Mapping a New Path

When most people think of a job as a fisheries technician, they generally think that means working with fish. When I started with Texas Parks & Wildlife Department (TPWD) I did just that; I identified, measured, and counted fish collected in sampling gear and brought in by anglers. Now, however, I am a fisheries technician that works with a different type of fish, a sonar fish. I was not expecting to work this close with complex technology, but when the opportunity arose to learn a new technique to assess habitat changes over time, I was excited to become a part of it.

The opportunity to start this new program was given to TPWD via a grant from the National Oceanographic and Atmospheric Administration (NOAA) with funds from a congressional appropriation to provide aid for fisheries disaster relief and recovery from Hurricanes Rita and Katrina. Now, the Coastal Fisheries Division of TPWD has an Oyster Mapping and Restoration component made up of myself, Jennie Rohrer, Bill Rodney, and David Westbrook. The main objective of the program is to map and identify submerged habitat in Galveston Bay, Sabine Lake and the nearshore Gulf of Mexico. With this baseline information it will allow us to assess impacts resulting from hurricanes, vessel groundings, oil spills, etc.

At the beginning of the Oyster Mapping and Restoration Project, we compared different habitats and organisms found within Texas' estuaries and determined that monitoring oysters would be the best indicator species to evaluate habitat changes over time. The reason for choosing oysters as our keystone species is two fold. First, oysters are an ecologically, recreationally and commercially important aquatic species that provides nursery, refuge and foraging habitat for numerous fish and invertebrates. Water filtering capabilities of oysters also make them a valuable species in our bays. Second, oyster reefs are a permanent structure on the bay bottom making them susceptible to fast moving waters from natural disasters that can lead to sedimentation or scouring of oyster reefs. Changes in oyster habitat can be detected with images created by our new side scan sonar system, a Teledyne Benthos C3D. This sonar allows us to not only detect changes to oyster reefs, but also map shallow water habitats and determine water depths and bottom contours.

Side scan sonar creates an image of the bay bottom based off the absorptive and reflective acoustic properties of each habitat substrate. Unfortunately, side scan sonar can not indicate whether an area is mud, sand, oyster shell, rocks, etc. Using a variety of ground truthing methods, including oyster dredges, sounding poles and underwater video, we can identify different bottom types. As a result, we have concluded that dense areas, like oyster reefs, appear lighter in the sonar image, while less dense areas, like mud, appear darker. Once multiple images and ground truthing data are collected, we can begin to distinguish oyster reef complexes and other habitat types as well as identify objects of interest.

Following Hurricane Ike, our side scan sonar images were effective in assessing habitat changes. Passage of the storm and retreating storm surge led to significant sediment transport in Galveston Bay, resulting in widespread impact to oyster habitat. On average we found a 60% loss of oyster reefs throughout Galveston Bay caused by storm surge derived sediment and debris. Estimates in East Galveston Bay, however, show an 80% loss of oyster habitat (Figure 4). Interestingly, the images do not only reveal negative impacts from Hurricane Ike. For example, oyster reefs near the Houston Ship Channel were scoured by retreating storm surge, resulting in higher elevations of reefs relative to the surrounding bay bottom which will expose oysters to better water flow over the reef and the phytoplankton upon which they feed. The images have also allowed us to determine the best methods for assisting in recovery that include helping with debris removal and aiding in current and future oyster reef restoration projects.

With the success of the Oyster Mapping and Restoration Project thus far, we hope to eventually expand the project to survey the entire Texas coast allowing improved damage assessments for the entire state. Although we do not wish for more hurricanes, technology has better prepared us to assess damages and aid in a quick recovery that will not only benefit the oysters, but everyone who uses the water.