An acoustic-optic comparison of fish assemblages at a Rigs-to-Reefs habitat and coral reef in the Gulf of Thailand

The number of decommissioned oil and gas platforms will increase over the coming decades. In the tropics and sub-tropics, platforms may operate near biodiverse natural habitats, such as coral reefs. Abundant and diverse fish communities often aggregate around platforms, with different decommissioning options posing variable ecological effects on these assemblages. One decommissioning option is to repurpose platforms into artificial reefs, a process commonly termed "Rigs-To-Reefs" (RTRs). Many RTR programs attempt to retain or enhance habitat availability and associated fish assemblages. To help demonstrate the importance of RTR projects, the fish habitat value of RTRs can be estimated by comparing the fish communities of RTRs to those of adjacent natural habitats. Surveying fish habitats in turbid waters necessitates the use of methods that can quantify the abun-dance, biomass, and length of fishes irrespective of visibility. Using an imaging sonar (1.2. MHz) and imple-menting a novel, pixel-based approach analogous to echo-integration, this study estimated the relative biomass density and length distribution of fishes at seven toppled oil and gas platform jackets in a Reefed Jacket Zone (RJZ) in the nearshore western Gulf of Thailand in comparison with a nearby coral reef. Biomass density was comparable at each habitat, despite extreme variability across the RJZ, attributed to the spatiotemporal sto-chasticity of larger, schooling fishes. However, fishes were larger at the RJZ, a trend supported by simultaneous stereo-video footage. The relative benefits and drawbacks of imaging sonar encountered in this study are also described.