but it could become a factor in dredged 

 areas (Frankenberg and Westerfield 1968). 



Current Velocity 



A positive effect of current velocity 

 on oyster feeding could be surmised from 

 the fact that oyster reefs tend to grow 

 outward toward the middle and more rapidly 

 flowing portion of a tidal stream. En- 

 hancement of oyster feeding as a function 

 of increase in current velocity was demon- 

 strated by Walne (1972). 



Given the above assumptions, the addi- 

 tional information most important for the 

 calculation of an energy budget for inter- 

 tidal oysters is the size-frequency (or 

 weight-frequency) distribution of reef 

 populations and the effect of weight on 

 the energy budget terms. 



All of the terms in the energy budget 

 equation for oysters are presumably af- 

 fected by the size (or weight) of individ- 

 uals by the following general equation: 



F = a W 



(2) 



where F = 



W = 



a and b = 



in energy 



the process rate 



or matter units 



the biomass of the oyster 



(g or kcal ) 



constants (represent the 



effects of temperature and 



the surface area-to-volume 



ratio, respectively) 



It is generally known that small oys- 

 ters ingest, egest, respire, grow (and 

 die) at higher rates than do large oys- 

 ters, and that these rates increase in all 

 oysters with increased temperature. Un- 

 fortunately, no general agreement exists 

 in the bioenergetics literature concerning 

 units of biomass. Table A-1 lists some 

 conversion factors for oyster biomass that 

 were compiled from various sources. The 

 numbers are only approximate because the 

 allometric relationships can change with 

 gonadal state or with tidal elevation of 

 the population. Dame (1972a) found that 

 intertidal oysters in North Inlet, South 

 Carolina, had a significantly higher ratio 

 of shell weight to dry meat weight than 

 subtidal oysters had. 



Because small oysters process energy 

 at relatively higher rates than large 



ones, it is important to document the size 

 (biomass) frequency of intertidal oysters 

 in the study area. Bahr (1974) separated 

 reef oysters at Doboy Sound, Georgia, into 

 32 size classes at 5-mm intervals (2 to 

 157 mm). He found that the oyster popula- 

 tion in the central (higher) portion of 

 several old reefs typically showed a log 

 normal distribution, especially during the 

 late fall. Oysters in the smallest five 

 size classes (up to 19 mm) dominated the 

 population, and oysters above 100 mm were 

 rare. Dame (1976) reported a similar size- 

 frequency distribution of reef oysters in 

 South Carolina, but with generally lower 

 overall populations and reduced dominance 

 of small size classes. Figures A-3 and 

 A-4 illustrate the temporal changes in 

 size-frequency distributions of reef oys- 

 ters in these two respective studies. 



The equation that describes the size- 

 frequency distribution of reef oysters in 

 Doboy Sound, Georgia (Bahr 1974) is as 

 follows: 



log^Q Y = -0.02 X^. + 2.32 



(3) 



where Y = the number of oysters per 



0.1 



1^ in size class X,- 



X. (i = 2, 7, 12. ..157) = 5-mm 

 ^ size class 



The relation between individual oys- 

 ter size and biomass from Bahr (unpub- 

 lished data) is described by another 

 regression equation as follows: 



0.02 X -1.8 



(4) 



where Y = logjg afdw (g) of total 



oysters including shell, 



X = height of each oyster in mm 



The r of this relationship is 0.84 

 with 78 degrees of freedom. The experi- 

 mental animals were collected at eight 

 different times, including all seasons. 



To simplify the computation of the 

 energy budget of the reef oyster popula- 

 tion. Equations 3 and 4 were used to 

 describe a typical reef oyster population, 

 intermediate in both numbers and biomass. 

 Thus, the numerical dominance of small 

 oysters is offset by the higher biomass of 

 (rare) large oysters, and oysters from 40 

 to 80 mm in height (mean 60 mm, or 0.25 g 

 afdw) are functionally typical (See Figure 



97 



