Understanding patterns of fish backscatter, size and density around petroleum platforms of the U.S. Gulf of Mexico using hydroacoustic data

Hydroacoustic surveys were conducted at 56 different oil and gas platforms in the U.S. Gulf of Mexico (GOM), along with a commensurate collection of physio-chemical data, in order to characterize patterns of fish distribution around platforms. Relative acoustic measurements of Mean Volume Backscatter “MVBS”, Target Strength “TS” and fish density were examined. These acoustic variables were employed to test if patterns of MVBS, TS and density around GOM platforms differed among seabed depth-based categories (Coastal: 0-30 m; Offshore: 31-60 m; Bluewater: >60 m), and how these patterns varied as a function of the distance to platforms. The acoustic measurements were also examined across the study area to determine the influence of specific environmental factors and platform features (using generalized additive mixed models (GAMMs)), and whether hypoxic waters compressed fish distribution in the water column. Significantly greater mean fish densities and MVBS occurred at Coastal sites and these variables decreased with increasing seabed depth, whilst mean TS was highest at Offshore sites. Greatest values of fish MVBS and density were found at the shortest distances (0-25 m) from platforms. GAMMs suggested that salinity and temperature had a significant effect on fish density, TS, and MVBS. The number of other platforms within 5 km also had a significant effect on MVBS. Dissolved oxygen concentration has a significant effect on fish density. Finally, we found that fish densities and MVBS were significantly higher in the depth layers immediately above hypoxic layers, demonstrating vertical compression of fishes in the water column when hypoxia occurs. Our findings provide further evidence of the influence of environmental conditions and platform features on the regional patterns of fish density, TS and MVBS around platforms, and the influence of platforms on these metrics on a local scale across the GOM. The study highlights the applicability of using hydroacoustics in conjunction with modelling to assess fish distributions around artificial habitats.