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Benthic habitat mapping and assessment in the Wilmington-East Wind Energy Call Area : final report

Abstract

The Bureau of Ocean Energy Management (BOEM) is responsible for oversight and management of the development of energy resources on the Outer Continental Shelf (OCS). In 2012, BOEM identified three Wind Energy Call Areas and later defined Wind Energy Areas on the OCS of North Carolina. Presently, sufficient uncertainty exists regarding cumulative impacts to ecosystem services such as essential fish habitat and maritime cultural resources as a result of the construction or operation of offshore energy facilities to merit preliminary studies. From rocky outcrops to shipwrecks, hardbottom habitats serve as essential fish habitat for reef fisheries off of North Carolina and along the southeast OCS. This project accomplished the primary objective of describing and delineating rocky outcrops, within the Wilmington-East Call Area. The delineation of rocky outcrops and artificial hardbottom habitats guided an intensive diver visual assessment characterizing the benthic and fish communities, the seasonal changes in communities, and influences of sand and sediment movement around hardbottom habitats. This report is the result of a collaborative effort between the University of North Carolina Institute of Marine Sciences and NOAA’s National Centers for Coastal Ocean Science, and the Bureau of Ocean Energy Management.

Key findings are:
Delineation of hardbottom habitats and shipwrecks

  • We provide the first complete coverage by hydrographic sidescan and multibeam sonar of the Wilmington-East Call Area. These GIS products show a varied seafloor interpreted as sand shoals, pavement, rocky outcrops, ledges, and shipwrecks. The pattern of seafloor sediments is consistent with the geological framework of Long Bay and nearby Frying Pan Shoals.
  • The distribution of rocky outcrops is clustered in patches in discrete regions of the study area. The distribution and clusters of notable outcrops appears to conform to areas of fishing uses previously identified by stakeholders.
  • While clusters and isolated hardbottom features are present in the Wind Energy Area, large clusters of delineated rocky outcrops occur in the southern and eastern regions of the study area, outside of the Wilmington-East Wind Energy Area, which defines the OCS blocks that may be available for wind energy lease and development.
  • Five shipwrecks have been confirmed with their position accuracy improved. Two potential new shipwrecks were found within the study area.

Benthic habitats and fish communities on hardbottom in the study area

  • The community composition and physical structure formed by invertebrates and macroalgae in the study area are diverse and in some ways distinct from those seen in neighboring Onslow Bay.
  • The fish community composition is similar to that seen in neighboring Onslow Bay.
  • Complex, high relief hardbottom and the associated benthic communities support higher numbers of species and biomass of large apex predators, including species in the snapper-grouper management complex. 
  • Remotely sensed fish distributions using echosounders show patterns of high fish densities that conform spatially to the distribution of hardbottom habitats. During the day, the majority of fish were within 150 m of the hardbottom. At night, fish distribution extended as far as 1000 m from the hardbottom features.

Seasonal dynamics in sediment cover, benthic and fish communities

  • Repeated surveys over natural and artificial hardbottom habitats characterized seasonal changes in the biological communities, particularly related to structural complexity and sediment dynamics.
  • The benthic community in the study area, in Long Bay’s highly dynamic sedimentary environment, experiences more frequent burial and abrasion by sediment than sites further north in Onslow Bay.
  • Fish community metrics related to seafloor complexity revealed that reef fish use a wide range of hardbottom habitat types.

Shipwrecks are used much like hardbottom habitat supporting reef fish

  • Shipwrecks provide substrate for a diverse assemblage of attached biological communities that support high diversity of reef fishes.
  • The fish community composition differs between shipwrecks and natural reefs due to the presence of large aggregations of planktivorous fishes near shipwrecks.

This report follows decades of research on the importance of hardbottom habitats on the southern Atlantic OCS that support the ocean ecology and economies of North Carolina and other southeastern US coastal states. This study represents an important baseline condition of US south Atlantic benthic habitats offshore Wilmington, NC and of their value to fishes, in preparation for offshore development of wind energy facilities.