Environmental ▼News Oxygen deficiency disrupts fish reproduction
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carpio) in two large vessels filled with normoxic (7 milligrams per liter [mg/L] O2) or hypoxic (1 mg/L O2) water. After two months of chronic exposure to limited oxygen, they found that estradiol serum levels in female fish and testosterone levels in male fish were only 20% of the sex hormone levels of fish in the ARS/USDA
seminal study conducted at the City University of Hong Kong shows that the sex hormone levels in fish exposed to low dissolved oxygen levels lead to impaired reproduction (Environ. Sci. Technol. 2003, 37, 1137–1141). Endocrine disruption by hypoxic stress may now have to be considered a serious environmental problem, because the oxygen budgets of large areas in coastal, estuarine, and inland water ecosystems worldwide have been adversely affected by eutrophication. Cultural or anthropogenic eutrophication arises when excessive amounts of nutrients, mainly from sewage and agricultural runoff, stimulate algal growth. The increase in algal biomass subsequently leads to more organic matter sinking into the benthic water layers. Bacteria decompose the organic matter at a lake’s bottom, consuming large amounts of oxygen. The breakdown of the dead algal cells also liberates ions, increasing the density of the water and thus impeding mixing with the more oxygen-rich surface waters. Hypoxic or even anoxic conditions are also encountered in the benthic layers of marine areas with poor water exchange. “Such areas extend over thousands of square kilometers on our planet and are thus several orders of magnitude wider spread than areas polluted by endocrine-disrupting chemicals,” stresses Rudolf Wu, first author of the study. Compared to other aquatic organisms, like crustaceans and molluscs, fish are the most sensitive to hypoxic stress. While reduction of food intake and growth in response to low oxygen levels is relatively well documented, the effects of hypoxia on fish reproduction and development have not been studied. Wu and his co-workers placed immature adult carps (Cyprinus
Researchers find that oxygen deficiency in carp may act as an endocrine disrupter.
normoxic control group. In addition, the percentage of total body weight that is sexual organs, which is a standard measure for sexual maturation, was less than half in hypoxic males and one third in hypoxic females. Reduced reproductive hormone levels and slower gonadal development also impaired the development of spermatids and egg cells. The eggs of 12-week old sexually mature hypoxic females contained less yolk, and the sperm of hypoxic males of the same age showed decreased motility. Lower gamete quality, in turn, resulted in a decline of important reproductive parameters, such as fertilization, hatching success, and 24-hour post-hatching survival rates of the offspring. Whereas 92% of the eggs produced by nor-
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moxic females gave rise to living offspring, the overall survival rate of eggs from oxygen-starved carps dramatically dropped to 4%. Impaired fish reproduction therefore could be an important mechanism accounting for the population decline and observed changes in fish species composition in hypoxic environments, the authors claim. Hypoxia is a new mechanism for endocrine disruption. Unlike other endocrine disruptors or “modulators”, which tend to bioconcentrate in the food chain, hypoxia acts as an environmental stress only on the organisms exposed to it, explains Charles Tyler, head of the Environmental and Molecular Fish Biology Group at the University of Exeter, United Kingdom. Another difference is that “the endocrine modulators specifically interact with the synthesis, perception, or transformation of steroid hormones, or mimic their actions, whereas aquatic hypoxia has broader effects as a general stressor,” says Tyler. He wouldn’t be surprised if, in addition to the changes in reproductive hormone levels, other parts of the fish endocrinological system would be altered, and expects increased stress hormone (cortisol) levels in hypoxic fish. “Also, other general stressors such as heat or space confinement have been shown to disturb the endocrinology of fish,” Tyler adds. However, no other environmental variable has changed so drastically in such a short period as dissolved oxygen. Although hypoxic and anoxic environments have existed throughout geological time, their presence appears to be increasing as a result of human activities. “Both cost-effective nutrient removal technologies, as well as adequate changes of farming and land use practices, are urgently required,” warns Wu. —ORI SCHIPPER © 2003 American Chemical Society
