Assessing Southern Flounder Population in the Northern Gulf of Mexico
Type of DegreeMaster's Thesis
Fisheries and Allied Aquacultures
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Abstract Despite Southern Flounder being a commercially and recreationally important flatfish species in the northern Gulf of Mexico (GOM), our understanding is poorly described of its population dynamics. I collected Southern Flounder from three sites along the northern GOM-- Barataria Bay, LA, Mobile Bay, AL, and Apalachicola Bay, FL-- and used sagittal otoliths to estimate age-and-growth, as well as mortality. The lifetime variation in the elemental composition of otoliths (also was assayed to infer patterns of past habitat use throughout life, particularly as related to salinity) among regions. Male Southern Flounder collected from Alabama coastal waters grew faster and had a greater estimated von Bertalanffy length asymptote than did male Southern Flounder from Louisiana or Florida coastal waters. Female Southern Flounder collected from Louisiana and Alabama coastal waters grew faster than fish from Florida coastal waters, but the von Bertalanffy length asymptote estimate for Southern Flounder collected from Florida was larger than those for Louisiana and Alabama coastal waters. Length-frequency distributions differed across all three populations, with fish being larger in Alabama versus both Louisiana and Florida. Weight-length relationships also differed across these populations, with Southern Flounder from Louisiana and Alabama coastal waters putting on more weight per length than fish from Florida coastal waters. Based on catch-curve analysis, total annual mortality for these populations was 82% in Louisiana, 66% in Alabama, and 54% in Florida. Results from age-and-growth analyses of Southern Flounder from these three populations indicate they are exhibiting different growth patterns. Laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) was used to assay otolith chemical composition through time in an effort to explore lifetime habitat use and potential population differences in chemical constituents, as well as to assess whether otoliths of Southern Flounder along the northern GOM were exposed to oil as a result of the 2010 Deepwater Horizon (DWH) oil spill. None of the elements thought to be markers indicative of the DWH oil spill met the limit of detection criteria for any population, indicating that fish collected in my study did not accumulate elevated levels of elements previously identified from the oiling event. I also used changes in otolith chemical signatures, with particular emphasis on Sr88, along the growth axis to examine past salinity exposure, and thus past environment experienced by fish collected along the northern GOM during their otolith formation. Based on otolith microchemistry, a group of Southern Flounder from each population had low Sr88 concentrations during the development of their otolith core, indicating a period of freshwater residency after ingress from the marine environment. Concentrations of Ba137 represent an effective marker for distinguishing Southern Flounder collected from Louisiana when compared to Alabama or Florida. In order to test for possible effects of the DWH oil spill of April 2010 on the Southern Flounder population structure from areas that experienced heavier oiling (i.e., Louisiana, and to a much lesser extent Alabama), growth and condition estimates from my study were compared to historical studies conducted from the same areas. When compared to pre-oil spill data, I found that male Southern Flounder collected from Louisiana reached a larger theoretical maximum length (L∞) at a slower rate, while females reached a smaller L∞ at a faster rate after the oiling event. Pre-oil comparisons of condition suggest that Southern Flounder from Louisiana put on more weight per length than did fish collected in my study. Alabama comparisons suggest that males reached a larger L∞ at a faster rate, while females reached a smaller L∞ at a faster rate after the oiling event. No differences of condition between pre-oil and post-oil data were observed from this region. Results from this study suggest that Southern Flounder populations across the northern GOM are growing differently from one another and further support the need for freshwater habitats to be included when identifying nursery habitat for this important flatfish species. More work is needed to evaluate the importance of such habitats for their migratory life history strategies and how these freshwater habitats affect the overall population dynamics of Southern Flounder.