Despite the numerous recognized benefits provided by oysters, reef loss over the past century has been staggering—just 15% of native oyster reef ecosystems are estimated to remain globally, compared to historic levels. Oyster population declines are related to a combination of factors including degradation of water quality and quantity, storms, oil and chemical spills, destructive or unsustainable fishing practices, and disease. Loss of reefs results in the loss of associated ecological and economic benefits provided by oysters. Because free-swimming larval oysters depend on the shells of older generations for attachment and growth, when reef structure is removed, the fundamental building blocks supporting reef sustainability are lost. 

Habitat restoration has become an important tool to ameliorate the effects of degraded coastal ecosystems. For oyster reef restoration efforts to be successful and sustainable, a key first step is selection of suitable sites that support long-term growth and survival of oysters. Site selection is critical as it can greatly influence mortality factors and determine the ultimate success of the restoration project. 

Habitat suitability index (HSI) methods can be used to integrate spatial information and are a common tool used by natural resource managers for habitat mapping, conservation and restoration planning. HSIs can be used to characterize locations based on their potential for successful habitat restoration, reducing uncertainty in the site selection process. HSIs also provide an objective and quantitative tool for evaluating project success (e.g. spat, oyster density) post-construction. 

The aim of this project was to create a standardized HSI framework to guide oyster reef restoration in all Texas bay systems. By integrating bay-wide, long-term data on oyster populations and environmental variables, we have created maps that allow users to prioritize restoration actions based on their potential for success. In addition, we created maps that illustrate area-specific metrics of success (spat, oyster density), in order to more effectively evaluate accomplishment of project goals.

If you want access to the maps scroll down to the last page to download them.

Water quality and oyster data were extracted from the Texas Parks and Wildlife Department (TPWD) long-term Fisheries Independent Monitoring Program for all estuaries in Texas.

We calculated normal conditions as well as long-term variability for each water quality variable and interpolated the values to create spatial representations of salinity, turbidity, dissolved oxygen, and temperature.

Spatial representations of water quality were integrated with oyster presence data to create the Oyster Reef Restoration Habitat Suitability Index, which characterizes conditions based on suitability for oyster reef restoration

The Reef Quality Index (RQI) illustrates area-specific metrics of success (newly settled oyster spat, oyster density), in order to more effectively evaluate the achievement of project goals.

We again used oyster data for all estuaries in Texas, extracted from the TPWD long-term Fisheries Independent Monitoring Program. We created polygons to represent the spatial extent of oyster reefs, and then calculated the average number of live oysters (> 25 mm shell height), dead oysters (> 25 mm), and spat (≤ 25 mm) for each reef polygon. Data were then used to calculate the Reef Quality Index, detailed in   Beseres-Pollack et. al (2011). 

The Oyster Reef Restoration HSI map to the right illustrates conditions based on their favorability for oyster reef restoration, ranked according to long-term water quality in each bay system. Cooler colors represent areas more suitable for oyster reef restoration and warmer colors represent less suitable areas. Practitioners can use these maps to compare and evaluate potential locations for reef restoration to maximize project success.

Notice that the map to the right is an interactive map that can be zoomed in and out on and moved locate an area of interest. Just double click on a location to zoom in on an area or you can click the plus sign on the bottom right-hand corner. You can zoom out by clicking on the minus sign in bottom right-hand corner. You can also pan North, South East and West by by holding down the mouse button and moving you mouse to the desired location. Single click on the Oyster Reef Restoration Habitat Suitability Index to identify what the rank is for a certain location or consult the Legend above.

The Oyster Reef Restoration RQI map to the right illustrates conditions on oyster reefs based on their favorability for oyster reef restoration, ranked according to long-term abundances of live oysters (> 25 mm shell height) and spat (≤ 25 mm shell height). Darker colors represent areas more suitable for oyster reef restoration and lighter colors represent less suitable areas. If practitioners are interested in siting a restoration project adjacent to a natural reef, these maps can be used to compare and evaluate potential locations to maximize project success. 

Notice that the map to the right is an interactive map that can be zoomed in and out on and moved locate an area of interest. Just double click on a location to zoom in on an area or you can click the plus sign on the bottom right-hand corner. You can zoom out by clicking on the minus sign in bottom right-hand corner. You can also pan North, South East and West by holding down the mouse button and moving you mouse to the desired location. Single click on the Oyster Reef Restoration Reef Quality Index to identify what the rank is for a certain location or consult the Legend above. 

Once an oyster reef restoration project has been completed, practitioners can compare the number of spat on their restored reef to that of nearby natural reefs to assess project success. Darker colors represent areas with higher long-term abundances of spat oysters (≤ 25 mm shell height) and lighter colors represent lower long-term abundances of spat oysters

To use this map, a subset of five live oysters and five dead oysters (> 25 mm shell height) from your restoration site should be examined and the number of spat (shell length ≤ 25 mm) attached to each shell should be counted. Calculate the average number of spat per shell and compare that to that of nearby natural reefs using the map. A successfully restored reef would be expected to have average numbers of spat that are similar to, or exceed that of nearby natural reefs within 1-2 years after restoration.

Notice that the map to the right is an interactive map that can be zoomed in and out on and moved locate an area of interest. Just double click on a location to zoom in on an area or you can click the plus sign on the bottom right-hand corner. You can zoom out by clicking on the minus sign in bottom right-hand corner. You can also pan North, South East and West by holding down the mouse button and moving you mouse to the desired location. Single click on the map to identify what the long-term average number of spat for a certain location or consult the Legend above.

The HSI approach was effective in identifying suitable sites for oyster reef restoration, is simple to use, and provides guidance for restoration practitioners and resource managers to maximize return on restoration investments. Although focused on oyster reefs in Texas, this framework may be adapted to different habitats and/or regions of the U.S. to address similar management challenges.