Lack of oxygen stunts fish reproduction.

Seasonal hypoxia, when dissolved oxygen concentrations in water drop below 2 milligrams per liter, is a normal summer occurrence in estuaries. Over the past 20 years, however, pollution has increased the severity and frequency of hypoxia in waters worldwide. That trend could put a crimp in the reproductive capacity of coastal fish, new research shows.

Earlier laboratory studies indicated that in species that can survive hypoxia, reproduction shuts down (SN: 3/1/03, p. 132). Peter Thomas and his colleagues at the University of Texas Marine Science Institute in Port Aransas have now shown that some Atlantic croakers (Micropogonias undulatus) in the Gulf of Mexico have underdeveloped sperm and eggs. The fish were living in parts of Florida's Pensacola Bay that are hypoxic for extended periods each year.

"The extent of impairment was profound in comparison to what you normally find with pollution," Thomas says. "It was much worse, and it affects both sexes." Laboratory experiments showed that hypoxia disrupts hormones that stimulate reproduction, the researchers report online and in an upcoming Proceedings of the Royal Society of London B.

Shutting down reproduction is probably a survival strategy that croakers developed to cope with brief periods of hypoxia, Thomas says. The fish's reproductive problems could imply similar difficulties for other fish populations in areas such as the Chesapeake Bay as well as elsewhere in the Gulf.--S.W.