Reef Fish Demographics on Artificial Reefs: The Effects of Reef Size on Biomass and Spatial Distribution

It is well known that reef-fish aggregate on artificial reefs, but the extent to which these structures can be used in a management context has been fiercely debated.  This is due in part to the lack of quantitative information on how artificial reefs affect the surrounding fish assemblage.  In particular, it is important to determine what affect the scale of artificial reefs will have on the size of the associated reef fish assemblage.  The Gulf of Mexico has approximately 3500 oil and gas platforms that serve as artificial reefs.  In previous years, many of these structures had been modified and converted into artificial reefs upon decommissioning.  The goal of this study was to examine the differences in reef fish biomass and spatial distribution around standing and toppled oil and gas platforms in shelf waters of the northern Gulf of Mexico.  A multi-frequency mobile hydroacoustic approach was utilized to measure water column backscatter via a series of ten transects conducted at two standing and two toppled platforms.   Standing platforms are approximately 50m x 50m extending to the surface (80 - 100m depth) while toppled platforms are cut off 30m below the surface and deposited on the seafloor, giving a footprint of 50 – 100m wide.  Acoustic backscatter data were used to determine relative fish biomass associated with each type of structure.  Data were binned by distance intervals from the structure and depth intervals to determine three-dimensional spatial distributions around the structures.  Preliminary results indicate that mean volume backscattering (proportional to fish biomass) is approximately three times higher at the standing platform sites.  In addition, we observed a difference in the vertical and horizontal spatial distributions between structures.  At standing platforms, there are two distinct peaks in fish biomass with depth, one 20 – 40m in depth, and one 60 – 90m in depth.  Distribution of fish biomass around toppled platforms is more uniform with depth.  There are also two distinct peaks in fish biomass with distance from the structure at the standing platforms, one at 20m and one 60 – 100m from the sites.  At the toppled platform, fish biomass peaks directly adjacent to the structure (0 – 20m) and declines with distance.   Our results suggest that the spatial configuration of artificial habitats plays an important role in the observed distribution of biomass associated with these habitats.