Megan K. La Peyre and Jerome F. La Peyre
Louisiana remains one of the largest U.S. suppliers of oysters, producing, on average, more than 34 percent of U.S. landings, a value of more than $68 million in 2016, according to the Louisiana Department of Wildlife and Fisheries. The oyster industry represents an integral aspect of Louisiana culture and coastal ecosystem. The reefs formed by oysters also provide valuable fish habitat, protect shorelines from erosion and may help in improving water quality.
With multiple natural and human-made disasters, combined with ongoing large-scale re-engineering of coastal Louisiana, oyster populations and the industry dependent on them have faced dramatic fluctuations. While Gulf Coast oyster populations are known for their high ability to settle as larvae onto the substrate (recruitment) and rapid growth rates, often reaching market size within less than 18 months, variations in growth and mortality can have significant effects on oyster production. Understanding the growth and mortality rates of oysters and how they change with environmental conditions are critically important in ensuring the long-term sustainability of oyster populations and are necessary for developing realistic management models and predicting future restoration and climate-related effects.
For the past 15 years, research conducted through the LSU AgCenter School of Renewable Natural Resources and School of Animal Sciences has quantified the effects of changing coastal conditions on eastern oyster growth and mortality. Measuring growth and mortality rates of individually tagged oysters either placed in the field or exposed to controlled changes in salinity and temperature in the laboratory has demonstrated that Louisiana oysters are most sensitive to lowered salinities when water temperatures are high – above 86 degrees Fahrenheit. This response varies over an oyster’s life. Newly recruited oysters less than three months old were more tolerant of low-salinity and high-temperature conditions, while larger market-size oysters experienced significant mortality at high temperatures and even greater mortality when low salinity levels coincided with the high temperatures.
Given that Louisiana estuaries normally experience high variations in salinity throughout the year, the researchers further examined how exposure to short versus long periods of lowered salinity might affect oyster growth and mortality. Overall, Louisiana oysters were relatively tolerant to short low salinity events, particularly during spring and winter time periods, although growth rates were decreased.
Explicitly examining growth and mortality using Louisiana oyster populations provides critical data because these populations appear to be more tolerant of low salinity and high temperature compared to Atlantic Coast populations of the same species. Recent analysis of 40 years of Louisiana Department of Wildlife and Fisheries monitoring data from public oyster reefs, with collaborators at the University of New Orleans, the School of Renewable Natural Resources and LDWF, showed that the optimal combinations of temperature and salinity where Louisiana oysters experience maximum net growth are skewed toward lower salinities and higher water temperatures that were used in oyster models. Outside of that optimal range, oysters experience increased mortality and reduced growth.
Having a realistic understanding of how Louisiana oyster populations respond to environmental conditions enables development of models to predict the effects of harvest, restoration and climate on riverine inflow and weather events. Most recently, these data were used to develop an energetic budget model of the eastern oyster that incorporated salinity for the first time. This model may now be used to assess potential management, restoration and climate scenarios on oyster growth within coastal Louisiana, providing a useful tool to inform decisions.
Current work is examining the reproductive stage of oysters. Reproductive capacity and recruitment of oysters remains the missing piece of the puzzle. School of Renewable Natural Resources researchers are examining oyster recruitment, survival and growth at different water depths within southeast Louisiana waters. Data from this project will provide insight into how changes in hydrology (salinity, water depth, turbidity) may influence oyster recruitment and their subsequent survival and growth to adult size. This work, combined with efforts to assess reef reproductive capacity, will provide some means to fully understand and model the effects of changing environmental conditions on oyster population sustainability.
These data help refine the growth and mortality models used to ensure sustainable reefs for long-term harvest and ecological function, benefiting not only the oyster and its associated ecological role but also the oyster industry.
Megan K. La Peyre is with the U.S. Geological Survey, Louisiana Fish and Wildlife Cooperative Research Unit and is an adjunct professor in the School of Renewable Natural Resources; and Jerome F. La Peyre is a professor in the School of Animal Sciences.
(This article appears in the fall 2018 issue of Louisiana Agriculture.)
A Louisiana oyster. Photo by Kevin Savoie