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Fishery Bulletin 104(3) 



The lower Chesapeake Bay, Mid-Atlantic Bight, and 

 adjacent coastal lagoon systems serve as the primary 

 summer nurseries for sandbar sharks in the Northwest 

 Atlantic (Musick et al., 1993). Juvenile sandbar sharks 

 return for four to ten years to these nursery grounds, 

 where they enjoy the benefits of generally high food 

 availability and limited exposure to large shark preda- 

 tors (Musick and Colvocoresses, 1986; Grubbs et al., 

 in press). Juvenile sandbar sharks in the nurseries 

 are exposed to seasonal water temperature variations, 

 as well as shorter time-scale fluctuations associated 

 with their vertical movements and day to day variation. 

 The minimum seasonal temperatures (=15°C) occur in 

 mid or late May, whereas the maximum temperatures 

 (=28°C) are reached in surface waters in July and Au- 

 gust (Merson and Pratt, 2001). Throughout the day, 

 sandbar sharks perform frequent vertical excursions 

 and thus experience surface and bottom water tem- 

 peratures that can differ by up to 5°C (Grubbs, 2001). 

 Similarly, in Virginia's Eastern Shore lagoons, juvenile 

 sandbar sharks venture onto broad, warm tidal flats at 

 high tide and return to deeper, cooler channels as the 

 tide recedes (ConrathM. 



To improve bioenergetics models and to define criti- 

 cal habitat and the current suitability of nursery areas 

 more accurately, standard (SMR) and routine metabolic 

 rates (RMR) of juvenile sandbar sharks were measured 

 over a relevant range of body masses (=1 to 10 kg) and 

 temperatures (18-28°C) (Merson and Pratt, 2001). This 

 is the first direct measurement of SMR, and the first 

 comparison of paired SMR and RMR, in a continuously 

 active carcharhiniform species. 



Materials and methods 



All experiments were conducted at the Virginia Insti- 

 tute of Marine Science Eastern Shore Laboratory from 

 June through September 2002. Sandbar sharks (57-124 

 cm total length; 1.025-10.355 kg) were captured by 

 using hook and line from the surrounding tidal lagoon 

 system and maintained in shoreside tanks (temperature 

 22-29°C, salinity 34-36%^). Individuals were fasted 

 for at least 48 hours prior to use in an experiment to 

 reduce any confounding effects of specific dynamic action 

 (Medved, 1985). 



Standard metabolic rates 



Because sandbar sharks are continuously active obligate 

 ram ventilators, SMR measurements were obtained 

 from chemically immobilized and artificially ventilated 

 animals maintained in flow-through, sealed box res- 

 pirometers (Brill, 1979, 1987; Leonard et al., 1999). 

 Respirometers were constructed of 0.85 cm thick acrylic, 

 sized to accommodate the fish being studied, and cov- 



Conrath, C. 2004. Personal commun. Virginia Institute 

 of Marine Science. 1208 Create Road, Gloucester Point, VA 

 23062. 



ered with black plastic to minimize visual disturbance. 

 Aerated and filtered seawater from a constant pressure 

 head tank passed through the mouth and over the gills 

 of the sharks, was mixed in the chamber by a small 

 recirculating pump, and exited the respirometer by a 

 hose mounted at the top. Water leaving the respirom- 

 eter was collected, re-aerated, and mixed with a small 

 amount of fresh filtered seawater to help maintain a 

 constant temperature. Turnover rate for the system was 

 20-30%/hour (Steffensen, 1989). 



Sharks were netted, injected with 0.4-1.8 mg/kg of 

 the neuromuscular blocking agent pancuronium bromide 

 through the caudal vein, and returned to the holding 

 tank until they were unable to swim (typically 1-2 min). 

 They were then placed supine on a moist towel and ven- 

 tilated with aerated seawater while electrocardiogram 

 (EKG) wire leads were inserted subcutaneously over the 

 pectoral girdle to monitor heart rate. Individuals were 

 also given an intramuscular injection (0.2-1.2 mg/kg) 

 of steroid anesthetic Saffan" (alphaxalone and alph- 

 adolone; Pitman-Moore, Uxbridge, UK) (Oswald, 1978). 

 Two 20-gauge hypodermic needles were inserted into 

 the dorsal musculature and used to administer supple- 

 mentary doses of pancuronium bromide and Saffan® 

 whenever any slight tail movement was observed. 



The partial pressure of oxygen (POo, mm Hg) in the 

 seawater was measured with a polarographic oxygen 

 electrode (Radiometer A/S, Copenhagen, Denmark) 

 mounted in a water-jacketed cuvette (maintained at the 

 experimental temperature) and connected to a digital 

 oxygen meter (Cameron Instruments Company, Port 

 Aransas, TX). All equipment was calibrated to man- 

 ufacturer's specifications. Oxygen level in the inflow 

 water was measured hourly, and outflow water was 

 otherwise monitored continuously. Water temperature, 

 PO.,, and heart rate were recorded every 10 seconds 

 with a computerized data acquisition system (Daqbook 

 120 with a DBK52 thermocouple expansion card; lotech. 

 Inc., Cleveland, OH). The inflow ventilation volume (V , 

 L/min) was adjusted to keep oxygen extraction between 

 10% and 20%. Measured PO,, values were converted 

 to oxygen content (mg 0.,/L) following Richards (1965) 

 and Dejours (1975). Standard metabolic rate (mg OJh) 

 was calculated by using the Fick principle (Steffensen, 

 1989). Because the effects of pancuronium bromide were 

 not reversible, at the end of each experiment individuals 

 were euthanized with a massive overdose of sodium pen- 

 tobarbital injected into the caudal vein. Sex was then 

 determined and they were weighed to the nearest five 

 grams and measured (precaudal and total lengths). 



Standard metabolic rate data for each individual were 

 plotted against time and averaged over all hours (range 

 1-7 hours) after the outflowing PO, stabilized. Standard 

 metabolic rate measurements were obtained at 24 ±1°C 

 on 33 of the 34 animals. In addition, 18 animals were 

 exposed to acute temperature changes (to 18 ±1^C or 28 

 ±1°C, or to both). Temperature change rates averaged 

 4.5 ±0.6°C per hour and 6.4 ±1.1°C per hour for cool- 

 ing and heating, respectively, although these were not 

 statistically different it =-1.46, df=36, P=0.15). Data 



