352 



Fishery Bulletin 92(2). 1994 



feet of these mortality sources is implicit in the value 

 chosen for k d and the size class range used (k,l). 



Environmental forcing 



Monthly-averaged time series of temperature mea- 

 sured in Galveston Bay (Soniat and Ray, 1985) and 

 Chesapeake Bay (Galtsoff et al., 1947) were used as 

 input to the model (Fig. 2). Each time series is two 

 years in length and each shows temperature trends 

 expected for mid-latitude temperate bays: cool in fall 

 and spring and warm in summer. For a 6-year simu- 

 lation, the 2-year time series shown in Figure 2 was 

 repeated three times. Salinity values were held con- 

 stant throughout the year at 24%c to simplify the dis- 

 crimination between salinity and temperature effects. 

 Monthly-averaged values of food concentration 

 were also input into the model (Fig. 2). However, 



unlike the temperature time series, idealized time 

 series, constructed to illustrate particular types of 

 food availability, were used. This approach was used 

 so that the occurrence and magnitude of features 

 such as the spring and fall phytoplankton blooms 

 could be manipulated (Fig. 2, A and C) or eliminated 

 (Fig. 2B). However, the general characteristics of the 

 idealized food time series are representative of mea- 

 sured values (Soniat et al., 1984; Berg and Newell, 

 1986). The basic idealized food time series consisted 

 of low winter levels (0.5 mg-L" 1 ), higher summer lev- 

 els (0.75 mg-L" 1 ) and still higher values for two 

 months in the spring and fall to simulate spring and 

 fall bloom levels (1.25 mg-L" 1 ). A summary of the 

 environmental conditions used for each simulation 

 is given in Table 2. 



E 



8 



1.4 

 1.2 

 1.0 

 0.8 

 0.6 

 0.4 



B 



40 

 30 

 20 

 10 

 



12 



18 



24 



30 



36 42 



48 



54 



60 



66 



72 



Q. 



E 



30 36 42 

 Julian Month 



Figure 2 



Monthly-averaged time series of temperature and food concentration used as 

 environmental forcing for the oyster population model. (A) Temperatures from 

 Galveston Bay (Soniat and Ray, 1985) and plankton biomass for years having 

 blooms in March-April and August-September. (B) Temperatures from 

 Galveston Bay and plankton biomass for years having one bloom in August- 

 September. (C) Temperatures from Chesapeake Bay with plankton biomass for 

 years having blooms in April-May and September-October. See Table 2 for a 

 description of the food time series used. 



