Sheepshead Foraging Patterns at Oil and Gas Platforms in the Northern Gulf of Mexico

Abstract

In the northern Gulf of Mexico, there are over 2,000 oil and gas platforms (hereafter, "platforms") that are de facto artificial reefs. The ecological importance of platforms is the subject of debate, but little information exists on the extent to which fish feed on platform-dwelling benthic organisms. We used a camera array to characterize the frequency and duration of incidents in which Sheepshead Archosargus probatocephalus foraged on platformdwelling benthic organisms. We also characterized abundances and distributions of select platform-dwelling benthic organisms (barnacles, blennies, and stone crabs). Sheepshead foraging was more strongly associated with dissolved oxygen (DO, mg/L; measured at each camera) and time to slack tide (high or low) than other biological (prey densities), physiochemical, locational, or temporal variables. The number of Sheepshead foraging incidents increased by 18.9 incidents·m −2 ·h −1 (95% confidence interval [CI] = 4.6-33.3) when DO increased by 1.0 mg/L; foraging incidents increased by 4.5 incidents·m −2 ·h −1 (95% CI = 1.6-7.4) when time to slack tide increased by 1.0 h. The time Sheepshead spent foraging increased by 245.4 s·m −2 ·h −1 ( 95% CI = 144.1-346.7) when DO increased by 1.0 mg/L, and time spent foraging increased by 78.0 s·m −2 ·h −1 (95% CI = 60.5-95.5) when time to slack tide increased by 1.0 h. Abundances of barnacles and blennies decreased with increasing depth. The presence of hypoxia near the seafloor during the late summer appeared to cause barnacle mortality, which shifted barnacle depth distributions upward (i.e., 1-3 m shallower). Platform-dwelling benthic organisms persisted throughout the hypoxia season and served as prey for Sheepshead.

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

“…Indeed, a concurrent study found that Sheepshead frequently forage on platform‐dwelling organisms (Reeves et al. ). It is notable that Sheepshead muscle isotopic values were highly variable and were not tightly clustered around barnacles.…”

“…Sheepshead were collected at platforms adjacent to those where stone crabs, barnacles, algae, and sediment were collected to avoid biasing concurrent studies of Sheepshead abundances and grazing behaviors (Reeves et al. , ). Sheepshead were collected using hook and line and spears; stone crabs, barnacles, and macroalgae were collected from the platform's main pilings.…”

“…Sheepshead, Gulf stone crabs, barnacles (Balanidae), platform associated macroalgae, and sediment were collected from 14 platforms. Sheepshead were collected at platforms adjacent to those where stone crabs, barnacles, algae, and sediment were collected to avoid biasing (Reeves et al 2018b(Reeves et al , 2018c. Sheepshead were collected using hook and line and spears; stone crabs, barnacles, and macroalgae were collected from the platform's main pilings.…”

“…Moreover, barnacles likely made an important contribution to Sheepshead and stone crab diets because they represented~64% of all Sheepshead gut contents, and past studies indicated that barnacles are common prey of Sheepshead (Hoskin 1980;Overstreet and Heard 1982;Hoskin and Reed 1984;Sedberry 1987) and stone crabs (Powell and Gunter 1968). Indeed, a concurrent study found that Sheepshead frequently forage on platform-dwelling organisms (Reeves et al 2018b). It is notable that Sheepshead muscle isotopic values were highly variable and were not tightly clustered around barnacles.…”

“…Sheepshead and stone crabs were studied because they are abundant at shoal and seaward platforms throughout the summer (Reeves et al 2017(Reeves et al , 2018c. Benthic invertebrates, including sessile and mobile organisms, are the most common prey of Sheepshead (Overstreet and Heard 1982;Sedberry 1987) and stone crabs (Bender 1971), and Sheepshead are known to forage on platform-dwelling prey (Reeves et al 2018b). Both species support small commercial fisheries in the Gulf of Mexico, and mean annual landings from 2006 to 2015 were 601 metric tons for Sheepshead and 2,112 metric tons of stone crab claws (Menippe adina and M. mercenaria); however, these organisms are not typically fished at platforms.…”