‘Dead Zone’ fish struggle to reproduce

U. TEXAS-AUSTIN (US) — Atlantic croaker living in the large Gulf of Mexico “Dead Zone,” off the coast of Louisiana, are showing signs of severe reproductive problems.

Researchers say there’s potential for long-term effects on the fish’s population abundance. Findings are reported in Proceedings of the Royal Society B.

Males were found to produce dramatically fewer sperm and females fewer eggs. In addition, females in the hypoxic Dead Zone waters were masculinized—some 20 percent produced sperm in their ovaries. The sex ratio was also heavily skewed toward males in the hypoxic area.

“This is the first evidence of impairment of reproductive output across a wide region in a fish population inhabiting a hypoxic coastal zone,” says study co-author Peter Thomas, professor of marine science at the University of Texas at Austin.

The occurrence of low oxygen (hypoxic) waters in coastal regions worldwide has increased dramatically over the past 25 years, but its impact on marine animals is just beginning to be understood.

The northern Gulf of Mexico “Dead Zone,” off the coast of Louisiana, is the second largest seasonal hypoxic zone in the world, averaging 6,447-square-miles in size annually. Coastal areas affected by hypoxia are often economically and environmentally important for maintaining fish populations and fisheries.

Thomas and co-author Md. Saydur Rahman collected and studied fish from sites throughout the northern Gulf of Mexico hypoxic zone and compared them with fish collected from areas with normal oxygen levels.

Masculinization of the female croaker and other reproductive problems were associated with declines in neuroendocrine function and ovarian and brain expression of an enzyme called aromatase that converts androgens (male sex hormones) to estrogens (female sex hormones). This was found to be the case in both field and laboratory experiments.

“Our results show that ovarian masculinization is a specific response to hypoxia and is due to decreased aromatase activity,” Thomas says.

Left: Ovaries in Atlantic croaker are smaller if the fish are found in the Dead Zone’s hypoxic waters. Right: Female croaker were found to produce sperm in hypoxic water. (Credit: University of Texas)

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