The effects of oyster harvest on resident oyster reef communities and reef structure in coastal Louisiana proper

Harvest of the eastern oyster (Crassostrea virginica) is a primary contributor to oyster reef habitat disturbance in the northern Gulf of Mexico. The impacts of oyster dredging on reef substrate and resident fauna have not been thoroughly examined on the extensive sub-tidal oyster reefs of Louisiana. Several reef structure and resident community metrics were compared on unharvested and harvested reefs during the spring, summer, and fall of 2010. Unharvested reefs had higher amounts of oyster clusters, solid reef substrate, and more large oysters, while harvested reefs had higher amounts of loose shell, mixed shell/mud substrate, and elevated chlorophyll-a levels. Overall, faunal densities did not differ with harvest status and dominant species were similar, although greater invertebrate diversity was found on harvested reefs. Several resident species were found to primarily associate with live oysters [freckled blenny (Hypsoblennius ionthas) and skilletfish (Gobiesox strumosus)] and chlorophyll-a levels [Harris mud crab (Rhithropanopeus harrisii) and snapping shrimp (Alpheus sp.)], indicating the importance of live oysters in determining reef microhabitat preferences by regulating types of available food sources. Condition (weight:length ratio) of naked gobies (Gobiosoma bosc) was greater on unharvested reefs, while other common fish species showed no difference. Large interstitial spaces associated with oyster clusters appear to enable several fish species to reach larger sizes at unharvested reefs and promote retention of age = 0 G. bosc. Stable isotope values (del 13C and del 15N) of dominant species and basal food sources were used to compare food web characteristics between sites. Non-pelagic source contributions and trophic positions of dominant species were elevated at harvested sites. Trophic order did not differ suggesting that no major shifts in feeding behavior occur at harvested reefs with the exception of zooplankton (trophic position increased substantially at harvested sites). While not changing total refuge capacity, oyster harvest appears to decrease the number of large oysters and also fragment solid reef area, resulting in elevated phytoplankton abundance, decreased benthopelagic coupling, and increased habitat heterogeneity. A larger forage base in the water column and mixed shell/mud substrate could account for increased invertebrate diversity and trophic position elevations on harvested oyster reefs