Modelling Oyster Population Response to Variation in Freshwater Input

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oyster fisheries, statistical models, estuarine circulation, river flow, predation, mortality rates, growth rates, Gulf of Mexico

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This paper describes the linkage of a three-dimensional hydrodynamic circulation model with descriptive and experimental biological data concerning oyster (Crassostrea virginica) population dynamics in the Apalachicola Estuary (Florida, U.S.A.). Our intent was to determine the direct and indirect role of Apalachicola River flow in the maintenance of oyster production. Results of a monthly field sampling programme conducted on the oyster reefs in the Apalachicola system during 1985–1986 were used to develop statistical models relating several life-history characteristics of oysters to physical-chemical aspects of water quality. The same life-history characteristics were related statistically to output from a circulation model of Apalachicola Bay. Highest oyster densities and overall bar growth were found in the vicinity of the confluence of high salinity water moving westwards from St George Sound and river-dominated (low salinity) water moving south and eastwards from East Bay. With the exception of models for oyster mortality, the predictive capability of results from the parallel modelling efforts was low. A time-averaged model was developed for oyster mortality during the summer of 1985 by running a regression analysis with averaged predictors derived from the hydrodynamic model and observed (experimental) mortality rates throughout the estuary. A geographic information system was then used to depict the results spatially and to compare the extent of expected mortality in 1985 and 1986. High salinity, relatively low-velocity current patterns, and the proximity of a given oyster bar to entry points of saline Gulf water into the bay were important factors that contribute to increased oyster mortality. Mortality was a major determinant of oyster production in the Apalachicola Estuary with predation as a significant aspect of such mortality. By influencing salinity levels and current patterns throughout the bay, the Apalachicola River was important in controlling such mortality. Oyster production rates in the Apalachicola system depend on a combination of variables that are directly and indirectly associated with freshwater input as modified by wind, tidal factors, and the physiography of the bay. River flow reduction, whether through naturally occurring droughts, through increased upstream anthropogenous (consumptive) water use, or a combination of the two, could have serious adverse consequences for oyster populations. By coupling hydrodynamic modelling with descriptive and experimental biological data, we were able to determine the effects of potential freshwater diversions on oyster production in Apalachicola Bay.

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Citation / Publisher Attribution

Estuarine, Coastal and Shelf Science, v. 50, issue 5, p. 655-672