Finding the Sweet Spot: Studying Oyster Habitat Suitability in the Pontchartrain Basin

09.06.2017 | In Science, Wildlife & Birds
By Michael Hopkins, Ph.D., Coastal Sustainability Program, Lake Pontchartrain Basin Foundation

Eastern oysters (Crassostrea virginica) have been cultivated in Louisiana’s coastal waters since the mid-1800s. For the last 35 years, the industry has produced more oysters in Louisiana than any other state. In Louisiana, the oyster industry is smaller in size than the pogy or shrimp fisheries but is similar in value to crawfish or alligator harvests. 

Understanding oyster habitat dynamics over space and time in the Pontchartrain Basin is important because of the cultural and commercial significance as well as the inevitable adaptation to coastal change. Furthermore, unharvested oyster reefs are an integral part of the coastal landscape and can act as living shorelines that help stabilize shoreline erosion. Sustainable oyster reefs help maintain the integrity of coastal features like the Biloxi Marsh, which is a buffer against storm surge from tropical systems. 

The best places for oysters

The Lake Pontchartrain Basin Foundation (LPBF) has been studying oyster habitat suitability dynamics throughout the Pontchartrain Basin since 2016. That year, LPBF released a report on oyster suitability for 2013-2015 that analyzed salinity data, which is gathered biweekly for LPBF’s hydrocoast maps. This data was used to determine where the best areas for oyster habitat were located in the basin. 

Map of oyster suitability in the Pontchartrain Basin. Suitability classes indicate where salinity is favorable (green) and unfavorable (red) for oysters.

In July 2017, LPBF released a new oyster suitability report, which serves as an update to the previous report and includes salinity data from 2016, as well as 2013-2015, to better understand spatial and temporal changes in oyster suitability over the past four years.

Map of Soniat class 1 polygon outlines for the years 2013-2016. Polygon outlines are the most suitable areas (Soniat class 1) that are only located within the Biloxi Marsh. These outlines illustrate how the most suitable areas change year to year.

In general, findings show that 2016 was not too dissimilar from previous years. Overall, the most suitable areas on private leases were located within the Biloxi Marsh. One striking difference, however, is the movement of the most suitable areas toward the southeast. From 2013 to 2015, the most suitable areas progressively moved toward the northwest by several miles each year. However, in 2016, the most suitable areas were located farther east than in 2013. Several hydrological events may be partly responsible for the observed change, including the opening of the Bonnet Carré Spillway in January 2016 and the catastrophic floods north of Lake Pontchartrain in March and August of 2016.

Where oysters are harvested may be different

Oyster boat in-transit with cultch barge in the Biloxi Marsh. Photograph courtesy of Pat Quigley, Gulf Coast Air Photo.

New to the 2017 report is an analysis of oyster harvesting boats, which LPBF periodically counts in the Pontchartrain Basin. This data reveals that some areas, which experience optimal salinity conditions, appear to be underutilized by oyster harvesters. In other words, in places where salinity is optimal, very few or no boats harvested those areas in 2016.

*Density of boats that were actively harvesting oysters on private leases in 2016. ** Numbers are given in units of area of water.

It is likely that several factors aside from salinity, such as substrate type and hypoxia, do place a limit on the true extent of oyster habitat suitability in the Pontchartrain Basin. In the future, LPBF will continue to analyze salinity data to understand oyster habitat dynamics. LPBF is also evaluating options to enhance oyster propagation in the eastern Biloxi Marsh and Chandeleur Sound. The Biloxi Marsh provides buffer from storm surge and is an example of an important coastal component outlined in LPBF’s Multiple Lines of Defense strategy. Robust oyster development here can help stabilize the entire marsh, and, therefore, help provide hurricane surge reduction for the region.

Actively harvesting oyster boats.

Actively harvesting oyster boat

For more information on this report, please contact the Lake Pontchartrain Basin Foundation.