Dowd et a\ Consumption rates of Corcharchinus plumbeus in Chesapeake Bay 



335 



Growth (G) Growth (G) is the change in energy stored 

 in biomass and can be subdivided into somatic and 

 reproductive growth outputs. We assumed the latter to 

 be negligible because all the age classes in the sandbar 

 shark bioenergetics model are at least 8 years from 

 the age at maturity (Casey et al., 1985; Sminkey and 

 Musick, 1995). 



We employed a von Bertalanffy growth equation 

 (Sminkey and Musick, 1995), based on a validated ag- 

 ing technique for sandbar sharks (Branstetter, 1987), to 

 represent the precaudal length (PCL) of sharks of age jy 

 (y=0-5 yr) upon immigration (or birth) on May 15: 



l,„ = l4i-.-^-'-"') 



(5) 



where L^ = 164 cm; 



K = 0.089; and 

 fi, = -3.8 years. 



The PCL at emigration (L^g) was determined by 



(6) 



where p = the proportion of annual growth in PCL that 

 occurs in the Chesapeake Bay nursery. 



Analysis of vertebral rings indicates that annual growth 

 of juvenile sandbar sharks occurs in two distinct phases: 

 one period of rapid growth in the summer nurseries 

 during which the sharks achieve roughly 75% of their 

 annual growth in length, followed by a period of reduced 

 somatic growth during the winter (Sminkey and Musick, 

 1995). Therefore, we assumed a p of 0.75 as an initial 

 estimate. Limited tag-return data support this seasonal 

 growth pattern. One juvenile (67 cm total length [TL] at 

 tagging) was recaptured 0.5 km from the tagging loca- 

 tion within the summer nursery in September 1998 by 

 VIMS scientists; it had grown 3 cm TL after 44 days at 

 liberty. Similarly, a juvenile sandbar shark of similar 

 size that had been tagged and recaptured by NMFS 

 scientists grew 3 cm in fork length (FL) (48-51 cm FL) 

 over 62 days at liberty between mid-July and mid-Sep- 

 tember (Casey et al., 1985). In Delaware Bay, two sand- 

 bar sharks recaptured during the same summer grew 

 3 cm FL (45 cm flat tagging and 1 cm FL) (no size given) 

 in 40 and 47 days at liberty, respectively (Merson and 

 Pratt, 2001). In comparison, another juvenile (66 cm TL) 

 was tagged in Chesapeake Bay in September 1995 and 

 recaptured by VIMS scientists during the subsequent 

 immigration period. This shark was at liberty for 225 

 days and grew only 3.5 cm TL during that time. 



Both Medved et al. (1988) and Kohler et al. (1995) 

 published equations relating mass to length for sandbar 

 sharks. Because preliminary runs of the model dem- 

 onstrated that these length-mass relationships yielded 

 very similar results, we used the equation produced by 

 Kohler et al. (1995) because it was derived from a larger 

 number of individuals: 



Fork length (FL) is in centimeters and mass (M) is in 

 grams. Lengths were converted from PCL to FL and vice 

 versa by using the regression (VIMS^): 



FL = 1.0791 PCL + 2.78. (« = 4385; r2 = 0.99) 



(8) 



Specific growth rate (grams added per gram of body 

 mass per day) was modeled by assuming that the mass 

 of the shark increased by a constant proportion (x) in 

 each of the n days of the simulation: 



0=1 



(9) 



M = 0.0109 FL3 oi24_ 



(7) 



M,j is the mass of the shark at the beginning of day D. 

 No data exist to support an alternative pattern (e.g., 

 growth varying with temperature or dissolved oxygen 

 levels). 



The mass of the shark on the first and last day (My 

 and Ml,-, respectively) of the simulated nursery season 

 was determined by using Equations 5-8. Fitted val- 

 ues for X in Equation 9 were on the order of 0.1-0.5% 

 increases in mass per day. We used these values to 

 calculate daily growth increments in grams per day 

 and then multiplied by 5400 J/g of body mass (Cortes 

 and Gruber, 1990; Lowe, 2002) to determine the daily 

 increase in energy content. 



Waste loss in feces (f ) and excretions (L/) A generally 

 accepted value for total waste loss to excretions and 

 fecal waste for carnivorous fishes and elasmobranchs is 

 27 ±3% of consumed energy (C) (Brett and Groves, 1979; 

 e.g., Sundstrom and Gruber, 1998; Lowe, 2002; Schindler 

 et al., 2002). This value was assumed for the sandbar 

 shark in the present study, divided into F=0.20C and 

 t/=0.07C. Juvenile A^. brevirostris have fecal waste losses 

 between 38.1% and 16.9% (Wetherbee and Gruber, 1993), 

 and excretory losses average 7% of ingested energy for a 

 number of teleosts (Brett and Groves, 1979). 



Water temperature data Surface and bottom water 

 temperatures were obtained from the Chesapeake Bay 

 Program's water quality database- for seven monitoring 

 stations within the core sandbar shark nursery area in 

 Chesapeake Bay for 1996-2002. Temperature measure- 

 ments were averaged over all stations and over all years 

 for each day of the simulation. The surface and bottom 

 temperature readings were also averaged to obtain a 

 mean water temperature for each day of the simulation 

 in an average year. The simulated temperatures ranged 

 from 16.8° to 27.9°C over the summer nursery season 

 (mean 23.0' ±0.2°C). 



Diet composition data Recent data detail the ontoge- 

 netic patterns of juvenile sandbar shark diet composition 

 in and around Chesapeake Bay for sharks captured with 

 longline and gillnet gears (Ellis, 2003). Diet data are 

 represented by the index of relative importance. Index 

 of relative importance combines the frequency, weight, 

 and number of each prey type and is considered to have 



