Powell et al.: Modeling oyster populations 



365 



•S 30 - 



o 20 



10 



■d 30 " 



o 20 



 March 

 B June 

 E3 August 



1 ) B September 



(3 December 



D February 



k 



1 23456789 10 

 Size Class 



were exposed to mortality. In the latter 

 case, lowered recruitment no longer bal- 

 anced the higher rates of mortality. 



In cases where mortality was imposed 

 for time periods of less than one year, 

 mortality restricted to the six summer 

 months (April-September) nearly al- 

 ways resulted in decreased population 

 density compared to mortality restricted 

 to the winter months. Rarely did the two 

 yield similar results. Never did summer 

 mortality have a lesser impact. The ef- 

 fect was noted at different latitudes, in 

 populations having mortality restricted 

 to a variety of differing size classes, and 

 in populations varying in larval sur- 

 vivorship. However, adult mortality was 

 required. Extending mortality into the 

 juvenile size classes minimized the effect. 



Nearly all reports of population 

 crashes in oyster populations result from 

 adult summer mortality, recruitment 

 failure, or floods. Most predators and 

 parasites are most effective in the sum- 

 mer. The series of simulations presented 

 here suggests that the explanation for 

 the importance of adult summer mortal- 

 ity does not necessarily reside in the fact 

 that the most significant agents of adult 

 mortality (except the fishery) operate 

 most effectively in the summer. Al- 

 though this may well be true, the oys- 

 ter itself would appear to be more sus- 

 ceptible to mortality in the summer. 

 That is, a greater chance of population 

 crashes in the summer may be physi- 

 ologically preordained. One potentially 

 important mechanism causing this in- 

 creased susceptibility is the temperature 

 control on the partitioning of somatic 

 tissue and reproductive tissue in the 

 winter, spring, and summer. Fewer in- 

 dividuals are present in the adult size 



Figure 14 



A comparison of the changes in size-fre- 

 quency distribution through the year in 

 simulated Crassostrea virginica populations 

 having size classes 3 and larger exposed to 

 mortality. Compare to Figure 13 where mor- 

 tality was restricted to size classes 5 and 

 larger. (A and B) mortality restricted to the 

 winter; (C and D) mortality restricted to the 

 summer. More information in Figure 3 and 

 Table 2, cases 31 and 32. 



