EFFECTS OF HABITAT AND FLUCTUATING TEMPERATURES ON SOUTHERN FLOUNDER SEX RATIOS

Southern flounder (Paralichthys lethostigma) exhibit environmental sex determination (ESD), where environmental factors can influence sex and masculinize populations. Flounder aquaculture as well as recreational and commercial fisheries are dependent upon females due to their faster growth and larger size relative to males, so it is important that populations not become male biased. Impacts of the environment on sex occurs during early juvenile development (30 - 65 mm) and sex reversal (masculinization) is limited to the XX female genotype. Previous data show that temperature extremes (18°C and 28°C) and background color can influence sex determination, with suboptimal parameters promoting male development. Ideal conditions at 23°C will produce at most 50% females, with much lower percentages possible due to masculinization of XX females. We also show that ESD in these fish is in part mediated by cortisol, as treatment with this stress hormone masculinizes fish. As such, environmental variables that induce glucocorticoid signaling during sex determination could impact population sex ratios. As temperatures naturally fluctuate in the wild, and in systems that are not temperature controlled (outdoor tanks/ponds), the present study first examined the effects of temperature fluctuations on sex determination in wild populations of southern flounder in North Carolina, USA. We then assessed whether the patterns of temperature variation associated with male bias sex ratios in the wild would reproduce male skewed sex ratios under laboratory conditions.

We show that northern sites (Pamlico River) had near 50:50 sex ratios from 2014-2016 (52%, 37%, and 61% male). The Neuse River, an intermediate location, produced male biased sex ratios from 2012-2016 (88%, 82%, 76%, 59%, and 82% male). In the southern locations, south of the New River, sex ratios were male skewed from 2014-2016 (88%, 86%, and 81% male). The habitats that produced male skewed sex ratios were associated with warmer temperatures, with an average of 4°C max difference between daily temperature readings across habitats. In Swanquarter Bay, a nursery habitat in the Pamlico area with 50:50 sex ratios, the average temperature over the sex determination period was 23.3°C, consistent with the temperature that promotes female development in the laboratory. Hence, we determined whether a 4°C increase or decrease under the naturally fluctuating temperature profile observed at this site would produce male-biased sex ratios with fish reared in controlled systems. We show 98% males in a constant 27°C system, while sex ratios were 83%, 65%, and 100% male in 19°C, 23°C, and 27°C fluctuating systems respectively (Fig. 1). These results indicate that controlled temperature fluctuations in the laboratory produce sex ratios consistent with that found in natural habitats.