OG-53

Two decades after Emerald River Dam was built, non of the eight fish species native to the Emerald River was still reproducing adequately in the river below the dam. Since the dam reduced the annual range of water temperature in the river below the dam from 50 degressa to 6 degrees, scientists have hypothesized that sharply rising water temperatures must be involved in signaling the native species to begin the reproductive cycle.

Which of the following statements, if turem would most strengthen the scientists' hypothesis?

A.  The native fish species were still able to reproduce only in side strems of the river below the dam where the annula temperature range remains approximately 50 degrees.

B. Before the dam was bulit, The Emerald River annualy overflowed its banks, creating backwaters that were critical breeding areas for the native species of fish.

C. The lowest recorded temperature of the Emerald River before the dam was bulit was 34 degrees, whereas the lowest recorded temperature of the river after the dam was bulit has been 43 degrees.

D. Nonnative species of fish, introduced into the Emerald River after the dame was built, have begun competing with the decling native fish species for food and space.

E. Five of the fish species native to teh Emerald River are not native to any other river in North America.

I was confused by OG 53,  can someone can explain the hypothesis "sharply rising water temperatures must be involved in ...." Since the paragraph just mentioned the change of RANGE of temperatue, reducing from 50 to 6, not  the absolute temperature.

Why OG said that "for the hypothesis to be tenable it is important that the fish in stream in the River area that retain a wide temp difference have not lose their ability to reproduce "

Thanks in advance.

Phenomenum: None of the eight fish species was still reproducing adequately in the river below the dam

Cause : Dam was built, the dam reduced the water temp from 50 degressa to 6 degrees (here mentions the temperature change)

Effect: sharply rising water temperatures must be involved in signaling the native species to begin the reproductive cycle, which means that rising water temperatures might help native species to reproduce.

I think that the Key is A

I didn't check OG, but as what you ask Why OG said that "for the hypothesis to be tenable it is important that the fish in stream in the River area that retain a wide temp difference have not lose their ability to reproduce", it might mean that the fish itself can't lose the ability to reproduce, the cause is not the Fish, but the water temp.

Hope it works!

I appreciated your response.

The passage only metioned the RANGE of water temperature reduced from 50 to 6.   For my understanding, the range of temperature, for instance, before the dam was built, was -20 ~ +30 degrees; after that, was -4 ~ +2 degrees.

Please correct me if it is wrong.  Thanks.

UP

in my opinion, what OG said -for the hypothesis to be tenable it is important that the fish in stream in the River area that retain a wide temp difference have not lose their ability to reproduce- is the assumption of the hypothesis.

without this assumption, there're plenty of other factors to affect the fish and then make fish not reproductive anymore.

second, the "50 degrees" range means a sharp rising water temperature.

take this for example.

before the dam is built, the temperature range is 50 degree, which mean probably in summer the fish suffer the high water temperature, and begin to reproduce. after the dam is built, however, the temperature range becomes only 6 degrees, then leading the fish no signal for the reproduction, a signal that mean sharply raising water temperature.

lip00ff2002 and vincent0330,

Thank both of you for help.

有点问题,答案A没有问题,可是我觉得D也是可以的,他说没有其他鱼和本土鱼竞争食物和空间,这不是典型的排除了他因解释的削弱,为什么不可以呢?请教.

conclusion: decrease in range of temperature leads to deline of natives' reproduction.

choice D: competition from nonnatives leads to the above result, although both after the construction of the Dam. It is a typical answer to weakening question.