Powell et al.: Modeling oyster populations 



359 



of days (180) with and without mortality. Regard- 

 less of the mortality rate, when mortality is re- 

 stricted to size classes 5 and larger, populations 



Number of Individuals —  

 Mortality (no/month) - - 



Spawn (kcal) 

 Number of Adults 



30 36 42 

 Julian Month 



3000 



2000 



1000 "> 



10000 



8000 



B 



IlJ 



12 3 4 5 

 Julian Year 



2345678910 

 Size Class 



Figure 8 



Simulated time development and population distribution of a 

 Galveston Bay Crassostrea virginica population exposed to a 

 continuous mortality rate of 99% imposed on all size classes. 

 (A) Monthly averaged values of the number of individuals, the 

 number of adults (j=4, 10), and the monthly reproductive ef- 

 fort in kcal for the 6-year simulation. (B) The yearly reproduc- 

 tive effort (number of kcal spawned). (C) The final size class 

 distribution in the population at day 2,160. Further informa- 

 tion in Figure 3 and Table 2, case 10. 



30 36 42 

 Julian Month 



3000 



suffer a greater reduction in density when mortal- 

 ity is restricted to the summer (compare Fig. 11B 

 and 12B; Table 6). Summer mortality depresses re- 

 productive effort and depressed reproductive 

 effort, continued over time, results in lower 

 population density 



Examining the population size-frequency 

 distribution over the year for simulated oys- 

 ter populations suffering winter (Fig. 13, A 

 and B) and summer (Fig. 13, C and D) mor- 

 tality suggests an explanation for the more 

 detrimental effect of summer mortality on 

 population density. Figure 13 shows snap- 

 shots of the population's size-frequency dis- 

 tribution at various times during the year. 

 When mortality is imposed only during the 

 winter, the population size-frequency distri- 

 bution shifts to larger size classes in the 

 summer in response to increased growth 

 rate produced by warmer temperatures. 

 Therefore, during the fall spawning season 

 the population is dominated by the larger 

 size classes that account for much of the 

 reproductive effort. Winter mortality then 

 shifts the population size-frequency distribu- 

 tion back to smaller individuals (Fig. 13B) 

 and the cycle begins again. Hence, winter 

 mortality allows the population to replace, 

 during the next summer and fall, the indi- 

 viduals that are lost. 



In contrast, restricting mortality to sum- 

 mer months produces a population size-fre- 

 quency distribution that varies little over a 

 year (Fig. 13, C and D). The variation that 

 does occur is a shift towards smaller indi- 

 viduals in the summer. For example, more 

 individuals are found in size classes 6 and 

 7 in September in populations that experi- 



- 2000 



- 1000 



10000 



8000 



6000 



5 4000 



2000 



B 



ll.... 



12 3 4 5 6 

 Julian Year 



2345678910 

 Size Class 



Figure 9 



Simulated time development and population dis- 

 tribution of a Galveston Bay Crassostrea vir- 

 ginica population exposed to a continuous mor- 

 tality rate of 99.9% restricted to size classes 3 

 and larger and in which the food time series con- 

 tained only the fall bloom. The case is compa- 

 rable to Figure 7 in which two blooms occurred. 

 (A) Monthly averaged values of the number of 

 individuals, the number of adults (j=4, 10), and 

 the monthly reproductive effort in kcal for the 

 6-year simulation. (B) The yearly reproductive ef- 

 fort (number of kcal spawned). (C) The final size 

 class distribution in the population at day 2,160. 

 Further information in Figure 3 and Table 2, 

 case 14. 



