Dowd et al ; Consumption rates of Carcharchinus plumbeus In Chesapeake Bay 



339 



1.49%BM/d, M=1.7 kg, Stillwell and Kohler, 1993). This 

 difference was partly due to the incorporation of species- 

 specific routine metabolic rate data into our model, which 

 were 8-15% higher than values from the spiny dogfish 

 (Squalus acanthias) used in earlier models. Earlier models 

 also estimated daily ration at a mean temperature over 

 the entire year, whereas our model incorporated seasonal 

 temperature shifts and the resulting effects on metabolic 

 rate using the Qjq. Test runs of our model were used to 

 predict daily rations over the winter, assuming that the 

 diet composition was the same, 25% of annual growth 

 occurred in the winter (Sminkey and Musick, 1995), and 

 average water temperature was 14°C (Springer, 1960). 

 These model runs predicted daily rations less than half 

 (<l%BM/d) of those estimated for the summer nursery 

 season. More data, however, are needed on the biology of 

 sandbar sharks in the winter nursery grounds in order to 

 develop an accurate year-round bioenergetics model. 



Sandbar shark daily consumption rates have also been 

 estimated by using meal size and frequency, as well as 

 gastric evacuation rates. Our models predicted consump- 

 tion rates (1.30-2.17 %BM/d) support estimates based on 

 meal size and frequency. The reconstructed meal size 

 for juvenile sandbar sharks in Chincoteague Bay, based 

 on stage of digestion estimates, was 4.23 ±0.31% BM 

 (Medved et al., 1988). Given the sandbar shark's 70-92 

 hour gastric evacuation rate (Medved, 1985), as well as 

 the high proportion of sharks landed with empty stom- 

 achs (17.9-20.0%) (Medved and Marshall, 1981; Medved 

 et al., 1985; Stillwell and Kohler, 1993; Ellis, 2003), it 

 seems likely that 48-72 hours pass between significant 

 feeding events (Medved et al., 1985). Therefore, the re- 

 constructed meal sizes correspond to daily consumption 

 rates of 2.12-1.41% BM/d. In contrast, gastric evacua- 

 tion models predicted juvenile sandbar shark daily ra- 

 tions (0.93% BM/d to 1.07% BMd; Medved et al., 1988) 

 lower than our bioenergetics model. However, the data 

 probably violated the gastric evacuation models' assump- 

 tions of continuous feeding and that time between meals 

 exceeds digestion time (reviewed by Cortes, 1997). 



The estimated sandbar shark daily rations are compa- 

 rable to those for other active shark species. For exam- 

 ple, the estimated daily rations for a 1-kg N. brevirostris 

 and a 0.76-kg S. lewini were 2.62% BM/d and 2.9-3.9% 

 BM/d, respectively (Gruber, 1985; Lowe, 2002). The 

 sandbar shark daily rations were averaged over the 

 entire simulated nursery season, during which tem- 

 perature fluctuated by 10°C. Predicted daily rations in 

 mid-summer were frequently higher than 3.0% BM/d. 



The predicted mean gross conversion efficiency from 

 our model (0.10-0.16) was similar to estimates for 

 bull sharks (Carcharhinus leucas) fed to satiation in 

 captivity (0.05-0.12, Schmid and Murru, 1994) and 

 for juvenile lemon sharks {N. brevirostris) in the wild 

 (0.10-0.13, Cortes and Gruber, 1994). 



Parameter uncertainty 



The largest potential sources of error in the model were 

 L^, K, SMRa, and SMRb (Fig. 1). Fortunately, the von 



Bertalanffy growth parameters (L^, K) and the SMR 

 allometric scaling parameters (SMRa and SMRb) are 

 among the best known for juvenile sandbar sharks, 

 and the initial estimates used are considered reliable. 

 Metabolic rate may also be impacted by osmoregulatory 

 costs incurred by penetrating the less saline regions 

 (-20-25 ppt) of the Chesapeake Bay nursery area (Chan 

 and Wong, 1977; Meloni et al., 2002). Future studies 

 should investigate this possibility. Other confounding 

 factors which will alter metabolic rate estimates associ- 

 ated with routine swimming behavior include movement 

 of the animals with dominant tidal currents or burst 

 swimming followed by oxygen debt repayments (or both 

 factors) (e.g., Kerr, 1982; Boisclair and Leggett, 1989). 

 Although these factors may affect ACT estimates, field 

 tracking data from juvenile sandbar sharks indicate that 

 mean rates of movement (converted to body lengths per 

 second, BL/s) in the wild (0.23 BL/s, Huish and Bene- 

 dict3; 0.46 BL/s, Medved and Marshall, 1983; 0.59 BL/s, 

 Grubbs, 2001) are comparable with laboratory swimming 

 speeds used to estimate the ACT (mean 0.55 BL/s; Dowd 

 et al, 2006). 



The effects of temperature on metabolism were not 

 important in the error analyses, but two points mer- 

 it consideration. Seasonal (e.g., winter vs. summer) 

 metabolic rate Qjg may be lower than Qjq in response 

 to acute temperature changes (Carlson and Parsons, 

 1999); future studies should address this possibility in 

 sandbar sharks. The averaging of surface and bottom 

 water temperatures in the model potentially obfuscated 

 short-term changes in metabolic rate caused by sharks 

 crossing the thermocline. Energetic implications of such 

 short-term movements could be investigated with more 

 detailed spatial models, but such an approach lies out- 

 side the scope of the present study. 



Uncertainty in the fecal waste parameter accounted 

 for a large portion of the variance in the stochastic 

 model outputs, indicating that F should be investigated 

 in sandbar sharks to refine the bioenergetics model. 

 The effects of the slow gastric evacuation rate of the 

 sandbar shark on the magnitude of the waste and SDA 

 parameters are unknown. 



One of the implicit assumptions of our model is that 

 all energy spent is derived from food. Because juvenile 

 sandbar sharks in the Chesapeake Bay nursery appear 

 to grow steadily and rapidly (Sminkey and Musick, 

 1995), the assumption that the vast majority of energy 

 is derived from food and not from energy reserves is 

 probably justified. However, little is known about the 

 feeding habits of sandbar sharks during their seasonal 

 migrations or during their time in the winter nursery. 

 At these times stored energy may play a greater role in 

 the energy budget. Seasonal changes in energy content 

 occur in Atlantic sharpnose sharks (Rhizoprionodon 



^ Huish and Benedict (1977) published their results under 

 the species name for the dusky shark [Carcharhinus obscu- 

 rus), but Grubbs (2001) noted that the size of the animals 

 tracked was smaller than the size at birth for C. obscurus. 

 Misidentification of the congeneric sandbar and dusky sharks 

 is common. 



