From Science News

   New research suggests that low oxygen in freshwater ecosystems can disrupt animal's endocrine systems. Researchers say this might explain the ongoing decline in some fish and amphibian species. Pesticides, components of plastics, and other chemical pollutants, mimic natural hormones such as estrogen. Scientists have linked such contaminants to reproductive failures and oddities ranging from deformities in frogs, to sex changes in fish. These disrupters are also suspected as possible cause of some breast cancer in women. Human activities that overload fresh water with plant nutrients, such as fertilizers and detergents reduce dissolved oxygen in lakes and rivers. It was found that oxygen-starved fish have an altered metabolism and remain smaller than normal. Adult carp, a species tolerant of low oxygen, were still found to have hormonal changes that inhibited sperm production. In female fish, the concentration of sex hormones was reduced along with egg production. The low oxygen halved the success of sperm at fertilizing eggs and slashed the number of eggs that hatched into healthy offspring. Rudolf Wu of the City University of Hong Kong said there is good reason to believe that low oxygen concentrations may also disrupt hormones in other fish and amphibians.