Powell et al.: Modeling oyster populations 



357 



Furthermore, as mortality extends into lower size 

 classes, the size-frequency distribution shifts to 

 larger sizes (Figs. 6C, 7C, 8C). The effect is signifi- 



es 100000 



Number of Individuals 

 Mortality (no/month) 



• I. i. JV. 



A/*Ji 



6 12 18 24 



h 



/\ 

 n 

 / i 



A . 



30 36 42 

 Julian Month 



H 

 |l 



i i 

 J j_i. 



-H 



fi 



i 



i 



A 



8000 



cant because only in cases where mortality is high 

 do oysters grow large enough to reach marketable 

 size for the oyster fishery (size class 6 and larger). 

 Removal of smaller individuals increases the 

 available food supply for the survivors, 

 thereby allowing some to attain market-size. 



6000 



c 

 3 



4000 ™ 

 00 



2000 



48 54 60 66 72 



20000 



10000- 



2 3 4 5 

 Julian Year 



23456789 10 

 Size Class 



Figure 5 



Simulated time development and population distribution of a 

 Galveston Bay Crassostrea virginica population exposed to a 

 continuous mortality rate of 99.9% per year on size classes 5 

 and larger. (A) Monthly-averaged values of the number of in- 

 dividuals, the number of adults (j=4, 10), and the monthly re- 

 productive effort in kcal for the 6-year simulation. (B) The 

 yearly reproductive effort (number of kcal spawned). (C) The 

 final size class distribution in the population at day 2,160. 

 Further explanation in Figure 2 and Table 2, case 6. 



Effect of food supply 



Interactions between food supply and mor- 

 tality rate are potentially important in de- 

 termining population density and size-fre- 

 quency distribution. In years in which a 

 spring bloom is reduced or fails to occur (Fig. 

 2B), the available food spectrum is shifted 

 in time and total food supply for the year is 

 reduced. In Figure 9, we examine the effect 

 of the failure of the spring bloom. Figure 9 

 can be compared directly with Figure 7, the 

 two differing only in food supply. A failed 

 spring bloom shifts the food spectrum as 

 well as decreasing the total food available 

 over the year. 



Hofmann et al. (1992) showed that the 

 food supply time series used for Figure 9 

 results in a strong fall spawning pulse. With 

 an imposed yearly mortality of 99.9% in size 

 classes 3 and larger and no spring bloom, 

 the simulated oyster populations (Fig. 9) are 

 not substantially different from those shown 

 in Figures 6—8. However the simulated oyster 

 population shown in Figure 9 is characterized 

 by a stronger fall spawning pulse, as expected, 

 whereas the previous simulations generally 

 had spawning more evenly distributed over 

 the spawning season. The population still 

 reaches a stable distribution and the size-fre- 

 quency distribution includes individuals in the 

 larger size classes (Fig. 9C). Thus, continuous 

 yearly mortality overrides the effects of varia- 

 tions in the timing of food supply. 



Figure 6 



Simulated time development and population dis- 

 tribution of a Galveston Bay Crassostrea 

 virginica population exposed to a continuous 

 mortality rate of 99% per year restricted to size 

 classes 3 and larger. (A) Monthly-averaged val- 

 ues of the number of individuals, the number of 

 adults (7=4, 10), and the monthly reproductive 

 effort in kcal for the 6-year simulation. (B) The 

 yearly reproductive effort (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 8. 



