Understanding the Success and Failure of Oyster Populations: The Importance of Sampled Variables and Sample Timing

One of the primary obstacles to understanding why some oyster populations are successful and others are not is the complex interaction of environmental variables with oyster physiology and with such population variables as the rates of recruitment and juvenile mortality. A numerical model is useful in investigating how population structure originates out of this complexity. We have monitored a suite of environmental conditions over an environmental gradient to document the importance of short time-scale variations in such variables as food supply, turbidity, and salinity. Then, using a coupled oyster disease population dynamics model, we examine the need for short rime-scale monitoring. We evaluate the usefulness of several measures of food supply by comparing field observations and model simulations. Finally, we evaluate the ability of a model to reproduce field observations that derive from a complex interplay of environmental variables and address the problem of the time-history of populations. Our results stress the need to evaluate the complex interactions of environmental variables with a numerical model and, conversely, the need to evaluate the success of modeling against field observations of the results of complex processes. Model simulations of oyster populations only approached field observations when the environmental variables were measured weekly, rather than monthly. Oyster food supply was estimated from measures of total particulate organic matter, phytoplankton biomass estimated from chlorophyll a, and total labile organic matter estimated from a regression between chlorophyll a and total labile carbohydrate, lipid, and protein. Only the third measure provided simulations comparable to field observations. Model simulations also only approached field observations when a multiyear time series was used. The simulations show that the most recent year exerts the strongest influence on oyster population attributes, but that the longer time-history modulates the effect. The results emphasize that year-to-pear changes in environment contribute substantially to observed population attributes and that multiyear environmental time series are important in describing the time-history of relatively long-lived species

Understanding the Success and Failure of Oyster Populations: Periodicities of Perkinsus Marinus, and Oyster Recruitment, Mortality, and Size

Ten-year time series (1992 to 2002) of salinity, Dermo disease, and size-class structure and mortality measured for an eastern oyster (Crassostrea virginica) population at a reef in Bay Tambour, Terrebonne Parish, LA, were analyzed using wavelet techniques to determine dominant frequencies and correlations. Along the Gulf Coast of the United States, Dermo disease (caused by Perkinsus marinus) responds to the El Nino-Southern Oscillation (ENSO) climate signal through its response to salinity. During the La Nina portion of ENSO, decreased rainfall leads to an increase in salinity, which triggers a rise in Dermo disease prevalence and intensity, producing increased oyster mortality. Although disease responds to the 4-y periodicity of ENSO and salinity, the oyster population dynamics do not appear to be controlled by disease at this site. A significant 4-y coherency exists between recruitment and salinity, with recruitment being higher during periods of high salinity. \u27Recruit numbers and submarket numbers also exhibit a strong 4-y periodicity. However, a relationship between the recruit time series and the subsequent change in market-size abundance did not exist. The complexity of postsettlement processes and the extended time over which these processes interact decrease the predictability of the recruit-to-market transition. Even the strong pulse of recruits associated with La Nina and its locally elevated salinities did not result in an exceptional abundance of market oysters. Understanding the environmental and biotic factors that favor the production of large oysters is critical because large oysters not only supply the fishery, but, upon their death, contribute the bulk of the shell required for reef sustainability

Abundance And Vertical Flux Of Pseudo-Nitzschia In The Northern Gulf Of Mexico

Many species of the ubiquitous pennate diatom genus Pseudo-nitzschia have recently been discovered to produce domoic acid, a potent neurotoxin which causes Amnesic Shellfish Poisoning (ASP). Pseudo-nitzschia spp. were extremely abundant (up to 10(8) cells l(-1); present in 67% of 2195 samples) from 1990 to 1994 on the Louisiana and Texas, USA, continental shelves and moderately abundant (up to 10(5) cells l(-1); present in 18% of 192 samples) over oyster beds in the Terrebonne Bay estuary in Louisiana in 1993 and 1994. On the shelf there was a strong seasonal cycle with maxima every spring for 5 yr and sometimes in the fall, which were probably related to river flow, water column stability, and nutrient availability. In contrast, in the estuary there was no apparent seasonal cycle in abundance, but the time series of data is relatively short and the environment highly variable. At one site on the shelf, where sediment traps were deployed from spring to fall and sampled at frequent intervals in both 1990 and 1991, approximately 50% of the Pseudo-nitzschia spp. cells present in the water sank into sediment traps. Pseudo-nitzschia spp. were also abundant in surficial sediments. The species of Pseudo-nitzschia present, during this study were not routinely identified with the methods employed. However, toxin-producing P. multiseries has been identified previously from Galveston Bay, Texas, and cells from a bloom on the shelf in June 1993 were identified by scanning electron microscopy as P. pseudodelicatissima, which is sometimes toxic. Although there have been no known outbreaks of ASP in this area, historical data suggests that Pseudo-nitzschia spp,abundance may have increased on the shelf since the 1950s. It is hypothesized that the increase is due to doubling of the nutrient loading from the Mississippi and Atchafalaya rivers and increased eutrophication on the shelf