Doctor of Philosophy (PhD)
Oceanography and Coastal Sciences
The ecological and economic value of sandy shoals off the Louisiana coast is not well understood. During three years of comprehensive benthic sampling and environmental measurements I studied the Ship, Trinity, Tiger Shoal Complex (STTSC), which comprises changing and discrete benthic habitats including high relief sandy shoals, and muddier, mostly deeper off-shoal areas, prone to hypoxia. Benthic macrofaunal assemblages of shoals included endemic species, and shoal communities were significantly different from each other and the muddier offshoal habitat, contributing to northern Gulf of Mexico (GoM) regional biodiversity. Sand percentage was the most influential environmental parameter shaping macrofaunal community composition across the region. My study revealed several more potential shoal-based functions such as providing a conduit for GoM sandy-habitat metapopulations, serving as an oxygenated benthic refuge from seasonal bottom water hypoxia, and functioning as offshore blue crab (Callinectes sapidus) spawning grounds. I discovered unexpectedly high concentrations of spawning female blue crabs, greatly expanding what was previously understood about blue crab reproductive migrations. Blue crab abundances were significantly higher on Ship and Trinity Shoals than the surrounding muddier and deeper seafloor. STTSC blue crabs compared favorably with those from nationally recognized spawning grounds in terms of condition factor (an index of health), abundance, and fecundity. This work is the first to use an ecological field study to predict the number of days (~21) between successive spawns for blue crabs, suggesting STTSC blue crabs produce at least seven broods per spawning season (~April– October). My morphometric predictors of crab weight were 12 to 16% better than the traditionally used method. In addition, I used natural abundance isotopes (δ13C, δ15N) to link blue crabs from the STTSC to the inshore blue crab fishery. I analyzed isotopic variations in crab muscle and ovary tissue and found relationships with salinity and proximity to the Atchafalaya River, indicating that crabs predominately migrate directly offshore from their home estuary, including from low salinity environments. Isotopic analysis also suggests that crabs utilize offshore prey resources and do not re-enter inshore estuaries during the spawning season but rather remain offshore for the season, continually spawning and hatching their eggs.
Document Availability at the Time of Submission
Release the entire work immediately for access worldwide.
Gelpi Jr., Carey George, "Function and diversity of the Ship, Trinity, and Tiger Shoal Complex, with emphasis on macrofauna and spawning blue crabs (Callinectes sapidus)" (2012). LSU Doctoral Dissertations. 937.