Barnacle settlement and growth at oil and gas platforms in the northern Gulf of Mexico

Nearshore oil and gas platforms (platforms) off Louisiana’s coast occupy some of the most productive waters in the United States. Platforms are de facto artificial reefs with diverse fouling and fish communities, but little is known about the spatial dynamics of secondary production at platforms. In 2015 and 2016, we used settlement plates to compare barnacle production along a 43.9 km landward–seaward transect at depths of 2 and 7 m. Amphibalanus reticulatus settlement was greater at 2 than at 7 m and decreased with distance from shore at rates of −2.0 to −86.2 ind. m−2 d−1 km−1, depending on depth and year. A. reticulatus growth rates were greater closer to shore, at 2 vs. 7 m, and in 2016 vs. 2015. Height growth ranged from 0.02 to 0.13 mm d−1, but width growth was not associated with any variables. A. reticulatus total and tissue weight gains were similar to growth rates and ranged from 0.43–8.17 and 0.13–1.82 mg d−1, respectively. The ratio of tissue to total weight was greater in 2016 than 2015 and was similar for smaller barnacles, regardless of distance. However, distance had a strong negative effect on this ratio for larger barnacles and peaked at ~25 to 30%. Greater barnacle settlement and growth rates in near-shore waters suggest that prolific primary and secondary production translate to increased production of fouling organisms. Moreover, greater barnacle production at 2 than at 7 m depth suggests the vertical dimension of platforms is an important aspect of their ecological value.