Stray and spawn in nearby 35 streams low levels of

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stray and spawn in nearby (35) streams. Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be (40) repopulated should the fish there disappear. Yet high rates of straying can be problematic because misdirected fish may interbreed with the existing stock (45) to such a degree that any local adaptations that are present become diluted. Straying rates remain relatively low when environmental conditions are (50) stable, but can increase dramati- cally when streams suffer severe disturbance. The 1980 volcanic eruption of Mount Saint Helens, 19
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for example, sent mud and debris (55) into several tributaries of the Columbia River. For the next couple of years, steelhead trout (a species included among the salmonids) returning from the (60) sea to spawn were forced to find alternative streams. As a consequence, their rates of straying, initially 16 percent, rose to more than 40 percent (65) overall. Although no one has quantified changes in the rate of straying as a result of the disturbances caused by humans, there is no (70) reason to suspect that the effect would be qualitatively different than what was seen in the aftermath of the Mount Saint Helens eruption. Such a dra- (75) matic increase in straying from damaged areas to more pristine streams results in substantial gene flow, which can in turn lower the overall fitness of subsequent generations. -------------------------------------------------------------------------------- Q9: The primary purpose of the passage is to A. argue against a conventional explanation for the extinction of certain salmon populations and suggest an alternative B. correct a common misunderstanding about the behavior of salmon in response to environmental degradation caused by human activity C. compare the effects of human activity on salmon populations with the effects of natural disturbances on salmon populations D. differentiate the particular effects of various human activities on salmon habitats E. describe how environmental degradation can cause changes in salmon populations that extend beyond a numerical reduction Answer: -------------------------------------------------------------------------------- Q10: It can be inferred from the passage that the occasional failure of some salmon to return to their natal streams in order to spawn provides a mechanism by which 20
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