Oil and Gas Platforms Provide Refugia for Stone Crabs (Menippe spp.) Living within the Northern Gulf of Mexico's Hypoxic Zone

Oil and gas platforms (rigs) may provide key settlement sites for stone crabs in nearshore Louisiana because they provide substrate in normoxic surface waters when bottoms are hypoxic.  Stone crab habitat use was compared at rigs on and off Ship Shoal, a drowned barrier island that tends to be normoxic while surrounding areas are hypoxic.  Densities were estimated with visual counts, and populations were characterized by removing subsamples for identification, sexing, and measurements.  Stone crab densities at rigs were higher on than off shoal (mean 4.0 vs. 1.8/m²; P<0.05), and carapace width where 50% of females were ovigerous (CW₅₀) was 23 mm larger on the shoal (P<0.05).  Stone crabs on and off the shoal did not significantly differ in mean carapace width (P>0.05), size-class distribution (P>0.05), or sex ratio (P>0.05).  Rigs on and off Ship Shoal provide important refuge and foraging substrate in normoxic waters; however higher density and CW₅₀ on the shoal suggest that shoal rigs are of higher relative importance.  During this study, 3.3 million cubic yards of sediment were dredged from Ship Shoal, and 2 of 24 sampled rigs were removed.  Further depletion of these substrates may negatively affect stone crab populations in nearshore Louisiana.