Date of Award

August 2020

Document Type


Degree Name

Master of Science (MS)


Forestry and Environmental Conservation

Committee Member

Troy Farmer

Committee Member

Lindsay Campbell

Committee Member

Michael Childress


Southern Flounder (Paralichthys lethostigma) is an economically important species that uses habitats across salinity gradients along the Atlantic Ocean and Gulf of Mexico. During winter, adults spawn offshore, and larvae migrate to estuaries. During spring, larvae settle, grow, and metamorphose into juveniles. Juveniles and adults continue to use estuaries until adults mature and migrate offshore to spawn. This presumed, migratory life history indicates that Southern Flounder is an estuarine-dependent species. As with many estuarine-dependent species, growth, condition, and juvenile recruitment are highly variable across time and space. In response to declines across the species range, state managers have imposed a series of increasingly stringent regulations on Southern Flounder fisheries. However, recent stock assessments show no signs of recovery thus far, suggesting that factors other than fishing mortlity, such as environmental conditions and habitat-use patterns, may be contributing to the decline. My objectives were to 1) develop an index of juvenile Southern Flounder abundance to investigate relationships with environmental factors, and 2) use back-calculation and condition indices to investigate how observed patterns in growth and condition relate to habitat-use patterns.

We developed an index of juvenile Southern Flounder abundance in Mobile Bay (1981 – 2018) and Perdido Bay (1988 – 2018) using historical Alabama survey data. Generalized additive models tested mechanistic hypotheses by relating environmental variables to juvenile abundance for short- and long-term analyses in Mobile and Perdido Bays. Models that included winter covariates were selected as best for all three analyses, suggesting that environmental conditions during spawning and larval stages explain the most variation in year-class strength. Specifically, westerly winds, river discharge, and intermediate winter durations were positively correlated with juvenile abundance, and recent suboptimal conditions helped to explain dramatic declines in juvenile recruitment.

We used 313 otoliths from Southern Flounder collected in Alabama’s coastal waters in 2004 – 2007 and 2018 – 2019 to investigate how age-specific contingent types impacted age-specific back-calculated growth rates and condition. We used linear mixed effects models with various random effect structures to account for age, growth, year, and individual effects. Age-0 and first-year estuarine and transient contingents had higher growth rates than freshwater contingents, but there was no difference in growth among contingents during the second year of life. Age-0 and age-1 estuarine and transient contingents had higher condition than freshwater contingents, but there was no difference in condition for age-2 Southern Flounder.

Over the next century, the Gulf of Mexico is expected to see increased drought conditions, more intense storms, and warmer winter temperatures. This could impact the amount of river discharge that enters the estuary, thus impacting growth rates and overall abundance of young Southern Flounder. Warmer winters could interfere with reproductive success and the number of recruits surviving to the juvenile stage. As recruitment dynamics strongly influence adult abundance, our results should help inform expectations for Alabama’s Southern Flounder fishery in response to changing environmental conditions. Growth and condition are products of good estuarine habitat, and our results could assist in identifying high-quality estuarine habitats that could be used for recently developed stocking programs and continued habitat restoration efforts in Alabama.



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