Much of the debate about the world’s fish stocks has focused on over-fishing, but a more insidious threat has largely gone unnoticed until now. A study by researchers from City University in Hong Kong raises new concerns about vast areas of the world’s oceans, known as “dead zones,” that lack sufficient oxygen to sustain most sea life. Fish trapped in these zones often die, but the new study has found that these fish are also at a considerable reproductive disadvantage.
The study, appearing in Environmental Science & Technology, found that oxygen depletion, primarily caused by agricultural run-off and pollution, could spark the development of far more male fish than female, thereby threatening some species with extinction.
Led by Rudolf Wu, the researchers found that low levels of dissolved oxygen in seawater, also known as hypoxia, can induce sex changes in embryonic fish, leading to an overabundance of males. As these mostly male fish mature, it is unlikely they will be able to reproduce in sufficient numbers to maintain sustainable populations.
The researchers found that low levels of dissolved oxygen (less than 2 parts per million), down-regulated the activity of certain genes that control the production of sex hormones and sexual differentiation in embryonic zebra fish. As a result, 75 percent of the fish developed male characteristics. Wu said that with normal oxygen levels (around 5 parts per million), the usual sex ratio of zebra fish is about 60 percent male and 40 percent female. “Reproductive success is the single most important factor in the sustainability of species,” Wu added. “In many places, the areas affected by hypoxia are usually larger than the spawning and nursery grounds of fish. Even though some tolerant species can survive in hypoxic zones, they may not be able to migrate out of the zone and their reproduction will be impaired.”
Hypoxia occurs when excessive amounts of plant nutrients, such as fertilizers, run-off into the ocean. These nitrogen nutrients trigger the growth of algae and phytoplankton which deplete most of the oxygen in the surrounding water when they decompose. Such dead zones can develop naturally, but studies suggest that in many coastal areas, hypoxia is primarily caused by agricultural run-off and the discharge of domestic and industrial wastewaters. The United Nations Environmental Program estimates that nearly 150 permanent and recurring dead zones exist worldwide, including 43 in U.S. coastal waters.
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