Trophic Ecology of Sheepshead and Stone Crabs at Oil and Gas Platforms in the Northern Gulf of Mexico's Hypoxic Zone

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AbstractOil and gas platforms (platforms) may provide important habitat for reef‐associated organisms in the Gulf of Mexico's hypoxic zone. While summer hypoxia typically affects the bottom 3.0 m of the water column, platforms span from the bottom to above the surface and provide hard substrate in oxygenated waters overlaying hypoxia. However, little is known about the effects of hypoxia on organisms living at these platforms. We compared diets of Sheepshead Archosargus probatocephalus and Gulf stone crabs Menippe adina during early and late summer at platforms with mild (shoal platforms) and intense hypoxia (seaward platforms) using a combination of stable isotopes (δ13C and δ15N) and gut contents analyses. We hypothesized that diets at seaward platforms would shift from bottom to platform‐dwelling prey during late summer when hypoxia had fully developed and decimated benthic communities, but no such effect was found. Barnacles (Balanidae) were the most common prey found in Sheepshead guts for all samples collected (64.1% of gut contents). Sheepshead gut contents showed high predation of mantis shrimp (Squillidae; 35.2% of gut contents) soon after the formation of hypoxia at seaward platforms, likely reflecting Sheepshead exploiting benthic prey that were vulnerable because of hypoxia. These findings indicate that hypoxia affected the behavior of mantis shrimp in a way that altered their availability for Sheepshead associated with platforms. Comparisons of Sheepshead and stone crab isotope values (δ13C and δ15N) to those of their potential prey and basal sources suggested a high dependence on barnacles (proxy for water column particulate organic matter) and filamentous algae throughout the summer. However, comparisons of isotope values in early and late summer indicated no apparent effect of hypoxia. High contributions of barnacles to Sheepshead diet showed that platform‐dwelling prey were an important resource for Sheepshead.