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Reading Passage "Oxygen" Duplicated Below


The levels of dissolved oxygen in the world's ocean waters are declining precipitously. Just like humans, fish need oxygen to survive, and so in ocean areas experiencing significant levels of oxygen scarcity, or hypoxia, fish populations are plummeting. Although some hypoxic areas, called Dead zones, occur naturally, hypoxia in coastal areas and inland waters is caused mainly by agricultural run-off and by discharge of industrial wastewaters. More than one-hundred permanent dead zones, many covering thousands of square miles, exist worldwide today.

Since reproductive success is the most critical factor in the sustainability of any species, the extent of this threat to marine life and genetic diversity can hardly be overstated. After several months in hypoxic waters, female fish produce fewer eggs. Moreover, hypoxic conditions serve to alter the normal ratio between two particular hormones manufactured during the embryonic stage when a fish's gender is determined. During gestation and under these conditions, the mother produces more testosterone (and less estradiol), which inhibits the development of female reproductive organs and other female characteristics in the embryo, while promoting the development of male traits.

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