Mikulas, Joseph John (2009-05). Habitat use, growth, and mortality of post-settlement lane snapper (Lutjanus synagris) on natural banks in the northwestern Gulf of Mexico. Master's Thesis.
Thesis
Three low-relief banks (Heald Bank, Sabine Bank, Freeport Rocks) in the northwestern Gulf of Mexico were evaluated as lane snapper (Lutjanus synagris Linnaeus, 1758) nursery habitat. Trawl surveys were conducted in three habitat types (inshore mud, shell ridge, offshore mud), designated by side-scan sonar surveys, to determine patterns of distribution and abundance. Heald Bank and Sabine Bank were trawled in 2003 while Freeport Rocks was trawled in 2000 (Freeport A) and 2004 (Freeport B). Density of lane snapper was higher on Sabine Bank (20.8 +- 2.8 ind ha-1) than on Heald Bank (1.1 +- 0.4 ind ha-1), Freeport A (12.7 +- 2.3 ind ha-1) or Freeport B (3.0 +- 1.0 ind ha-1). Habitat-specific differences in density were observed, although patterns were not consistent among banks. Highest densities of lane snapper were found on Heald Bank's offshore habitat (1.7 +- 1.0 ind ha-1), Sabine Bank's ridge habitat (26.5 +- 6.9 ind ha-1), and on the inshore habitat of Freeport A and B (17.6 +- 4.9 ind ha-1 and 4.8 +- 3.6, respectively). Otolith microstructure analysis was performed on lane snapper collected in trawl surveys to determine age, hatch-date distribution, growth and mortality of new recruits. Hatch dates ranged from May 1 to August 31, peaking in June for Freeport (A and B) and in July for Heald Bank and Sabine Bank. Growth rates varied from 0.90 mm d-1 at Heald Bank to 1.27 mm d-1 at Sabine Bank, and rates were highest on the ridge habitat of Sabine Bank (1.31 mm d-1). Mortality of post-settlement lane snapper was higher on Sabine Bank (15.2% d-1; Z = 0.165), than on Freeport A (9.2% d- 1; Z = 0.097), and was greatest on the ridge habitat of Sabine Bank (24 % d-1; Z = 0.275). Recruitment potential (G : Z), evaluated on habitats at Sabine Bank, was highest on the offshore habitat, with a value greater than 1.0, indicating higher potential contribution to the adult population. Results indicate Heald Bank, Sabine Bank, and Freeport Rocks all serve as settlement habitat of lane snapper, which appear to be capable of successful settlement across a variety of habitats and banks.
