Variation in the density and species composition of fishes associated with three petroleum platforms using dual beam hydroacoustics

The 4000 petroleum platforms in the northern Gulf of Mexico have significantly impacted the marine community in the region and constitute the largest artificial reef system in the world. They vary in depth from a few meters to over 400 m and range geographically throughout the northern Gulf of Mexico. Despite the number and ubiquitous placement of these structures, little information existed on associated fisheries resources due to difficulties in sampling petroleum platforms with traditional fisheries methods. Hydroacoustics enabled researchers to accurately assess the fisheries resources associated with these structures. From 1994 to 1996 dual-beam hydroacoustics were employed on quarterly research trips to measure the density and in situ target strengths of fishes associated with petroleum platforms in 20, 60 and 219 m of water. Species identification was provided by visual point counts using a remotely operated underwater vehicle. Density varied significantly with platform, distance from the platform, depth, platform side and time of the year. Platforms on the continental shelf had a near field area of influence of approximately 18 m, while near field area of influence for the site on the continental slope was 10 m but the relationship was not as well defined as other sites. Average estimated abundance and 95% confidence intervals over the study period were 26 347 (±3636) at the 60 m site, 13 444 (±4578) at the 20 m site and 11 224 (±2618) at the 219 m site. Fishes were distributed throughout the water column at the sites on the continental shelf while at the 219 m site over 88% of the fishes were found in the upper 60 m. Visual point count surveys detected 10 species, 18 species and 14 species at the 20, 60 and 219 m sites, respectively. Despite the large number of fishes found at each site, 6–7 species constituted over 95% of all the fishes observed at each site and species composition was dominated by a single species at each site. The dominant species changed with site, and the most common species observed at each site included: Atlantic spadefish ( Chaetodipterus faber) at the 20 m site, blue runner ( Caranx crysos) at the 60 m site, and creole fish ( Paranthias furcifer) at the 219 m site.