Estimating the Blubber Content of Phocid Sea Morten Ryg, Christian Lydersen, Nina H. Markussen Division of General Physiology, University of Oslo, P.0. Box 7051, Blindern, 03 96 Oslo 3, Norway and Thomas G. Smith Fisheries and Oceans, Arctic Biological Station, Ste-Anne de Bedlevue, (Qu6bec) H9X 3R4 Canada and Nils Are C3ritsland Norwegian Polar Institute, Box F58, f 330 Oslo lufthavn, Norway; and Division of General Physiology, University of Osdo, P.O. BOX 705 I, Blindern, 03 9 6 Oslo 3, Norway Ryg, M., C. Lydersen, No H. Markussen, T. G. Smith, and N. A. @ribland. 1998. Estimating the blubber content of phocid seals. Can. J. Fish. Aquat. Sci. 47: 1223-1227. We have investigated the relationships between percent blubber content and xiphosteswal blubber thickness or girth-to-length ratios in ringed seal (Phoca hispida), harp seal (Phsca groenlandica), and grey seal (Halichoerus grypus). The blubber content was significantly correlated with blubber depths and girth-to-length ratios in all three species. In addition, we have developed an estimator for percent blubber content (the LMD-index) based on standard length (1, in meters), body mass (M, in kilograms), and on blubber thickness (d, in meters) measured at a defined position dsrsa9ly. From these variables the percent blubber content (B%) can be estimated by the expression B% = 4.44 + 5693 ad, with a standard error of the estimate of three percentage units. This index also gave reasonable estimates for blubber content in three harbour seals (Phoca vitulina) and eight bearded seals (Erignatur barbatus). On a 6tudi6 les relations entre le volume de petit lard (en pourcentage) et If6paisseur du petit lard de Ifappendice xiphoi'de ou les rapports entre Be volume de I'abdornen et la longueur du phsque annel4 (Phoca hispida), du phoqhse du Greenland (Phoca groeniandica) et dsr phoque gris (Halichoerus grypus). Le volume de petit lard 6tait nettement li6 A son epaisseur et au rapport entre le volume de I'abdomen et la longueur du corps chez ces trois especes. Be plus, sn a calcule un estimateur du volume de petit Bard en pourcentage (%'indice LMD) bas4 sur la longueur standard (L, en metre), la masse csrporelle (M, en kg) et l'epaisseur du petit lard (d, en metre) mesur6e A un point precis sur le dos. EBB se basant sur ces variables, on peut d4terrniner le volume de petit lard en pourcentage (B %) 21 I'aide de la formule B % =4,44 + 5693 L1M.d avec un 6cart-type de l'estimation de trois unites de pourcentage. Cet indice donne aussi des estimations aeceptables du volume de petit lard de trois phoques communs (Phoca vitulina) et de hu it phoques barbus (Erignatus barbatus). Received March 13, 3 989 Accepted january 18, 7 990 (JA088) B ecause heat loss to water is many times higher than to air, aquatic life poses special problems for homeothemic animals. In cetaceans and phocid seals, the intra- and subdermal fat tissues, called the blubber, foms the main protection against heat loss. In addition, the blubber fat serves as an energy reserve, although the relative importance of fat stored in the blubber and in the core (muscle, skeleton, and viscera) varies between species (Worthy and Lavigne 1987). The fat stores probably influence fertility in marine mammals (Lockyer 1986; Smith 1987 Testa 1987), as they do in terrestrial mammals (Hamilton and Blaxter 1980; Thomas 19821, and fat content varies throughout the year in conjunction with the reproductive cycle (Fed& and Anderson 1987 ; Ryg et a1. 1990). As a consequence, there is great interest in methods for determining body condition of marine mmmals. Direct rneasurement of blubber or fat mass by dissection or extraction of fat from the whole bdy is laborious, and it is common to use some kind of body condition index to evaluate nutritional condition. The two indicators most commonly used for seals are the xiph- Can. 9. Fish. Aquat. Sci., Vol. 47, 1990 Rep le 13 mars 1989 Accept6 le 7 8 janvier 1998 ostemal blubber thickness and the "condition index"; maximum girth divided by standard length times BOO (Sergeant 1973). The extent to which these indices are related to blubber content have not been quantitatively assessed. They appear to be relatively insensitive, even failing 'es pick out individuals that are, from inspection, close to death from starvation (T. G. Smith, pers. observ.). They also appear to be only weakly correlated with each other (Pitcher 1986). Better indices of body condition are therefore needed. Here we examine the relations between percentage blubber content and the xiphostemal blubber thickness, and the girth/ length ratio. We also develop a new index for calculating blubber content. This ' LMD-index' ' is calculated from the following variables: standard length (&), body mass (M), and blubber thickness (4). Using data from five seal species, we suggest that the LMD-index can be used to estimate blubber mass for phocid seals in general.
Materials and Methds Can. J. Fish. Aquat. Sci. Downloaded from www.nrcresearchpress.com by Peking University on 06/04/13 Collection sf Data Five species of seals were considered in the analysis. One hundred and thirty two ringed seals (Phoca hdspkda) were shot from the end of Febmxy to mid September in three consecutive years (6985 to 1987), 19 from the Barents sea and the rest from Kongsfjorden ('98'55 'N, 1 1"58 'E) on Svalbad. Norway. Eight bearded seals (Erignatus barbatus) were shot in Ksngsfjorden in May 1989. Thirty-eight grey seals (Halichoerus grypus) were shot on Anticosti Island in the Gulf of St. Lawrence, Canada from July to August 1987. Twenty harp seals (Phocw. groenlandiea) that had drowned in fishing nets on the northwest coast sf Noway were collected in January and February 198%. Three harbour seals (Phoca vituliaa) which had died during the North Sea epizootic in 1988 (Osterhaus and Vedder 2988) were also included in the total sample (the actual cause of death of these individuals was not confirmed). The animals were weighed, and standard measurements taken (American Society of Mammalogists 1967). Weighing of seals was done with spring balances to the nearest 100 g. The standard length (nose-to-tail length in a straight line) of the seals should ideally be measured before rigor sets in. The dead seals were placed belly upwards, and pulled gently by their hindlimbs to stretch them out. In this way one avoids either having the neck pulled into an S-shape. or overstretching the neck. In seals which had been frozen, we often had to forcibly straighten out the neck, which naay have introduced errors in the length measurements. Girths were measured around the snout and head and every 10 cm behind the snout, back to just in front of the anus. The sculp (skin with attached blubber from body and head, but excluding flippers) was carefully dissected from the body core, placed sn a flat surface, and stretched out so that sculp length was equal to the standard length of the animal. The blubber thickness was measured at every 10 cm across the length md width of the sculp. Thickness measurements did not include the skin. Blubber mass was determined by subtracting skin mass from sculp mass, after thoroughly removing the blubber from the skin. Data Analysis Blubber content, as percent sf body mass, was compared with the classical indices of body condition, xiphostemal blubber thickness, and "condition index," 188 x maximum girth/ standad body length sf the harp seals, grey seals, and ringed seals. The beaded and harbour seals were not used in these comp&sons because of the small sample sizes. Blubber content was also compared with the expression CM~, where L is standard body length in metres, M is body mass in kilogramd and d is dorsal blubber thickness in metres, measured at a standard position defined below. The rationale for testing this relationship was that the volume of blubber is equal to B =A-h&,,,,, where A is axial surface area and dm, is mean blubber thickness. Wyg et a%. (1988) showed that the axial surface area can be expressed as A = 0.112sfi, and that the blubber thicknesses at various parts on the body are linearly related to each other. Blubber volume can thus be expressed as Blubber = k-fl~ad, where the proportionality constant k depends upon which blubber thickness is chosen md the density of the blubber. Dividing by ass gives the percentage blubber content: B% = 10g9ke ad. Because of the small H. gry pus Blubber thickness (cm) FIG. I. Relationship between percentage blubber content and sternum blubber thickness. sample sizes, the relations between blubber content and LIVIDindex were not calculated for bearded and harbour seals. Instead, measured blubber content was compared with the fat content predicted on the basis of the relation between blubber content and m ~ -derived d from the other three species. The position of choice to measure the blubber thickness is where it is most variable, following the recommendation of Lockyer et al. (1985) for whales. Ryg et al. (1988) found that the most variable location of blubber thickness in ringed seals is dorsally, about 60% of the standard body length behind the snout. The blubber thickness at this position (found by interpolating between the nearest points on the 10 x 18 crn grid) was used in the expression above. Least squares linear regression analysis was used to find the relationship between the various indices and the blubber content. Differences between the slopes and elevations sf regression lines were tested by analysis of covakimce (Snedecor and Cochran 196%). Other differences were tested with student's t- test. Results The blubber content, expressed as percent of total body weight, was significantly correlated @<0.01) with xiphoster- Can. J. fish. Ayuat. Sct., Val. 47, 1990
FIG. 2, Relationship between percentage blubber content and 100-girtWlength. nal blubber depth. (Fig. 1). The regressions between percentage blubber content and blubber depth were for ringed, harp, and grey seals: Pa hispida Percent blubber = 55 1 a d + 22.0 3 = 0.46** S, = 4.85 Pm groenlundica Percent blubber = 807-d + 4.35 3 = 0.70** S, = 4.01 H. g VPs Percent blubber = 497.d + 10.7? = 0.69** S, = 3.64 The percentage blubber content was significantly correlated @<0.81) with the condition index (100 x girthllength) (Fig. 2). The regressions between percentage blubber content and condition index were for ringed, harp, and grey seals: P. hkispida Percent blubber = 0.66-condition index - 16.29 = 0.39 = 5.17 2 d m (mlti kg-0,5) Rc. 3. Relationship behueen blubber content and m - d. 0 = Phoca hispida, @ - Phoca grsenlandica, = Halichoems grypaas. = Erignatus barbatus, A = Phsea vituliwa. P. groendandisa Percent blubber = 1.27econdition index - 67.3 1 3 = 0.55 S, = 4.91 H. grygus Percent blubber = 0.83 -condition index + 32.13 t2= 0.38 S,=5.10 Condition indices amel xiphostemal blubber thicknesses were significantly correlated in- all three species. The correlation coefficients were 0.7 1,0.83, and 0.59 for grey, harp, and ringed seals, respectively. Percentage blubber content and the expression -M.d were significantly correlated @<O.O%, Fig. 3). The regressions between percentage blubber content and were for ringed, h q, and grey seal: Ps hispida percent blubber = 5102aC~md + 8.53 3 =.0.82 S, = 2.80 P. ggroenkandca Percent blubber=5387.m*d+6.13 9=0.90 %,=2.35 H. gvpus Percent blubber = 5650.m~df 2.27? = 0.83 S, = 2.68 The slopes and elevations of the regression lines for harp, ringed, and grey seal species (Fig. 1) were not significantly different (0.1 BpBO.05). For the poled sample the regression equation was: Percent blubber = 56930-sdf4.44 $ = 0.88 S,= 3.02 In eight bearded seals, with blubber content ranging from 25 to 38% of body mass, observed blubber mass was not significantly different from the blubber mass estimated from the equation above (paired t-test, 9 = 0. '74). With one exception, where 09 Can. J. Fish. Aquat. Scb., Voi. 47, I990
the observed blubber content was 1 1 percentage units lower than estimated, the difference ranged from - 2.88 to 3.15 percentage units. The "exceptional" animal also had a very high condition index in relation to its actual blubber content, and we cannot exclude the possibility of an error in data recording. In thee harbour seals, the estimated and observed blubber content were 31.6 and 33.0; 26.2 and 26.1; and 29.5 and 32.9, respectively. The difference was not statistically significant. The positive y-intercept was significantly different from zero for all species except grey seal, and also for the pooled sample (Fig. 3). Discussion Knowledge of the mass of the blubber permits calculation of the minimum heat loss through the body surface (Ryg et al. 19881, and consequently gives a lower limit to the metabolic rate needed to maintain homesthemy. Provided the fat content of the blubber is known, it also gives an estimate of the energy reserves contained in the blubber. However, nutritional condition depends on more than the energy content of the blubber layer. Fat may also be stored in the core (i.e. muscles and viscera) (Worthy and Lavigne I 987), and to evaluate the condition one must also consider the normal seasonal fluctuations in body weight and body fat content. Blubber thickness, the condition index, ad our proposed LMD-index were all correlated with percent blubber mass. The relationship between blubber content and xiphostemal blubber thickness was different for the three species investigated, as a given blubber thichess was related to a lower blubber content in the grey seal than in the other seals. This is to be expected, as a given blubber thickness will constitute a smaller proportion of the body radius in larger animals. The LMD-index was a more precise estimator than the other two, provided a linear estimator of percentage fat content, and was species-indepewdent as far as we have ken able to ascertain. In contrast, the condition index probably reflects differences in the volume of the core as well as the blubber layer. Use of both indices could help to elucidate patterns of fat loss from blubber a d core. It is impt-tant to be aware of the limitations in use of indices. The S,, of 3% of the error of blubber mass prediction from the LMD-index gives a confidence internal for individual predictions of about 6%. This corresponds to a confidence interval sf individual heat loss estimates of + 28% at 220% blubber content, and & 15% at 550% blubber content (Ryg et al. 1988). One should also be careful not to use the LMD-index uncritically outside the range where it has been verified empirically. The value of the index must necessarily be zero for zero blubber content (since the blubber thickness then is zero), and the positive y-intercept therefore means that the line must curve downwards for Tow values. We do not know the reason for this nonlinearity at low values of blubber content, but it could be caused by nonlinear relations between blubber thickness in very lean seals. Our arguments for the LMD-index is that blubber thicknesses at different locations of the body are proportional, but this may not be the case outside the range of blubber contents that are found in our material. The statistical enor of the LMD-index depends on the errors in measurements of length, mass, and blubber thickness, and can be calculated by the expression where SM, SLY and S, are the standard deviations of the mass, length, and thickness measurements (Beviwgton 1969). Length measurements do not require more than an ordinary tape; however the standardization of the measurement is important (American Society of Mammalogists 11967). If quality weighing equipment is available, the body mass can be determined with an accuracy of about 1 %, even under field conditions. The index gives the blubber in percent of the mass as measured, and one should avoid correcting for blood loss before the index is calculated. Variations in the amount of blood loss will therefore introduce errors in the estimate of the blubber content of the live animal. According to the expression above the error in the blubber thickness measurement contributes four times more than emor in weight and length determinations. Since it should be possible to bring the variation coefficients for mass and length down to &out I %, and it is difficult to measure the usual range of 1 to 6 em blubber thickness with any greater accuracy than 1 mm, the statistical enor in the index will be dominated by the enor in measurement of blubber thickness. If the seal dissections are part of a commercial venture, measuring the dorsal blubber thickness may represent a problem because one will ruin the skin if it is cut in that position. It is therefore desirable to develop a simple and rapid method for measuring the dorsal blubber thickness of seals without ruining the skin. NOW-intrusive methods could possibly also be used on Hive animals. In conclusion, we have found that the two classical indices of condition in phocid seals, xiphostemal blubber thickness, and girth-to-length ratio, both are positively correlated with blubber mass as percent of body mass. We have also developed an index based on length, body mass, and dorsal blubber thickness, which can be used to estimate percent blubber mass of phocid seals in general. Acknowledgement The present work was supported financially by The Norwegian Council for Science and the Humanities (the ringed seal project under the PRO MARE programme), The Institute for Marine Research (csllection of harp seals), and The Norwegian Palar Institute (grants no. 18/85, 12/86 and 11/87). Grey seals were collected and processsed by the Arctic Biological Station, Department sf Fisheries and Oceans, Canada. The Polar Institute and Institute of Marine Research gave logistic help with field work and during collection of h q seals. A mmkr of persons helped with the practical wok: Don Allbright, Paul Aspholm, Chris Cuyler, Bj@m Haukelidszther, Wyb Hsek, Dystein Sigde, and T6~9 Wmen, we thank all of them. References AMERICAN SOCIETY OF MAMMALOGISTS 1967. Standard nleasurernents of seals. J. Mammal. 48: 459-462. BEVINGTQN, P. W. 1969. Data reduction and error analysis for the physical sciences. McGraw-Hill, New York, N.Y. FEDAK, M. A., AND S. S. ANDERSON. 1987. Estimating the energy requirements of seals from weight changes, p. 206-226. In A. C. Huntley, D. P. Costa, G. A. J. Worthy and M. A. Castellini [ed.] Maine mammal energetics. Society for Marine Marnmalogy. HAMILTON, W* J., AND K. L. BLAXTER. 1980. Reproduction in famed red deer. 1. Hind and stag fertility. J. Agkc. 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