...in metabolic rate with temperature) was 2.9 [+ or -] 0.2. Heart rate decreased with increasing body mass but increased with temperature at [Q.sub.10] of 1.8-2.2. Self-paired measures of SMR and RMR were obtained for 15 individuals. Routine metabolic rate averaged 1.8 [+ or -] 0.1 times the SMR and was not correlated with body mass. Assuming the maximum metabolic rate of sandbar sharks is 1.8-2.75 times the SMR (as is observed in other elasmobranch species), sandbar sharks are using between 34% and 100% of their metabolic scope just to sustain their routine continuous activity. This limitation may help to explain their slow individual and population growth rates, as well as the slow recoveries from overfishing of many shark stocks worldwide.

**********

Shark populations continue to suffer from overfishing throughout the Northwest Atlantic and worldwide (Baum et al., 2003). The sandbar shark (Carcharhinus plumbeus) can serve as a model for overfished coastal shark species, many of which share ecological and ecophysiological characteristics. After the rapid expansion of the Atlantic coastal commercial shark fishery in the mid-1980s, sandbar shark numbers declined 66% by 1991 (Musick et al., 1993; Sminkey and Musick, 1995). Like many of their K-selected relatives, sandbar sharks grow slowly and mature after a minimum of 13-15 years (Casey and Natanson, 1992; Sminkey and Musick, 1995). Demographic models of these species predict very slow rates of population increase even in the absence of fishing pressure, and elasticity analyses of these models demonstrate that the juvenile stage is the most critical life history stage (Sminkey and Musick, 1996; Cortes, 1999; Brewster-Geisz and Miller, 2000). It is necessary, therefore, to understand the actual and potential contributions of various juvenile nursery areas to recovery of the Northwest Atlantic sandbar shark population and to recovery of other coastal shark stocks (Branstetter, 1990).

Bioenergetics models can be used to assess the impacts and requirements of juvenile sharks as apex predators. Metabolic rate is the largest and most variable component of the energy budget for active fish species, and it is critical that it be determined accurately in order to construct realistic bioenergetics models (Ney, 1993). Systematic metabolic rate data for elasmobranchs are only rarely available, and previous models of sandbar shark bioenergetics have relied upon metabolic rate data from unrelated species (Medved et al., 1988; Stillwell and Kohler, 1993).

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 predators (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 ([approximately equal to]15[degrees]C) occur in mid or late May, whereas the maximum temperatures ([approximately equal to]28[degrees]C) are reached in surface waters in July and August (Merson and Pratt, 2001). Throughout the day, sandbar sharks perform frequent vertical excursions and thus experience surface and bottom water temperatures that can differ by up to 5[degrees]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 (Conrath (1)).

To improve bioenergetics models and to define critical 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 ([approximately equal to] 1 to 10 kg) and temperatures (18-28[degrees]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 Institute 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[degrees]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 respirometers (Brill, 1979, 1987; Leonard et al., 1999). Respirometers were constructed of 0.85 cm thick acrylic, sized to accommodate the fish being studied, and covered 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 respirometer 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 ventilated 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[R] (alphaxalone and alphadolone; Pitman-Moore, Uxbridge, UK) (Oswald, 1978). Two 20-gauge hypodermic needles were inserted into the dorsal musculature and used to administer supplementary doses of pancuronium bromide...

NOTE: All illustrations and photos have been removed from this article.



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