Dowd et al : Metabolic rates of luvenile Carcharhinus plumbeus 



325 



collection was not resumed until at least one hour after 

 the chamber had equilibrated to the new temperature 

 (Steffensen, 1989). In order to minimize any system- 

 atic errors, the direction of temperature change was 

 not always the same. To quantify the effects of acute 

 temperature changes on SMR and heart rate, Qm values 

 were calculated over the temperature ranges 18-24°C, 

 24-28°C, and 18-28°C by following the methods of 

 Schmidt-Nielsen (1997). 



Routine metabolic rates 



An annular respirometer (1250 L; diameter 167 cm) was 

 used to measure RMR (Bushnell et al., 1989; Parsons, 

 1990; Carlson et al., 1999). A cage (diameter 61 cm) was 

 placed in the center to force the sharks to swim around 

 the perimeter of the tank. Water temperature was con- 

 trolled during an experiment at 24-26°C by adjusting 

 room temperature. 



Sharks were transferred to the respirometer and 

 allowed to recover for 30-90 minutes. The tank was 

 sealed and data recording continued until oxygen con- 

 tent was reduced by 15% (=two hours). The tank water 

 was then re-oxygenated before the next measurement by 

 pumping the seawater through a membrane oxygenator 

 (Medtronics, Inc., Minneapolis, MN) (Steffensen et al., 

 1984). A complete RMR experiment consisted of one to 

 five iterations of this process. 



Oxygen concentration (mg OJh) was measured with a 

 YSI 57 oxygen meter by using a YSI 5739 polarographic 

 electrode oxygen-temperature probe (Yellow Spring In- 

 struments, Yellow Spring, OH). Water temperature and 

 oxygen content were recorded at 20-second intervals with 

 a computerized data acquisition system (model PCA-14, 

 Dianachart, Inc., Oak Ridge, NJ). Routine metabolic 

 rates (mg 0,,/h) were calculated from the rate of decline 

 in dissolved oxygen (mg 0.i/(Lxmin)) and the volume of 

 the respirometer (Steffensen, 1989). Swimming speeds 

 (in body lengths per second, BL/s) were determined 

 every 15-30 minutes by averaging the time required 

 for the shark to complete three to six laps. To account 

 for the increased costs of swimming in a circular path, 

 recorded RMRs were corrected to straight line estimates 

 (RMR^,) by following the method of Weihs (1981). 



Statistical analysis 



Routine and standard metabolic rate data at each tem- 

 perature were fitted to the allometric equation MR = 

 a-xM^ by using nonlinear, iterative Gauss-Newton 

 regression (Brill, 1979, 1987). This technique provides 

 more accurate estimates of the parameters than log- 

 transformed linear regression (Glass, 1969). The likeli- 

 hood ratio test statistic was used to test for differences 

 in the allometric exponents (b) among temperatures 

 and between SMR and RMR at 24-26°C. Analysis of 

 covariance (ANCOVA) of log-transformed metabolic 

 rate (with log-transformed mass as covariate) was also 

 used to test the equivalence of the exponents (b) in the 

 untransformed allometric equations. Differences in SMR 



0.91 



8 910 



3 4 

 Mass (kg) 



Figure 1 



Standard metabolic rates (mg 0.,/hr) of juvenile sandbar 

 sharks [Carcharhinus plumbeus) as determined by flow- 

 through box respirometry at 18 'C (D). 24"C (O), and 

 28 C (Tl. Lines show best-fit allometric equations at 

 each temperature. Error bars indicate ±1 standard error. 



and heart rate among temperatures were evaluated by 

 using ANCOVA. Analysis of covariance with mass as 

 covariate was used to test for differences in mean SMR 

 QlO's and heart rate QlO's among the three temperature 

 ranges. The relationship between the RMR-to-SMR ratio 

 and body mass was assessed with linear least squares 

 regression. Statistical analyses were performed in Sta- 

 tistica 6.1 (StatSoft, Inc., Tulsa, OK) and SAS, version 

 8.0 (The SAS Institute, Inc., Gary, NC), with P<0.05 

 taken as the limit for significance. All values reported 

 are means ± standard error of the mean. 



Results 



Standard metabolic rates 



Standard metabolic rate increased with body mass (range 

 1.025-10.355 kg) at all three temperatures (ANCOVA, 

 logmass, Fj -9=265.04, P< 0.001 ) (Fig. 1). The best-fit- 

 ting allometric equations relating SMR (mg Og/h) to 

 body mass (M, kg) were 



18°C: SMi? = 65 (±15) X ?; =16, 7-2=0.71 (1) 



24°C: SMi? = 120 (±17) X ?!=33, r2=0.84 (2) 



^0.79 I ±0.08 1 



28°C; SM7? = 207 (±28) X 7!=16, r2=0.87 (3) 



J^^0 63 (±0.071 



Standard metabolic rate increased with temperature 

 for each individual and overall (ANCOVA, temperature, 



