Powell et al.: Modeling oyster populations 



363 



GALVESTON BAY 



■£ 100000 



TO 



.-f 10000 



1000 

 100 



300 600 900 1200 1500 1800 2100 

 TIME (Days) 



Number of Individuals 

 Mortality (no/month) 



8000 



24 30 36 42 

 Julian Month 



15000 



« 10000 



5000 



2 3 4 5 6 

 Julian Year 



1 2345678910 

 Size Class 



Figure 1 1 



Simulated time development and population distribution of a 

 Galveston Bay Crassostrea virginica population exposed to a 

 continuous mortality rate of 99.9% restricted to size classes 5 

 and larger and in which mortality occurred only during the win- 

 ter. Compare Figure 12. (A) The number of individuals per size 

 class and reproductive effort per size class. Isolines, for number 

 of individuals, are the logarithms of the number of oysters 

 (log 10 N). Shading for the amount of reproductive effort (spawn) 

 represents the logarithm of cal (logical). (B) 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. (C) The yearly reproductive effort (number of kcal 

 spawned). (D) The final size class distribution in the population at 

 day 2,160. Further information in Figure 3 and Table 2, case 24. 



radic spawning pulses typically strongest 

 in midsummer. Like Galveston Bay popu- 

 lations, a shift in the timing of the spring 

 and fall blooms has little effect on the sea- 

 sonal changes in size-frequency distribu- 

 tion (Fig. 21) but considerable effect on the 

 resulting population density in some cases. 

 Populations experiencing winter mortality 

 are more affected by variations in the tim- 

 ing of the food supply than populations ex- 

 periencing summer mortality. Unlike 

 Galveston Bay populations, populations 

 experiencing summer mortality have lower 

 population densities than populations ex- 

 periencing winter mortality only when the 

 blooms occur in March/ April and August/ 

 September. Delaying the blooms by one 

 month results in little variation between 

 populations experiencing summer and win- 

 ter mortality. The most significant factor 

 producing differences between the 

 Galveston Bay and Chesapeake Bay popu- 

 lations is the cooler temperatures that 

 characterize Chesapeake Bay. This results 

 in reduced reproductive effort with more 

 net production going to support somatic 

 tissue growth (Table 7). 



Discussion 



The importance of mortality 



Unlike an oyster population, an oyster fish- 

 ery cannot persist without large adult in- 

 dividuals. One of the consistent messages 

 of this modeling exercise is the require- 

 ment of mortality for the population to 

 produce larger, market-size individuals. 

 Either adult or juvenile mortality will suf- 

 fice, as both juveniles and adults compete 

 for food (Powell et al., 1987). Low rates of 

 mortality result in crowding, food limita- 

 tion, and a stunted population. As mortal- 

 ity extends into the juvenile size classes, 

 and finally into the larval stages (modeled 

 as a reduction in recruitment, reduced re- 

 productive effort, or produced by the colder 

 temperatures of Chesapeake Bay) the 

 population on the average becomes skewed 

 more and more towards the larger adult 

 size classes. Frequently, this proportional 

 shift was sufficient to result in an increase 

 in adult density despite an overall lower 

 population density. An even higher rate of 

 mortality reversed this trend; the popula- 



