Warmer water linked to higher proportion of male flounder

APRIL 30, 2019

by D'lyn Ford, North Carolina State University

A juvenile southern flounder from Mill Creek in the southern region of a recent North Carolina State University study. Credit: North Carolina State University

If southern flounder live in warmer water during a critical window of early development, a higher percentage become male—more than 90 percent in some cases—research from North Carolina State University found. Having a high proportion of adult males over the long term could threaten both wild populations and the valuable commercial fishing industry, which depends on larger female flounder.

Field research and lab experiments showed that a four-degree Celsius difference in average watertemperature during juvenile development shifted the male-female ratio from about 50-50 to as much as 94-6, says Jamie Honeycutt, an NC State postdoctoral researcher and lead author of an article about the research in Scientific Reports. That difference is within the range of expected ocean temperature increases under climate change models.

Environmental factors such as water temperature influence sex determination in southern flounder, as well as in other fish and reptiles, Honeycutt explains. Flounder stick to shallow waters that serve as nurseries until after they become male or a female, hanging around estuaries until reaching maturity before returning to the ocean to spawn at about age 2.

"We think that southern flounder have a genetic sex-determining system similar to humans, who have two X chromosomes for a female and an X and a Y for a male. In flounder, if an individual is a genetic male, it is destined to be male," Honeycutt says. "However, if a genetic female is exposed to temperature extremes, then it can develop as a functioning male."

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