ADStr3Ct. — A time-dependent 

 energy-flow model was used to 

 examine how mortality affects oys- 

 ter populations over the latitudi- 

 nal gradient from Galveston Bay, 

 Texas, to Chesapeake Bay, Vir- 

 ginia. Simulations using different 

 mortality rates showed that mor- 

 tality is required for market-site 

 oysters to be a component of the 

 population's size-frequency distri- 

 bution; otherwise a population of 

 stunted individuals results. As 

 mortality extends into the juvenile 

 sizes, the population's size fre- 

 quency shifts toward the larger 

 sizes. In many cases adults in- 

 crease despite a decrease in over- 

 all population abundance. Simula- 

 tions, in which the timing of mor- 

 tality varied, showed that oyster 

 populations are more susceptible 

 to population declines when mor- 

 tality is restricted to the summer 

 months. Much higher rates of win- 

 ter mortality can be sustained. 

 Comparison of simulations of 

 Galveston Bay and Chesapeake 

 Bay showed that oyster popula- 

 tions are more susceptible to in- 

 tense population declines at higher 

 latitudes. The association of popu- 

 lation declines with disease agents 

 causing summer mortality and the 

 increased frequency of long-term 

 declines at high latitudes result 

 from the basic physiology of the 

 oyster and its population dynam- 

 ics cycle. Accordingly, management 

 decisions on size limits, seasons 

 and densities triggering early clo- 

 sure must differ across the latitu- 

 dinal gradient and in populations 

 experiencing different degrees of 

 summer and winter mortality rela- 

 tive to their recruitment rate. 

 More flexible size limits might be 

 an important management tool. 

 When fishing is the primary cause 

 of mortality, populations should be 

 managed more conservatively in 

 the summer. The latitudinal gra- 

 dient in resistance to mortality 

 requires more conservative man- 

 agement at higher latitudes and 

 different management philoso- 

 phies from those used in the Gulf 

 of Mexico. 



Modeling oyster populations. 

 IV: Rates of mortality, population 

 crashes, and management* 



Eric N. Powell 



Department of Oceanography. Texas A&M University 

 College Station. TX 77843 



John M. Klinck 

 Eileen E. Hofmann 



Center for Coastal Physical Oceanogrphy, Crittenton Hall 

 Old Dominican University. Norfolk. VA 23529 



Sammy M. Ray 



Department of Marine Biology, Texas A&M University 

 College Station, TX 77843 



One of the unfortunate character- 

 istics of oyster Crassostrea virgin- 

 ica populations is their susceptibil- 

 ity to periods of heavy mortality, 

 which can extend from a few months 

 to a few years in duration. Oyster 

 population abundances drop precipi- 

 tously during these times and may 

 remain low for extended periods 

 (Schlesselman, 1955; Engle, 1956; 

 Laird, 1961; Engle and Rosenfield, 

 1962). Why populations decline over 

 several years or crash over shorter 

 periods of time can usually be ex- 

 plained by killing floods (Andrews et 

 al., 1959; Soniat and Brody, 1988; 

 Soniat et al., 1989) or disease epi- 

 zootics (Needier and Logie, 1947; 

 Andrews and Hewatt, 1957; Mackin 

 and Hopkins, 1962) although preda- 

 tors and overfishing have occasion- 

 ally received some credit (Moore and 

 Pope, 1910; Menzel et al., 1957; 

 Quastet al., 1988). 



A review of the literature shows 

 that declines and crashes in oyster 

 populations have some interesting 

 characteristics (Mackin and Wray, 

 1950; Mackin et al., 1950; Menzel, 



1950, a and b; Menzel and Hop- 

 kins, 1953; Owen, 1953; Gunter, 

 1955; Mackin and Sparks, 1962; 

 Hofstetter et al., 1965; Copeland 

 and Hoese, 1966; Hofstetter, 1966; 

 Gilmore et al., 1975; and previously 

 cited references): 



1 With the exception of killing 

 floods, the times of the year with 

 the most intense mortality are 

 usually restricted to the summer 

 and early fall and to areas of 

 higher salinity. Warm tempera- 

 tures and high salinities pro- 

 mote the growth of the disease- 

 producing organisms Perkinsus 

 marinus and Haplosporidium 

 nelsoni (Ray and Chandler, 

 1955; Andrews and Hewatt, 

 1957) and predation by such 

 pests as the oyster drill, Thais 

 haemastoma (Garton and Stickle, 

 1980; Stickle, 1985). 



2 Population crashes or significant 

 declines have been documented 

 throughout the oyster's latitudi- 

 nal range. However, except for 

 permanent changes in salinity, 



Manuscript accepted 12 October 1993 

 Fishery Bulletin 92:347-373 (1994) 



Parts I-III have been published in the Journal of Shellfish Research (Part I in 11:387- 

 398; Part III in 11:399-416; Part II is in press). 



347 



