Fish hormones change when oxygen is scarce. (Sexual Hang-Up).

Oxygen deprivation tampers with sex hormones in fish and impairs reproduction, according to new research. The results suggest that low oxygen in freshwater ecosystems can disrupt animals' endocrine systems. Researchers say this link might explain the ongoing decline in some fish and amphibian species.

Various pesticides, components of plastics, and other chemical pollutants known collectively as endocrine disruptors mimic natural hormones such as estrogen. Scientists have linked such contaminants to reproductive failures in many animals and to oddities ranging from deformities in frogs to sex changes in fish. The disruptors are also suspected of underlying some breast cancer in women.

Researchers have found that other physical factors--including artificial lighting and magnetic fields--can similarly disrupt hormones (SN: 7/3/93, p. 10). Now, Rudolf S.S. Wu of the City University of Hong Kong and his colleagues add oxygen deficiency, or hypoxia, to the list of endocrine disruptors.

Human activities that overload fresh water with plant nutrients, such as components of fertilizers and detergents, reduce concentrations of dissolved oxygen in lakes and rivers. Wu and his colleagues found earlier that oxygen-starved fish have an altered metabolism and remain smaller than normal. The team wondered whether the almost suffocating conditions might also stunt fish reproduction.

To find out, the team placed immature adult carp--a species unusually tolerant of hypoxia--in lab conditions of either normal or one-seventh normal oxygen concentrations for 12 weeks. The researchers then ran the two groups through a battery of reproductive tests.

In a forthcoming Environmental Science and Technology, the team reports that hypoxia elevated concentrations of one form of estrogen in male fish and decreased testosterone. That hormonal shift inhibited testicular growth, as well as sperm production and motility.

In female fish, the same conditions reduced sex-hormone concentrations and egg production. Furthermore, the oxygen deficit halved the success of sperm at fertilizing eggs and slashed the number of eggs that hatched into healthy offspring.

"There is every good reason to believe that hypoxia may also cause endocrine disruption in other fish and amphibians," Wu says. "More sensitive species [than carp] may be affected to an even greater extent," he adds.

The researchers provide "good evidence that oxygen levels are an environmental factor that needs to be considered" for fish reproduction, says reproductive toxicologist James J. Nagler of the University of Idaho in Moscow. In some parts of the world, he adds, hypoxia might rival chemicals in its importance to endocrine disruption.

For cancer epidemiologist Richard G. Stevens of the University of Connecticut Health Center in Farmington, "the idea that oxygen [concentrations] could alter hormones is fascinating." Perhaps circumstances that reduce oxygen intake--for example, air pollution or high altitude--could affect people's hormone balance, he speculates.