146 



Fishery Bulletin 9! fl), 1993 



tions do exist, however. These include food consump- 

 tion studies for the lemon shark Negaprion brevirostris 

 by Graeber (1974), Gruber (1982), and Longval et al. 

 (1982); digestion rates in the blue shark Prionace 

 glauca by Tricas (1977); stomach evacuation and food 

 consumption experiments by Jones & Geen (1977) on 

 the spiny dogfish and Cortes & Gruber (1990) on the 

 lemon shark; stomach evacuation rates in captive sand- 

 bar sharks by Wass (1973); and stomach evacuation, 

 food consumption, and daily ration estimates in the 

 sandbar shark by Medved (1985) and Medved et al. 

 (1985, 1988). The data for the last three papers came 

 from experiments conducted on pups and small juve- 

 nile sandbar sharks maintained in a natural enclosure 

 in Chincoteague Bay, Virginia. Knowing the rate of 

 gastric evacuation is necessary for determining the 

 daily ration of a species. A number of factors, both 

 biological and physical, influence the evacuation rate 

 in fish (Langton 1977), but temperature, food type, 

 and predator size appear to have the greatest impact 

 (Windell 1966 and 1968, Pandian 1967, Edwards 1971, 

 Jones & Geen 1977, MacDonald et al. 1982, Medved et 

 al. 1985). Jones & Geen (1977) maintained spiny dog- 

 fish in aquaria at about 10° C and found that 5d were 

 required to evacuate a meal of herring. At the same 

 temperature, mature males required 10 d to evacuate 

 a full stomach of herring, and the time required was 

 probably influenced by the size of the dogfish. Wass 

 (1973) found that sandbar sharks (95-101 cm caudal 

 length) maintained in large experimental ponds (tem- 

 perature unknown) required at least 2-3 d to evacuate 

 their stomachs. In the natural environment around 

 the Hawaiian Islands where surface-water tempera- 

 tures average about 26° C, he suggested that 3-4+ d 

 might be needed, depending on whether the prey was 

 soft or hard and resistant to digestive enzymes. 



Prior to the feeding study, the sharks were starved 

 for 4d. What effect this had on their gastric evacua- 

 tion rate is unknown, but studies conducted on vari- 

 ous species of teleosts starved before feeding resulted 

 in slower evacuation rates (Windell 1967, Elliott 1972, 

 Jones 1974). Medved et al. ( 1985) were able to demon- 

 strate the variability in gastric evacuation rates that 

 can occur between food types. Soft blue crabs and At- 

 lantic menhaden Brevoortia tyrannus required 70.7 and 

 92.7h, respectively, (81.5 h avg.) to be depleted to 98% 

 of their original weight. The difference in depletion 

 time was probably the result of a combination of fac- 

 tors, including a natural lag phase in initial enzyme 

 action on the food items (Jennings 1972) and the resis- 

 tance of the skin and scales of the fish prey to diges- 

 tion (Windell 1967, Western 1971). Jobling (1987) also 

 reported that the different surface-to-volume ratio of 

 the two food types and their differing friability will 

 affect gastric evacuation. In addition, a concentration 



of fat in some fish flesh, such as found in menhaden 

 (Thayer et al. 1973), has been shown to delay gastric 

 evacuation (Quigley & Meschan 1941, Windell 1967, 

 Windell et al. 1969). 



In the present study, two approaches were used to 

 estimate daily ration. The first was by use of a calcu- 

 lated routine metabolic rate, and the second was the 

 basic energy equation of Winberg ( 1956), 



C = 1.37(R+G), 



where C = energy of food consumed, R = total energy 

 of metabolism, G = metabolic energy in terms of growth, 

 and the coefficient 1.37 represents the 27% of food 

 energy lost through excretion (Brett & Groves 1979). 

 This is a more recent and accurate value than the 20% 

 originally proposed by Winberg ( 1956). 



A metabolic rate for the sandbar shark has not been 

 determined. For our purposes, therefore, we assumed 

 that a routine metabolic rate of 49.2 mgOykg x h at 

 10°C for the spiny dogfish (Brett & Blackburn 1978) 

 was appropriate for the sandbar shark. Adjusting for 

 an increase in temperature to 18.5° C (J.C. Casey, 

 Narragansett Lab., NMFS Northeast Fish. Sci. Cent., 

 unpubl. longline data) and using a Q 1(1 of 2.2, we de- 

 rived a metabolic rate of 95.9mg0 2 /kg x h. Using an 

 oxycalorific equivalent of 3.25cal/mgOj cited for fishes 

 (Elliott & Davison 1975), the routine metabolic expen- 

 diture is 311.7 cal/kg x h or 7.48 kcal/kg x d. The 

 average sandbar shark at 34.0 kg BW would thus re- 

 quire 254.3 kcal/d. To compensate for the food energy 

 lost through excretion, the sharks would have to con- 

 sume 10.2 (7.48x1.37) kcal/kg x d. This would raise 

 the total daily average intake to 346.8 kcal/d (10.2 x 

 34.0). If we consider the average caloric value of the 

 foods eaten to be 1.195 kcal/g (Steimle & Terranova 

 1985), the energy intake in terms of food mass amounts 

 to 290.2 g/d (346.8/1.195) or 0.85% of average body 

 weight (BW). Yearly, this amounts to 105.9 kg (3.1 X 

 xBW). 



To employ the Winberg (1956) energy equation, we 

 calculated a value for G based on an average growth 

 in weight estimate of 3.72 g/d for juveniles and adults 

 (Casey & Natanson 1992). Using an average calorific 

 value of shark flesh of 1.01 kcal/g (Sidwell et al. 1974), 

 the daily increase in caloric content due to growth is 

 3. 75 kcal/d (3.72x1.01). Substituting the energy values 

 for metabolism and growth in the equation gives an 

 energy value for food consumed of 353.5 kcal/d 

 (1.37[254.3+3.75]l or 295.8 g/d (food energy = 1.195 kcal/ 

 g). Daily ration then is equal to 0.87% of BW/d and 3.2 

 x BW/yr. 



To estimate daily ration for the sandbar pups from 

 Chincoteague Bay, we assumed the metabolic rate 

 above for the spiny dogfish was appropriate. Correct- 



