Symposium Abstract: Using Side-Scan Sonar to Assess the Impact and Persistence of Natural and Anthropogenic Disturbance to Low-Relief Oyster Habitats in Coastal Louisiana
Y. Allen, C. Wilson, H. Roberts, and J. Supan
Traditional methods used to assess oyster reef distribution and condition are only able to provide subjective point information, which is often poorly georeferenced. Maps of oyster habitat in shallow waters are therefore typically extremely generalized, giving few details about the true distribution, character and dynamics of reefs. Sidescan sonar offers a significant advantage for oyster reef assessment in the turbid waters of coastal Louisiana. We used sidescan sonar in ultra-shallow (<2m) waters to completely image over 19,000 ha in Louisiana estuaries in advance of an impending freshwater diversion project. We also conducted four years of intense annual surveys in a more restricted area (320 ha) with a diversity of reef types and culture intensity to examine natural and anthropogenic impacts on oyster reef extent and character. Our intensive surveys identified older stable reefs that had not been actively worked. Shell abundance and structure on these reefs were high, but oyster meat productivity was low. Areas of intense oyster culture were characterized by low relief reefs that frequently showed distinct evidence of scarring from dredging and other anthropogenic sources. Smaller scars caused by oyster dredging typically healed through the within time period of our study while larger anthropogenic scarring did not diminish over the four years. We also deployed the sonar towfish over an area immediately before and after both seeding and harvesting to establish a quantitative relationship with sonar reflectance. These relationships can be further used to predict the impact of harvesting and seeding on the extent oyster habitat.