Date of Award

Spring 1996

Document Type


Degree Name

Doctor of Philosophy (PhD)


Ocean & Earth Sciences

Committee Director

Eileen E. Hofmann

Committee Member

Larry P. Atkinson

Committee Member

John M. Klinck

Committee Member

Roger Mann

Committee Member

Eric N. Powell


Three models were coupled to investigate the effects of changes in environmental conditions on the population structure of the Eastern oyster, Crassostrea virginica. The first model, a time-dependent model of the oyster population as described in Powell et al. (1992) and Hofmann et al. (1992, 1995), tracks the distribution, development, spawning, and mortality of sessile oyster populations. The post-settlement model incorporates mortality through parasitism, predation, food depletion and extremes in environmental conditions. The post-settlement model supplies the initial abundance of larvae spawned into the water column, and in turn is the recipient of cohorts of spawn which survive through metamorphosis. The second model, a time-dependent larval growth model, simulates larval growth and mortality. Larval growth rate, which determines the length of the planktonic period, is defined by food concentration, temperature, salinity and turbidity. The larval model includes the effects of adult density, environmental conditions, predation by zooplankton and reef availability on larval survivorship. The final model, a finite element hydrodynamic model, was developed by the U.S. Army Corps of Engineers to simulate the circulation in Galveston Bay, Texas. This model provides element-averaged flow rates to the post-settlement model and element-averaged salinities to both the post-settlement and larval models. The coupled post-settlement-larval model (the oyster model) runs within each element of the finite element grid with known reef.

The oyster-circulation model was first forced with five years of mean environmental conditions to provide a reference simulation for Galveston Bay. This reference simulation was then used as a baseline for comparison with other simulations, which considered the effects of increases and decreases in freshwater inflow and temperature on the population structure of the oyster. Additional simulations were used to investigate the effects of long term decreases in food concentration and turbidity on Galveston Bay oyster populations.

In general, the simulations suggest that salinity is the primary environmental factor controlling the spatial extent of oyster distribution within the estuary. Simulations also show that discrepancies between broodstock number and larval recruitment in the simulated populations arise because the environmental controls on the build-up of spawning material and the recruitment of larvae to the benthic community differ. Furthermore, simulated patterns of oyster reproduction and larval recruitment indicate that regions with predominantly low salinities require external sources of larvae. Finally, results indicate that the summer months, with higher salinities and warmer temperatures, have high levels of larval survivorship and subsequently the most significant recruitment events.

The results from this study allow predictions of the effects of environmental change on the status of oyster populations, both within Galveston Bay and within other estuarine systems supporting oyster populations, to be made.


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