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Undocumented Flounder Population Decline Discovered

Louisiana Sea Grant-funded research has identified a previously undocumented southern flounder population decline that spans the entire species’ range, and researchers are recommending regional collaboration to better understand why.

“If we see a species declining throughout its range, that should be cause for concern that something might be changing,” said Kenneth Erickson, former Louisiana State University (LSU) graduate student on the project and current 2021 Sea Grant Knauss Fellow. “Given that there are many other important species with a similar life history that rely on estuaries, the decline of southern flounder could be a warning sign for other ecosystem changes.”

Southern flounder are an important recreational and commercial coastal flatfish species common to South Atlantic and Gulf of Mexico states. In 2017, the state’s recreational southern flounder harvest had dwindled to 124,000 pounds — down from a high of 624,000 pounds in 2013. Originally, one of the researchers’ objectives was to provide data to update the stock assessment in Louisiana waters. But lead researcher Steve Midway, an assistant professor in the LSU Department of Oceanography and Coastal Sciences, discovered there were few flounder to be found.

Intrigued, Midway and Erickson (his former student) began looking at southern flounder populations across the species’ range from North Carolina to Texas. More than four decades of records covering 34 estuaries provided clear indications that the population decline is not isolated just to Louisiana. “Our findings show a clear range-wide decline of southern flounder, suggesting that local factors are unlikely to be the driving force behind the drop,” said Midway.

The exact cause for the decline is still unknown, but environmental factors such as water temperature could be responsible – especially when it comes to the species’ male/female population ratio. Juvenile southern flounder determine whether to become male or female based on water temperatures. Moderate water temperatures tend to result in a 50/50 male/female population split. Warmer waters produce more males than females.

“Male-dominated populations are a concern because with respect to egg production, females are typically the more limiting sex,” said Midway. “So, we hypothesized that warmer winters would be correlated to flounder declines, which we did see in many of the estuaries we studied. Those correlations are clear and worth additional investigation.”

Erickson and Midway have started sharing their research findings with fisheries managers, including through the scientific journal Global Change Biology. The article was published online in mid-March and can be found at https://onlinelibrary.wiley.com/doi/10.1111/gcb.15568.

“More scientific research should be conducted to pinpoint the drivers of this population decline,” Midway added. “In the meantime, while some states have made regulatory changes in response to their fishery’s decline, there should be a larger, coordinated effort to address the problem,” he said. “By sharing our findings with fishery managers, we’re encouraging them to create a larger collective to address the problem.”