Powell et al.: Modeling oyster populations 



355 



Effect of continuous mortality 



The first set of simulations considered the oyster 

 population that would be produced in Galveston Bay, 

 Texas, when continuous mortality (mortality 

 throughout the year) is imposed on size classes 5 

 and larger. Oyster size class 5 approximates the 2.5 

 in size limit often desired by the oyster fishery as 

 opposed to the standard size limit of 3 now enforced 

 in most areas. Over this series of simulations, the 

 rate of yearly mortality was varied from 50% to 

 99.9%, the two extremes being depicted in Figures 

 4 and 5. For an oyster population with no recruit- 

 ment, these rates would result in a reduction of the 



GALVESTON BAY 



i — i — i — i — i — <- 



300 600 900 1200 1500 1800 2100 

 TIME (Days) 



Number of Individuals 



SpaWn (kcal) 

 Number of Adults 



l| 



J L^_l J_A. 



12 18 24 30 36 42 48 54 60 66 72 

 Julian Month 



10000 



8000 



" 6000 



S 4000 



2000 



12 3 4 5 6 

 Julian Year 



8 910 



population by 0.5 and 0.999, respectively, in one 

 year. In our simulations, where recruitment and 

 mortality constantly change population abundance, 

 a 50% mortality rate does not necessarily result in 

 the loss of one-half of the individuals in the popula- 

 tion in one year. 



Over this series of simulations (Table 3, Figs. 4 

 and 5), as mortality rate increases from 50% to 

 99.9%, density declines by about 80% and the size- 

 frequency distribution shifts slightly to lower size 

 classes. Population reproductive effort declines as 

 the number of adults declines, but individual repro- 

 ductive effort increases. At the lower mortality rate, 

 spawning is primarily confined to a single strong 

 pulse in the fall. At the higher mortality 

 rate, spawning effort is distributed between 

 a spring and fall spawning peak; the fall 

 peak is stronger and extends over a longer 

 time (Fig. 4A vs. Fig. 5A). 



Moreover, spawning is higher in every 

 other year (Figs. 4C and 5C). In the tem- 

 perature time series for Galveston Bay (Fig. 



Figure 3 



Simulated time development and population dis- 

 tribution of a Galveston Bay Crassostrea vir- 

 ginica population with no mortality, allowing the 

 population to approach the carrying capacity of 

 the environment. (A) The number of individuals 

 per size class and reproductive effort per size 

 class. Values are plotted opposite the size class 

 designation, not halfway between; hence all in- 

 dividuals in size class 7 are opposite the grid 

 mark labeled 7 on day 1 of this simulation. Iso- 

 lines, for number of individuals, are the loga- 

 rithms of the number of oysters (log 10 N). Hence, 

 the zero contour corresponds to one individual. 

 Population concentrations less than this are in- 

 dicated by dashed lines; population concentra- 

 tions greater than this by solid lines. Shading for 

 the amount of reproductive effort (spawn) repre- 

 sents the logarithm of cal (logical) with the 

 darkest shades corresponding to highest values. 

 Contour interval is 0.5 for the number of indi- 

 viduals and 1.0 for reproductive effort. (B) 

 Monthly-averaged values of the number of indi- 

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

 monthly reproductive effort in kcal for the 6-year 

 simulation. Values can be converted into joules 

 by multiplying by 4.16y'cal _1 ; into biomass by us- 

 ing 6100 cal-g dry wt _1 ; and into the equivalent 

 number of fully developed eggs by 13 ngegg -1 

 X6.133X10" 6 cal-ng -1 . (C) The yearly reproductive 

 effort (number of kcal spawned). (D) The final 

 size class distribution in the population at day 

 2,160. Additional data and explanation in Table 

 2, case 1. 



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