. The thick black line represents predicted lipid contents being identical to the measured lipid contents. The red circles (each circle representing an individual seal) were obtained using the drag coefficient (CD) that minimised the sum of squared residuals between predicted and measured values (CD=0.69), while the blue crosses (again, each cross representing one individual seal) represent the analysis using a CD value that produced a regression slope of unity (CD=0.49; see text and Fig. 8). The regression lines for these two models are represented by the red and blue broken lines, respectively (see text for regression results). Individuals for which the first daily fitted drift rate (and predicted lipid content) occurs more than 10 days after departure (20918_99, 28497_99 and 28504_99; see Table 3), and/or for which the initial fitted drift rate (and predicted lipid content) was calculated from one isolated drift dive early in the record followed by a long gap in the data (2846_99 and 26627_99) were excluded from the graph and the analysis. Also, one individual (FirstOne_00) for whom the drift dive record shows an unusual pattern, and is thus probably subject to significant dive misclassifications, has also been excluded.

CiteULike

Complore

Connotea

Del.icio.us

Digg

Reddit

Technorati

Twitter