350 



Fishery Bulletin 92(2), 1994 



tion for filtration rate to oysters (Powell et al., 1992) 

 to obtain filtration rate as a function of biomass and 

 temperature (7): 



and 



^•0.96/^0.95 



FR, = —!- 



1 2.95 



^. =W 317 10 0.66 9> 



(4) 



(5) 



where filtration rate (FR) is in mL filtered ind : 

 min -1 and W- is the ash-free dry weight in g for each 

 size class. Equation 4 contains the temperature- 

 dependency described by Loosanoff (1958). 



Filtration rate was further modified by salinity as 

 described by Loosanoff (1958). Filtration rate de- 

 creases as salinity drops below 7.5%* and ceases at 

 3.5%*. In mathematical terms: 



at S > 7.5%* 



FR„,=FR, 



"SJ * "V 



at 3.5 < S < 7.5%c FR„ =FR,(S- 3.5)/ 4.0 



(6) 



at S < 3.5%o 



FR„=0. 



where S is ambient salinity and FR- is the filtration 

 rate obtained from Equation 4. 



The reduction in feeding efficiency at high particu- 

 late loads was included as a reduction in filtration 

 rate according to Loosanoff and Tommers (1948) 



r = (4.17xl(T 4 )(10 00418x ), 



(7) 



where x is the total particulate content (inorganic + 

 organic) in g-L -1 and x is the percent reduction in 

 filtration rate. 



Solving Equation 7 for the percent reduction in 

 filtration rate gives a modified expression for filtra- 

 tion rate of the form: 



FR V = FR sj 



1 0.01 



flog 



II) 



r + 3.38 



0.04 IS 



(8) 



Equation 8, if applied to total particulate content 

 (inorganic + organic), limits ingestion rate to ap- 

 proximately the maximum value found by Epifanio 

 and Ewart (1977). Therefore, an additional term to 

 lower ingestion efficiency at high food concentrations 

 was not used. 



The effect of oyster density on food availability 

 was parameterized from measurements given in 

 Lund (1957) as 



[k/f -l]e rd + l' 



(9) 



where f is the fractional reduction in food, d is oys- 

 ter density expressed as L filtered hr _1 m" 2 , and 

 f =0.001, an arbitrarily low number conforming to 

 the expectation that food supply is not affected by 

 low oyster density. For the high flow (59 L hr _1 ) con- 

 ditions given in Lund (1957), k = 0.31 and r = 

 1.36X10" 6 . For low flow (12 L hi-- 1 ) conditions, k = 

 0.57 and r = 9.746xl0~ 7 . Food availability at a given 

 oyster density is estimated as ( \-f) times the ambi- 

 ent food concentration. Filtration rate times the 

 ambient available food concentration then gives 

 oyster ingestion. Assimilation is obtained from in- 

 gestion using an assimilation efficiency of 0.75 

 (Powell et al., 1992). 



Respiration 



Oyster respiration as a function of temperature and 

 oyster weight was obtained from Dame (1972) as 



fl, =(69.7 +12.67) W, 



6-1 



(10) 



where R f is in uL 2 consumed hr 1 -g dry wt 1 and 

 b = 0.75. 



Salinity effects on oyster respiration were param- 

 eterized from data given in Shumway and Koehn 

 ( 1982) by obtaining a ratio (R r ) of respiration at 10%* 

 to respiration at 20%c, 



R 



' , and regressing this ratio against tem- 



R 



R 



20%r perature. This yielded two equations: 



at T < 20°C R r = 0.007T + 2.099; 

 atr>20°C R= 0.09157+ 1.324; 



(11) 



which were then used to obtain respiration rate as 

 follows: 



S > 157« 



Rtj =Rj\ 



10%c<S<15%- Rfj =fl,(l + [(15-S)(fl r -l)/5]); (12) 



S < 109k- 



Rj-j = RjR r . 



Shumway and Koehn (1982) identified effects of 

 salinity on respiration at 20%*; however, we used a 

 15%* cutoff to conform to Chanley's (1958) observa- 

 tions on oyster growth. 



Reproduction 



For adult oysters (/=4,10), net production was ap- 

 portioned into growth and reproduction by using a 

 temperature-dependent reproduction efficiency of 

 the form 



R, 



effj 



0.0547-0.729 



(13) 



