Southern Flounder (Paralicthys lethostigma) is an important species for recreational anglers and commercial fishermen alike in the state of North Carolina. Unfortunately, the flounder has experienced a notable decline in population in the last few decades, primarily due to overfishing. Despite strict regulations implemented by the N.C. Division of Marine Fisheries, populations have yet to recover. This may be due to an aspect of southern flounder biology: environmental sex determination.

Environmental sex determination (ESD) is when an organism is dependent on certain environmental conditions to develop into a male or female. Lab studies conducted on southern flounder juveniles have shown that ESD affects them when they are between 30 to 65 mm (1.1 - 2.6 in), and that to have a 50:50 split of females and males, temperatures had to be around 23 °C (73.4 °F). A field study conducted in 2019 here in N.C found that southern estuaries (south of the New River) warmed quicker than northern estuaries, and as such, juvenile flounder collected there had a larger ratio of males to females. Another lab study has shown that elevated cortisol (the “stress” hormone) also leads to increased masculinization, despite temperatures being kept at an optimal level, which could imply that other environmental conditions outside of temperature can affect flounder ESD.

All of this paints a picture that southern flounder populations in North Carolina could be male dominated. This, coupled with the fact that females are primarily targeted by commercial and recreational anglers due to their increased size, implies that the reason the population has not recovered could be due to drastically lowered reproductive output from a primarily male population.

Charles Veith, a graduate student at East Carolina University, plans to further investigate this phenomenon and how it ties into fisheries management of this species through a combination of field sampling, fisheries data analysis, and potentially lab experimentation with live juvenile flounder. The primary goals of this study is to discover which environmental variables affect ESD in southern flounder, improve analysis of fisheries data, and support adaptive size and harvest regulations in a changing world.

This graphic depicts the known life cycle of the Southern Flounder. Between steps 3 and 4, when juvenile flounder enter estuaries, is when ESD is suspected to occur in the wild. This would occur between the mid to late Spring. Thus, the temperatures of the estuaries they enter during this time is critical.

Effect of temperature on Southern Flounder sex determination Percentage of female southern flounder from replicate experiments (mean ± ) at temperatures of 18, 23 and 28 °C(n = 64, 53, and 49, respectively, with 20–34 fish/temperature/replicate).

Charles Veith, the 2025 NC BHA Conservation Grant Recipient. Charles is entering the PhD program at East Carolina University’s biology department this Fall, and in his free time is an avid angler.

 

References

Honeycutt, J. L. et al. Warmer waters masculinize wild populations of a fish with temperature-dependent sex determination. Sci Rep 9, (2019).

Luckenbach, J. A., Borski, R. J., Daniels, H. V. & Godwin, J. Sex determination in flatfishes: Mechanisms and environmental influences. Seminars in Cell & Developmental Biology 20, 256–263 (2009).

Luckenbach, J. A., Godwin, J., Daniels, H. V. & Borski, R. J. Gonadal differentiation and effects of temperature on sex determination in southern flounder (Paralichthys lethostigma). Aquaculture 216, 315–327 (2003).

Luckenbach, J. A. et al. Aromatase cytochrome P450: cloning, intron variation, and ontogeny of gene expression in southern flounder (Paralichthys lethostigma). J. Exp. Zool. 303A, 643–656 (2005).

Midway, S. R. et al. Southern Flounder: Major Milestones and Remaining Knowledge Gaps in Their Biology, Ecology, and Fishery Management. Reviews in Fisheries Science & Aquaculture 32, 450–478 (2024).