Southern Flounder, Paralichthys lethostigma

The Southern Flounder pictured here was caught locally on Padre Island, TX, using a hand gig, in February, 2024. All images and graphics © Sara Krebsbach 2024.

Southern Flounder, Paralichthys lethostigma, is an inshore GOM species with a fascinating life history and a distinctive set of morphological and physiological characteristics. Southern Flounder’s are euryhaline, bottom dwelling flat fish, that are laterally compressed, and adults have dorsal and anal fins that mirror one another, ending just before the caudal fin. Their flat bodies and fins, along with a small body cavity, the absence of a swim bladder, and varying pigmentation to match their environment, allow them to remain on the benthos with ease. They have a sub-terminal mouth shape with sharp teeth, representative of their diet of crustaceans and other bottom dwelling fish species. Additionally, adult Southern Flounders have both eyes on the top of their body, however, they aren’t born that way! As oviparous fall spawners, adult Southern Flounder leave the comfort of the nearshore benthos and head out to open waters in a mass migration determined by the lunar cycle, where they’ll produce up to 50,000 eggs per spawn. Eggs are hatched after 2-3 days, and larvae swim in an upright position with an eye on each side of their head. It isn’t until they have grown for 1-2 months and have grown to one inch that we see the migration of their right eye over to the left side of the body, which remains the ‘up side’ of the fish for the rest of its life. The last morphological characteristic to touch upon is that Southern Flounder are sexually dimorphic, meaning that males and females of the species look and act differently; Female Southern Flounder reach greater lengths and weights than their male counterparts, and generally inhabit nearshore and highly productive habitats in order to maintain their size and gain the energy to spawn. Southern Flounder exhibit a unique physiology that make them an interesting study. Southern Flounder demonstrate environmental sex determination (ESD), which in their case means that they depend on cool environmental temperatures during juvenile growth to differentiate into even male and female populations (Montalvo, A. J. 2010; Honeycutt et al. 2019).

Both sexual dimorphism and environmental sex determination contribute to their unique life history and ecological niche. Unfortunately, water temperature and environmental factors impact fishery management schemes. Female Southern Flounder contribute heavily to the recreational fishing industry as they grow almost double the size of their male counterparts. Not only do males rarely grow large enough to meet the current 2023 size limit of 15 inches, we are also seeing masculinizations of wild populations in the GOM due to higher sea surface temperatures (Montalvo, A. J. 2010). Consequently, overfishing and shrinking recruitment of this important and unique species has given them a ‘near threatened’ status by the International Union for Conservation of Nature (IUCN), as of their last assessment in 2015 (Munroe 2015). In an effort to stock the Southern Flounder fishery, Texas Parks and Wildlife have initiated a program to release 25,000 juveniles into Texas bays and estuaries per year (TPWD 2023). Flounder are released into the wild at roughly 60 days of age and before sexual differentiation occurs. Although Southern Flounder culture at the hatchery is focused on in the winter, they have been cultured into the summer months in the past several years, which is outside the natural breeding range. During the summer months, it is assumed that most individuals will differentiate into the male sex due to increased sea surface temperatures in the GOM. The lack of females in the population results in lower recruitment for the species and reduced the contribution to the restoration or conservation of the species. Similar to the Red Snapper, the Southern Flounder has also been rated by Monterey Bay Aquarium’s ‘Seafood Watch’ program in a 2023 report as ‘good alternative/yellow, primarily due to methods of catch, with ‘gigs, handlines, and hand operated poles/lines’ being the recommended type of fishing gear to catch Southern Flounder.

Glossary:

Benthos: The seafloor.

Ecological Niche: An organisms role in a broader complex community. This includes the environment they live in and how they utilize it’s resources, as well as their relationship with other organisms in their environment.

Euryhaline: A fish that is able to maintain its osmoregularity when moving between freshwater and saltwater environments.

Laterally Compressed: A fish that is compressed laterally, often resulting in a deep-bodied shape. The dorsal and anal fin remain medially.

Oviparous: Laying eggs in an external environment (outside of the female fish’s body, however, eggs can be fertilized internally or externally).

Recruitment: When a fish reaches the next stage of development, for example, when a larvae metamorphoses into a juvenile. In Fisheries, recruitment is often used to describe a fish who has grown to legal size to harvest.

Sub-terminal Mouth Shape: Mouth slightly pointed down, with it’s origin below the end of the snout.

References:

Hastings, P. A., Walker, H. J., & Galland, G. R. (2014). Fishes: A guide to their diversity. University of California Press.

Helfman, G. S., Collette, B. B., Facey, D. E., & Bowen, B. W. (2023). The diversity of fishes (3rd ed.). Wiley-Blackwell.

Honeycutt, J. L., Deck, C. A., Miller, S. C., Severance, M. E., Atkins, E. B., Luckenbach, J. A., Buckel, J. A., Daniels, H. V., Rice, J. A., Borski, R. J., & Godwin, J. (2019). Warmer waters masculinize wild populations of a fish with temperature-dependent sex determination. Scientific Reports, 9(1), 6527. https://doi.org/10.1038/s41598-019-42944-x

Montalvo, A. J. 2010. Sex determination in southern flounder, Paralichthys lethostigma from the Texas Gulf Coast and implications of climate change. (Unpublished doctoral dissertation). University of Texas at Austin.

Monterey Bay Aquarium Seafood Watch ®. Southern Flounder (US). Report ID 27899. June 5, 2023. https://www.seafoodwatch.org/recommendation/flounder/southern-flounder-38250?species=759

Ruschenberger, W. S. W., Milne-Edwards, H., & Comté, A. (1846). Elements of Herpetology, and of Ichthyology: Prepared for the Use of Schools and Colleges. Grigg & Elliot.