Oyster Biology and Habitat Suitability Analysis

Louisiana’s oyster industry has contributed to over 20% of the nation’s catch in pounds for 75% of years since 1950. It has long been a lucrative industry, rooted in culture, and supporting local families and businesses. The ongoing losses to Louisiana’s coastline necessitate manmade interventions that come in a variety of forms; diversions, dredging, shoreline armoring and reef building to name a few. These changes also affect the aquatic creatures that live in our estuary. The ones that can swim often migrate to areas to evade these coastal changes, but oysters are anchored to the bottom and unable to escape the effects of environmental change.

Wisely, oystermen often have multiple oyster leases, because they know that oyster productivity can shift around the coast. In addition, they have to enhance their reefs with cultch (rock/shell) for spat (baby oysters) to settle upon. Cultch is expensive so profitable cultch placement is important, and if the environment is inhospitable they risk financial losses if there are no oysters to harvest. Alternately, if the placement is in suitable habitat, they can turn a profit. Much like a doctor relies upon the expertise of nurses to assist in delivery of the best medical care, oystermen and management agencies can rely on scientists to assist them in making decisions about oyster reefs. Fishermen and wildlife management agencies need a useful tool to visualize the changes in oyster habitat suitability. Modeling the potential effects of these habitat changes is achieved using a Habitat Suitability Analysis (HSI). A HSI combines several habitat components that are fundamental to the species’ survival to produce a range of numeric values from 0.0 to 1.0 that characterize the suitability of the habitat for the organism of interest, in this case oysters. This modeling can assist in decisions regarding restoration efforts, fishery seasons, and bedding area choices increasing successful harvests and minimalizing financial risk.

The eastern oyster is familiar to the communities of the upper Gulf of Mexico. Oyster reefs can reduce coastal erosion, and provide habitat not only for themselves but, also for other aquatic animals. They are bottom dwelling filter feeders, requiring areas that have adequate water flow to provide food. Ideally, oysters need a clean, hard substrate to anchor upon so they don’t suffocate under sediment (although some do survive by growing vertically – these are known as coon oysters).  They also need oxygenated water, so the Gulf of Mexico seasonal hypoxic zone is a threat. Like many aquatic creatures salinity and temperature are the main driving factors for oyster survival and reproduction. Spawning season for Louisiana’s oysters is from April to September (warm months) and salinities between 18 and 22 are ideal to initiate reproduction. However, high salinity brings exposure to diseases such as “dermo” caused by a protozoan parasite, and predation by oyster drills. Therefore, fresh water fluxes are essential to oyster survival because oyster drills and dermo are not a threat in salinities below 15, yet too much fresh water can also be fatal – and that can be amplified by warmer temperatures. To put it briefly, there is no one perfect salinity. The oysters’ needs are well matched to the fluctuations of our dynamic estuary.

Modeling long term suitability can visualize changes in the estuary that will affect oysters during events such as spillway openings, storms and droughts. There have been several approaches that include dissolved oxygen, turbidity, substrate, temperature and salinity regimes but any analysis is only as good as the data that is available. Currently, surface salinity data is the most robust dataset available for the Pontchartrain Estuary and is widely used in oyster HSI modeling. However, differences in top and bottom salinity are common in both Chandeleur and Breton Sounds, and recording bottom salinity would create more accurate model results for those areas. Changes in our basin that affect the patterns of freshwater flow can be seen on the LPBF Hydrocoast maps. The Hydrocoast maps are a continuous set of snap shots of coastal salinity and over time, they show the long-term trends important for oyster propagation.  Therefore, the Hydrocoast maps provide the raw data for the HSI analysis.

For restorative efforts, it simplifies the model to remove the substrate factor as cultch is expected to be provided, and for existing reefs it is already in place. Turbidity data (a measurement of suspended particles) is not currently available at an adequate resolution for the entire estuary and therefore can’t be added into the model. This leaves temperature, salinity, and dissolved oxygen.

Below, the change in suitable habitat for oysters (green) can be seen moving down estuary towards the Gulf of Mexico during the operation of the Bonnet Carré Spillway in 2016 versus 2015, which was a non-spillway year characterized by drought, Tropical Storm Bill and coastal flooding that may have pushed higher salinity waters into the marsh.

Figure 1. 2015 Pontchartrain Basin Oyster Habitat Suitability Analysis in a Bonnet Carre Spillway non-operation year (green: good to red: unsuitable).

Figure 2. 2016 Pontchartrain Basin Oyster Habitat Suitability Analysis in a Bonnet Carré Spillway operation year (green: good to red: unsuitable).

Hypoxia surveys are done multiple times per year and their shapes (blue) can be overlaid onto salinity and temperature based HSI maps to show which areas are likely to be affected and unsuitable.

Figure 3 2016 Pontchartrain Basin Oyster Habitat Suitability Analysis in a Bonnet Carré Spillway operation year (green: good to red: unsuitable) with blue hypoxia overlay.

With climate change becoming more evident every day as the Louisiana coastline erodes there is a strong push to preserve the marshes, bayous and swamps that are paramount to our communities, and ways of life. Modeling can assist us in making better decisions when these projects come to the table enabling us to more effectively manage our natural resources and fisheries, and to adapt to the resultant changes.