Stillwell and Kohler: Food habits of Carcharhinus plumbeus off U.S. northeast coast 



147 



ing for temperature (25.0°C) and using a Q 10 of 2.2 

 resulted in a metabolic rate of 160.4 mgOVkg X h. 

 When converted to calories, the routine metabolic ex- 

 penditure is 521.3 cal/kg x h (160.4x3.25), or 12.5 kcal/ 

 kg x d. At this rate, the average sandbar pup (1.7 kg) 

 in this study would require 21.2 kcal/d (12.5x1.7) for 

 metabolic needs. When we consider the 27% of 

 food energy lost through excretion, the daily ration 

 would have to provide 17.1kcal/kg X d (12.5x1.37) or 

 29.0 kcal/d (17.1x1.7) for the average pup. 



Using a caloric value of 1.235 kcal/g for the foods 

 consumed by sandbar pups (Medved et al. 1988), the 

 daily ration required for routine metabolic needs would 

 be 23.4 g/d (29.0/1.235). This is equivalent to 1.38% of 

 the average BW and amounts to 5.0 times the average 

 BW per year. 



Daily growth in weight for sandbar pups was deter- 

 mined to be 1.74 g/d. This was based on the reexami- 

 nation of existing age data and newly collected infor- 

 mation by Casey & Natanson (1992). This amount of 

 daily growth in weight is equivalent to 1.75 kcal/d as- 

 suming an average caloric value of 1.01 kcal/g for shark 

 flesh (Sidwell et al. 1974). Substituting for metabolism 

 and growth in the Winberg equation gives an energy 

 value for food consumed of 31.4 kcal/d (1.37[21.2x 1.75]). 

 Taking the average caloric value of the food eaten 

 to be 1.235 kcal/g, the daily ration would be 25.4 g/d 

 (31.4/1.235) to meet the energy needs for routine meta- 

 bolic expenditure. This is equivalent to 1.49% of the 

 average BW and 5.4 times the average BW per year. 

 Our average estimate of daily ration (1.43% BW) for 

 sandbar pups above is in very close agreement to the 

 1.1% determined by Medved et al. (1988). 



The validity of using the metabolic rate of one spe- 

 cies for another is questionable and is especially so if 

 the original rate is determined by laboratory studies 

 and then extrapolated to studies in the wild. Not only 

 could significant differences exist between species, but 

 routine metabolic energy expended in the wild may be 

 considerably less than that expended under experi- 

 mental conditions. For instance, Medved et al. (1988) 

 have shown that a large proportion of sandbar shark 

 movement in Chincoteague Bay is accomplished by pas- 

 sively drifting with the currents. This behavior is most 

 likely a way to increase energy efficiency, since loco- 

 motion is metabolically costly for fish (Brett & Groves 

 1979). No doubt there are other ways by which sharks 

 conserve energy in the wild. With the limitations im- 

 posed by available data and literature values, we feel 

 our daily ration estimates are reasonable and compa- 

 rable with estimates for large sharks in other studies 

 (Table 6). When reliable information specific to the 

 metabolic rate of juvenile and adult sandbar sharks is 

 available, better estimates may be possible. 



Annual food consumption Estimates of annual food 

 consumption indicated that the pups can ingest 5.2 

 times their average body weight in a year. The com- 

 bined juveniles and adults, by comparison, will con- 

 sume 3 times their average BW annually (Table 6). 

 The gradual reduction in annual food consumption per 

 kilogram of body weight is a normal result of increas- 

 ing size. Brett & Groves (1979) show that at a young 

 stage, the rate at which fish generally accumulate body 

 mass exceeds the metabolic energy needs for mainte- 

 nance. Both rates are relatively high initially, but with 



