Two decades after the Emerald River Dam was built, none of the eight

Question

Two decades after the Emerald River Dam was built, none 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 degrees 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 true, would most strengthen the scientists’ hypothesis?

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

(B) Before the dam was built, the Emerald River annually 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 built was 34 degrees, whereas the lowest recorded temperature of the river after the dam was built has been 43 degrees.

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

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

shaselai · Answer

A. A says that the SAME species of fish are reproducing ~50degrees while the passage suggests the same species are not reproducing under the dam with temperature at 6 degrees. so raising the temperature up should get some reproduction going.

ykaiim · Answer

IMO A.

Premise - Decrease in temp >>> decline in fish population
Conclusion - Rise in temp causes fishes to reproduce.

Let'c check choices:
(A) The native fish species were still able to reproduce only in side streams of the river below the dam where the annual temperature range remains approximately 50 degrees.  Correct . ]

(B) Before the dam was built, the Emerald River annually overflowed its banks, creating backwaters that were critical breeding areas for the native species of fish.  Incorrect ]

(C) The lowest recorded temperature of the Emerald River before the dam was built was 34 degrees, whereas the lowest recorded temperature of the river after the dam was built has been 43 degrees.  Since the dam reduced the annual range of water temperature in the river below the dam from 50 degrees to 6 degrees.   Incorrect ]

(D) Nonnative species of fish, introduced into the Emerald River after the dam was built, have begun competing with the declining native fish species for food and space.  Incorrect ]

(E) Five of the fish species native to the Emerald River are not native to any other river in North America.  Incorrect ]

whamberto · Answer

I picked the answer A:

The conclusion is:  Scientists have hypothesized that sharply rising water temperatures must be involved in signaling the native species to begin the reproductive cycle.

A. This presents a strong reason to support their hypothesis. You are essentially performing a sensitivity test - you are just changing the temperature range and seeing how the fish react. All the other variables have been kept constant (ie. same fish, same river etc.). Therefore, if the same fish can reproduce in the same river with only a change in the temperature range, that is proof that the dam has changed the reproductive cycle.

B. First, I believe that this point is irrelevant because we are only concerned about the relationship between reproductive cycle in the river below the dam and the temperature of the river. Also, this answer choice could potentially point to another reason why reproductive cycles happened before the dam was built - in this case it would weaken the argument. In either case, this answer is incorrect.

C. This answer choice only gives you 2 specific temperatures. The first question I had was "where were temperatures recorded?" There is no explanation of that in this answer choice, so the temperature readings could actually have been taken before the dam - that would be irrelevant to this question since we are only concerned with the river after the dam. Also, we are only concerned with the  RANGE  of temperatures and not the temperature itself.

D. This could provide an alternate explanation as to why the fish are not reproducing. Therefore, this answer choice would actually weaken the argument.

E. What does this even tell us? This answer choice is irrelevant.

vnigam21 · Answer

Two decades after the Emerald River Dam was built, none 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 degrees 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 true, would most strengthen the scientists’ hypothesis?

(A) The native fish species were still able to reproduce only in side streams of the river below the dam where the annual temperature range remains approximately 50 degrees.
This supports what scientists have hypothesized.

(B) Before the dam was built, the Emerald River annually overflowed its banks, creating backwaters that were critical breeding areas for the native species of fish.
This option suggests that after the dam was built, Emerald river could not overflow and thus no backwaters created, thus it provides another cause for the breeding inadequacy of the native fish species of the Emerald river. It weakens.
Choice B undermines the hypothesis by suggesting a completely different hypothesis;

(C) The lowest recorded temperature of the Emerald River before the dam was built was 34 degrees, whereas the lowest recorded temperature of the river after the dam was built has been 43 degrees.
The argument is talking about the annual range of temperature, but this option is talking about the lowest recorded temperature. Therefore, just on the basis of lowest recorded temperature we cannot comment on the hypothesis of the scientists.

(D) Non-native species of fish, introduced into the Emerald River after the dam was built, have begun competing with the declining native fish species for food and space.
This option D relates a development after the native species began to decline, it does not bear on the hypothesis, which concerns the decline’s original cause;

(E) Five of the fish species native to the Emerald River are not native to any other river in North America.
Irrelevant. This option statement has no impact on the above scientist's hypothesis.

Can someone explain option C in more detail

AnubhavK · Answer

Correct choice is 'A' - The construction of a dam has significantly reduced the range of water temperatures in the river below the dam. Scientists have implicated this change in the failure of native fish species to reproduce adequately. Hence, statement 'A' strengthen Scientists' hypothesis.

adkikani · Answer

DavidTutorexamPAL   MartyTargetTestPrep   VeritasKarishma   nightblade354   GMATNinjaTwo   gmat1393

I faltered in understanding the argument and request your two cents.

Start with the question stem:

So, I need to strengthen the scientists's hypothesis which is:
sharply rising water temperatures must be involved in signaling the native species to begin the reproductive cycle.
Or, 
sharply rise in water temperatures led to fish begin their reproductive cycle.

But in above argument, final temperature is 6 deg and OE suggests that fish reproduce ONLY in temperatures at 50 deg C.

Does this mean building of dam led to reduction in reproduction of fish species?

DavidTutorexamPAL · Answer

Hey  adkikani ,

I think you may have missed the word 'range'.
The 'final temperature' is not 6 deg, rather the 'range of final tempratures' = max temp - min temp = 6deg.
So basically the hypothesis is that sharply increasing temps are required for reproduction, and the support is that now that the dam is built the range of temps is very small (so sharply increasing temp is an impossibility).
(A) supports this by saying that once you 'fix' the range of temperatures, you also 'fix' the reproduction.

EP2620 · Answer

Two decades after the Emerald River Dam was built, none 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 degrees 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 true, would most strengthen the scientists’ hypothesis?

Wow even though data says that this is a low difficulty question, I found the passage quite perplexing. What is the author even trying to say by mentioning:

1) Fishes are not reproducing "adequately"
2) The annual range of water temperate has reduced from 50 degrees to 6 degrees. So, maybe earlier if temperature used to vary from 50-100 deg, now it varies from 94-100 deg. This could also mean that earlier, if temp varied from 10-60 deg, it now varies from 25-31 deg

3) Hence, rising water temperatures "must be" giving some signals to fishes to reproduce

Wait what?! I couldn't really understand the reasoning in the first attempt...

Basically, statement 3 means that a sharply rising temperature (50-100 or 10-60)   would have told  the fishes to start reproducing  but since the water is fairly uniform now - temperature wise that is - the fishes don't get an external stimulus that tells them it's reproduction time! Hence, the population is not "adequate"

The scientists observe two changes are simply try to establish causality. Hmm seems lazy doesn't it!

Option A -  The native fish species were still able to reproduce  only  in side streams of the river below the dam where the annual temperature range remains approximately 50 degrees.  clearly strengthens this reasoning.

CEdward · Answer

Can someone transcribe the passage? I honestly had zero idea what this one was trying to say and I've done hundreds of CR questions.

mSKR · Answer

>> now fishes are not able to adequately reproduce because of dam.
Why?

>> because dam reduced the RANGE of temperature from 50 degree to 6 degree. It means now the range of water is lower. ( always say hot now 19 to 25 degree or 0 to 6 degree or 30-36 degree; range should be 6 ) fishes need 50 degree( say -25 to 25 degree) range of temperature to reproduce,

Scientists say that if we sharply increase water temperature then it would help native species to reproduce ( means ; currently say dam temperature range is 19 to 25 and sharply increase means 25 would become higher number , say 50; so the new range may become 19 to 50 ).As range become larger , there are chances that fishes would reproduce again.

A says: Fishes were able to reproduce in some water below dam where they could find range around 50 degree.
so it means 50 degree range still helps dolphins to reproduce even after dam was constructed. so wider range can play a significantly role for fishes to reproduce.

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Two decades after the Emerald River Dam was built, none of the eight

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Two decades after the Emerald River Dam was built, none of the eight

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