Oxygen deficiency is an endocrine disruptor in fish

A lack of oxygen in waters around the world could be doing more than just suffocating fish: It may be acting as an endocrine disruptor and impeding their ability to reproduce, posing a serious threat to the survival of many populations.

A new study of carp suggests that hypoxia, or oxygen deficiency, is an endocrine disruptor. The findings add a surprising member to the growing list of potential hormone-disturbing agents — a list that includes pesticides such as atrazine and DDT, various types of steroids and metals, and even ultraviolet light. And because it occurs across vast stretches of water around the world, hypoxia could be a greater concern than any of these.

The research is reported in the March 15 print edition of Environmental Science & Technology, a peer-reviewed journal of the American Chemical Society, the world’s largest scientific society.

Nutrients such as nitrogen and phosphorous are essential for healthy marine and freshwater environments, but an overload of nutrients can trigger excessive algal growth (or eutrophication), leading to a decrease in oxygen dissolved in the water. Excess nutrients can come from a wide range of human sources, including runoff from developed land, soil erosion, agricultural fertilizers and sewage discharges.

“The severity and spatial scale of hypoxia have increased in the last few decades,” says Rudolf Wu, Ph.D., a professor at the City University of Hong Kong and lead author of the paper. “Due to rapid human population growth, increases in nutrient input and global warming, the problem is likely to become worse in the coming years.”

Hypoxia has been implicated in fish kills around the globe, from the Caspian Sea to the Gulf of Mexico — where an area of hypoxia forms during the summer months that can cover more than 6,000 square miles, according to the Environmental Protection Agency. But this is the first time hypoxia has been linked to reproductive problems.

The researchers found that long-term exposure to hypoxia can impair fish reproduction through a variety of channels, including endocrine function, egg quality, hatching rate, fertilization and larval survival.

Immature adult carp (Cyprinus carpio) were separated into two groups of 50 fish each. One group was reared under normal conditions and the other was placed in a hypoxic system, receiving about 15 percent of the oxygen given to the first group.

During the 12-week experiment, researchers measured a number of parameters to monitor the development of the fish. They noticed major changes in hormone levels in the hypoxic fish, reduced production of sperm and eggs, and significant decreases in the size and weight of the sexual organs of the fish. Rates of fertilization and hatching in the hypoxic carp were also well below those of the normal group, but the most striking differences occurred in the overall rates of survival: About 90 percent of the larvae produced by the normal fish survived the experiment, compared to less than five percent of the hypoxic group.

This drastic decrease in reproductive output could cause major declines in fish populations around the world, leading eventually to extinction for some species, Wu predicts.

“There is every good reason to believe that hypoxia may also cause endocrine disruption in other fish and amphibian species, as well as in other animal groups,” Wu says. Carp are naturally tolerant to hypoxia, which means that more-sensitive species may be affected by this lack of oxygen to an even greater extent. “There is also some evidence to suggest that humans living in higher altitudes [where the air is thinner and has less oxygen] tend to have lower fertility, although this may be caused by other confounding factors,” Wu continues.

Most cases of endocrine disruption occur on a local scale, in a particular river or estuary where chemical contamination is found. Hypoxia, however, occurs over vast bodies of water around the world. “Our findings show that the problem of endocrine disruption caused by hypoxia is far more serious than any chemicals, in terms of spatial scales and natural populations potentially affected,” Wu says.