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Fishery Bulletin 104(3) 



The effects of acute temperature change on SMR were 

 consistent with published values for other elasmobranchs 

 (Qi(,=2-3; e.g., DuPreez et al., 1988; Carlson and Par- 

 sons, 1999; Miklos et al., 2003). The Q^ for the SMR in 

 elasmobranchs has also been reported to vary with the 

 temperature ranges assessed (Butler and Taylor, 1975; 

 Hopkins and Cech, 1994), but this was not the case 

 for sandbar sharks. It is important to note the distinc- 

 tion between acute temperature changes and seasonal 

 acclimatization when reporting Qjg values (Schmidt- 

 Nielsen, 1997). In the present study, sandbar sharks 

 were exposed to rapid temperature changes that mir- 

 rored short-term temperature fluctuations experienced 

 in the wild (SMR Qi,j=2.9 ±0.2). In seasonally acclima- 

 tized bonnethead sharks (Sphyrna tiburo), the effect 

 of seasonal temperature change on metabolic rate was 

 lower (Qj„=2.29-2.39; Carlson and Parsons, 1999). 



Cost of activity and routine energy expenditure 



The SMR is never realized in fish that must swim contin- 

 uously to maintain hydrostatic equilibrium or to venti- 

 late their gills. Measurement of SMR and RMR in active 

 species nevertheless allows insight into the division of 

 metabolic costs between swimming and maintenance 

 processes. For example, the average metabolic rate of 

 juvenile scalloped hammerhead sharks (Sphyrna lewini) 

 in the wild was 1.4 times the estimated SMR (Lowe, 

 2002). In sandbar sharks, the RMR to SMR ratio (1.8 

 ±0.1) and RMR^i to SMR ratio (1.6 ±0.1) were similar to 

 those observed and estimated for several elasmobranch 

 species (e.g., 1.5, Brett and Blackburn, 1978; 1.4, Nixon 

 and Gruber, 1988; 1.7, Carlson et al., 1999). In other 

 words, SMR comprises 56-63% of total metabolic rate 

 at routine activity levels. Because the allometric expo- 

 nents for RMR and SMR were not different at 24°C, we 

 conclude that the RMR-to-SMR ratio (and, therefore, 

 cost of activity) is also size independent, at least over 

 the size range of sandbar sharks tested. 



Our metabolic rate data span the size and tempera- 

 ture ranges relevant to the summer populations of ju- 

 venile sandbar sharks in Chesapeake Bay and other 

 western Atlantic nursery areas (Grubbs et al., in press; 

 Merson and Pratt, 2001). Bioenergetics models require 

 estimates of field activity and corresponding energetic 

 costs (Lowe, 2002). The swimming speeds of sandbar 

 sharks in the annular respirometer (mean 0.55 ±0.03 

 bl/s) were well within the range of activity levels ob- 

 served in nature (Grubbs, 2001). After the application 

 of an oxycalorific coefficient of 13.6 J/(mg O^) (Elliott 

 and Davison, 1975), the RMR and RMR^, for a 1-kg 

 sandbar shark at 24"C represent energy expenditures 

 of 69.7 and 63.4 kJ/day, respectively. These values are 

 comparable to those for the lemon shark {Negaprion 

 brevirostris, 67.7 kJ/day; Nixon and Gruber, 1988), S. 

 tiburo (80.2 kJ/day; Parsons, 1990), and S. lewini (96 

 kJ/day at ~28°C; Lowe, 2002) . The Q,„ values for SMR 

 obtained between 18° and 28°C demonstrate that juve- 

 nile sandbar shark metabolic demands change signifi- 

 cantly as ambient temperature changes, both on short 



time scales and over the course of the summer stay in 

 the nursery areas. 



Heart rates 



Heart rate decreased with increasing body mass but 

 increased with temperature (Fig. 2), as it does for other 

 ectothermic species (Schmidt-Nielsen, 1997). Heart 

 rates of juvenile sandbar sharks were comparable to 

 heart rates of two other shark species while swimming 

 (Scharold et al., 1989; Scharold and Gruber, 1991), 

 although the sandbar shark data should be interpreted 

 with caution. Pancuronium bromide has been shown to 

 exhibit vagolytic activity in mammals (Melnikov et al., 

 1999), but to our knowledge its effect on shark heart 

 rates is unknown and would depend on the resting vagal 

 tone. In the dogfish iScylior-hinus canicula), resting 

 vagal tone increased with temperature between 7°C and 

 17°C (Taylor et al., 1977). The resting vagal tone and 

 resulting elevation in heart rate after treatment with 

 pancuronium bromide could be significant in sandbar 

 sharks, especially at the higher temperatures. If so. 

 Figure 2 may reflect the effect of temperature and body 

 mass on intrinsic heart rate. 



Measuring SMR of immobilized sharks 



Standard metabolic rate is defined as the oxygen con- 

 sumption of a postabsorptive-stage fish at rest, and it is 

 considered the minimum metabolic cost of organismal 

 maintenance (Brett and Groves, 1979). Two methods 

 are commonly used to determine SMR. In the first, the 

 slope of a power-performance curve relating the loga- 

 rithm of oxygen consumption rate to relative swimming 

 speed is extrapolated back to zero activity (e.g., Lowe, 

 2001). However, extrapolation does not take into account 

 physiological differences between active and quiescent 

 fish, specifically the induction of anaerobic metabolism 

 during high-velocity swimming, and may misrepresent 

 SMR (Brett and Groves, 1979; Cech, 1990). Further, 

 swimming kinematics can be significantly altered in 

 a swim flume, leading to overestimates of SMR (Lowe, 

 1996, 2001). The second option for measuring SMR is to 

 confine the fish in a sealed or flow-through respirometer 

 (e.g.. Brill, 1987; Hopkins and Cech, 1994). This process 

 works well for sedentary species, but active fish will 

 struggle in such situations, requiring the use of paralytic 

 and sedative agents, as well as artificial ventilation. 



Several studies have confirmed, however, that the two 

 methods yield identical results (SMRs and allometric 

 exponents) in various fish species (e.g., yellowfin tuna 

 [Thunnus albacares]; kawakawa [Euthynnus affinis]; 

 skipjack tuna [Katsuwonus pelamis]; rainbow trout [On- 

 chorynchus mykiss\\ American shad [Alosa sapidissima]; 

 aholehole [Kiihlia sandvicensis] [Brill, 1979, 1987; Dew- 

 ar and Graham, 1994; Leonard et al., 1999)). Moreover, 

 treatment with anaesthetics has been shown to have no 

 effect on the SMR of little skate (Raja erinacea ; Hove 

 and Moss, 1997) or nursehound IScyliorhinus stellaris; 

 Baumgarten-Schumann and Piiper, 1968). 



