Population dynamics of the eastern oyster in the northern Gulf of Mexico

This project examined the economically and ecologically valuable eastern oyster (Crassostrea virginica) in the Northern Gulf of Mexico using field and laboratory experiments. Specifically, this project focused on natural rates of settlement, growth, and mortality in a variety of temperature and salinity regimes within Breton Sound, LA. Seed and market sized oysters in cages resting on the bottom, as well as settlement tiles, were monitored at four sites in Breton Sound, LA, along what is typically a salinity gradient ranging from ~5 to ~20. In April 2010, the Caernarvon Freshwater Diversion was fully opened with the goal of minimizing the impacts of the Deepwater Horizon oil spill on wetlands, resulting in extreme low salinity (\u3c 2) at all sites through August 2010. High seed and market-sized mortality and reduced condition were observed in oysters at all sites. Perkinsus marinus infection prevalence in surviving market oysters was low at all sites and all infection intensities were light. Settlement only occurred at the highest salinity site. In May 2011, the Mississippi River flooded to record levels, resulting in low salinity (\u3c10) at all sites through June 2011. This short period of low salinity minimized disease infection intensity and settlement at all sites. Oysters at the lowest salinity site experienced high mortality and low growth. Oysters at higher salinity sites experienced limited mortality, mostly from predation, along with higher growth and condition. While low salinity may be beneficial to oyster populations by reducing P. marinus infection levels, prolonged extreme low salinity through spring and summer appears to cause heavy mortality and negatively impact recruitment in the short term. While not only is it clear that the timing and duration of freshwater inputs will significantly affect the impacts on oyster resources, it is likely that large scale global climate patterns (including El Niño and La Niña events) will also modify how and to what extent freshwater events may affect Breton Sound. This project highlights the importance of understanding the timing and duration of low salinity events and their impacts on oyster life history stages

A Shell-Neutral Modeling Approach Yields Sustainable Oyster Harvest Estimates: A Retrospective Analysis of the Louisiana State Primary Seed Grounds

A numerical model is presented that defines a sustainability criterion as no net loss of shell, and calculates a sustainable harvest of seed (\u3c75 mm) and sack or market oysters (\u3e= 75 mm). Stock assessments of the Primary State Seed Grounds conducted east of the Mississippi from 2009 to 2011 show a general trend toward decreasing abundance of sack and seed oysters. Retrospective simulations provide estimates of annual sustainable harvests. Comparisons of simulated sustainable harvests with actual harvests show a trend toward unsustainable harvests toward the end of the time series. Stock assessments combined with shell-neutral models can be used to estimate sustainable harvest and manage cultch through shell planting when actual harvest exceeds sustainable harvest. For exclusive restoration efforts (no fishing allowed), the model provides a metric for restoration success namely, shell accretion. Oyster fisheries that remove shell versus reef restorations that promote shell accretion, although divergent in their goals, are convergent in their management; both require vigilant attention to shell budgets

Differences in extreme low salinity timing and duration differentially affect eastern oyster (Crassostrea virginica) size class growth and mortality in Breton Sound, LA

Understanding how different life history stages are impacted by extreme or stochastic environmental variation is critical for predicting and modeling organism population dynamics. This project examined recruitment, growth, and mortality of seed (25-75mm) and market (\u3e75mm) sized oysters along a salinity gradient over two years in Breton Sound, LA. In April 2010, management responses to the Deepwater Horizon oil spill resulted in extreme low salinity (\u3c5) at all sites through August 2010; in 2011, a 100-year Mississippi River flood event resulted in low salinity in late spring. Extended low salinity (\u3c5) during hot summer months (\u3e25°C) significantly and negatively impacted oyster recruitment, survival and growth in 2010, while low salinity (\u3c5) for a shorter period that did not extend into July (\u3c25°C) in 2011 had minimal impacts on oyster growth and mortality. In 2011, recruitment was limited, which may be due to a combination of low spring time salinities, high 2010 oyster mortality, minimal 2010 recruitment, cumulative effects from 10 years of declining oyster stock in the area, and poor cultch quality. In both 2010 and 2011, Perkinsus marinus infection prevalence remained low throughout the year at all sites and almost all infection intensities were light. Oyster plasma osmolality failed to match surrounding low salinity waters in 2010, while oysters appeared to osmoconform throughout 2011 indicating that the high mortality in 2010 may be due to extended valve closing and resulting starvation or asphyxiation in response to the combination of low salinity during high temperatures (\u3e25°C). With increasing management of our freshwater inputs to estuaries combined with predicted climate changes, how extreme events affect different life history stages is key to understanding variation in population demographics of commercially important species and predicting future populations. © 2013