To prevent a newly built dam on the Chiff River from blocking

Question

To prevent a newly built dam on the Chiff River from blocking the route of fish migrating to breeding grounds upstream, the dam includes a fish pass, a mechanism designed to allow fish through the dam. Before the construction of the dam and fish pass, several thousand fish a day swam upriver during spawning season. But in the first season after the project's completion, only 300 per day made the journey. Clearly, the fish pass is defective.

Which of the following, if true, most seriously weakens the argument?

(A) Fish that have migrated to the upstream breeding grounds do not return down the Chiff River again.

(B) On other rivers in the region, the construction of dams with fish passes has led to only small decreases in the number of fish migrating upstream.

(C) The construction of the dam stirred up potentially toxic river sediments that were carried downstream.

(D) Populations of migratory fish in the Chiff River have been declining slightly over the last 20 years.

(E) During spawning season, the dam releases sufficient water for migratory fish below the dam to swim upstream.

ID: 500276

VeritasPrepBrian · Answer

Good question - what's tricky with your hypothetical though is that we already know that the decline is much greater than 20%. The stimulus tells us that before the dam there were  several thousand  of fish that passed through that part of the river each day, and now we're down to  300 . So it's reasonable to say that we're looking at a 90+% decrease ("several thousand" has to be at least 3,000 and probably more), so "declined slightly" just can't be the explanation there.

AnthonyRitz · Answer

I agree with Brian's comment.

But beyond that, note that the timeline here is a little funky for D. The passage says "before the construction of the dam and fish pass" but D says "over the last 20 years." Since we have no reason to believe that the dam took 20+ years to complete -- and we cannot simply assume that it did -- the story of the drop-off would need to be even more abrupt (and thus even less explainable through "declining slightly" over a 20-year span).

Oh, and I don't really buy that "declining slightly over the last 20 years" could mean "declining slightly each year, so that over the 20-year span it adds up to a really substantial, not slight, decline." That seems like it's just twisting the language way too hard.

AashishGautam · Answer

I chose D, the reason being that since there has been a decline the population should decrease.

OG mentions that it doesn't explain the sudden decrease.

However, in my view, is there a sudden decrease at all?

300/day * 5 month season again runs in several thousands.

Can someone explain this!

Posted from my mobile device

GMATNinja · Answer

The passage tells us that before the dam was in place, "several thousand fish  a day  swam upriver during spawning season."

In the first season after the dam was completed, "only 300 per day made the journey."

If you take into account that several thousand fish  each day  dwindled to 300  each day , then yes, there is a sudden decrease in fish making the journey upstream.

I hope that helps!

zoezhuyan · Answer

dear experts,  DavidTutorexamPAL

GMATNinja ,  GMATNinjaTwo ,  VeritasKarishma ,  AnthonyRitz ,  CJAnish ,  MartyTargetTestPrep ,  AndrewN , VeritasPrepBrian 
 , fiftyoneverbal 
I was confused with B, 
 both other dams with passes had little decreases while Chiff River dam also with pass had significant decrease, that means both other rivers and Chiff River have possess, but have different decreases. so passes are not the reason led to decrease.

most of the posts say B strengthen, while I thought it is a weakness. OA is C, so I must miss something, but I have no idea, genuinely need you help.

thanks in advance

AndrewN · Answer

Hello,  zoezhuyan . Be careful in CR questions such as this one to stick to  exactly  what the argument says. This argument is simple:  the fish pass is defective . Why might that be argued? The passage simply tells us that the number of fish per day that swam upriver before the dam/fish pass project was completed versus after has decreased from  several thousand  to  300 . To weaken this argument, we need to look for information that allows for the possibility that  this particular  fish pass is  not  defective.

If, as choice (B) states, similar projects on other regional rivers have not led to significant decreases in the number of fish migrating upstream, then there may, in fact, be something problematic with this particular dam and/or fish pass. So, while we cannot eliminate the possibility that the dam is the problem rather than the fish pass, (B) leaves us wondering. It is worth noting that the article  the  in front of  fish pass  in the argument might have thrown you off. That is, we are not interested in whether the fish pass is defective in general, but in whether the fish pass on the Chiff River is defective. We cannot look to fish passes on other regional rivers to rule out the possibility that the one in discussion is faulty.

Meanwhile, choice (C) provides an alternative explanation, one that can logically help reconcile the numbers. If  toxic river sediments... were carried downstream  and the fish swim upstream, then perhaps these fish could not handle the toxins, and their numbers diminished  without respect to the fish pass . We still do not know for sure whether the fish pass is defective, but we have a much more compelling reason to doubt that such a contention is necessarily accurate.

I hope that helps. Thank you for thinking to ask me. (I enjoyed the question.)

- Andrew

CRACKGMATNUT · Answer

Just one thing here. How could one possibly conclude that these toxic substance were life threatening to fish. It could have been toxic for humans but not for fish. Even though I selected C because its more relevant than other option but still it draws no clear infrence and opens itself to some redundancy. To my belief this isnt an absolute weakner.

AndrewN · Answer

I agree,  Mayank2211333 , but you have to keep an eye on the question itself:

You said yourself that (C) "is more relevant," and if it makes the strongest case of the answers presented, you ought to choose it, plain and simple. In most CR questions, your goal is to find the most  reasonable  answer of the five, not one that presents an airtight case, and the closer you follow the linear logic of the passage—the less you bring in through outside association—the better off you will find yourself. (I used to overthink CR questions and find the answers somewhat arbitrary, but then I taught myself to follow the rules of GMAT™ logic.)

Good luck with your studies.

- Andrew

CRACKGMATNUT · Answer

Thanks Andrew. Can you help me with how to be aligned with linear logic? I mean that's like the biggest problem I'm facing with most of the CR question. Can you share those GMAT™ logic rules.

AndrewN · Answer

Hello again,  Mayank221133 . I would start with the  Ultimate CR Guide for Beginners , by  GMATNinja , as well as  Mod Nightblade's Quick Guide to CR Proficiency . Both posts have links to other helpful posts. These types of questions take a lot of review time for most people to understand, so when you practice CR questions, look them up in the forum to see which posts cut to the quick and put a finger on  exactly  why one answer choice stands alone as the correct one. Pay particular attention to what makes other answers incorrect. With consistent practice and a lot of review, you can align your thinking with this linear logic I keep bandying about in no time. Again, just appreciate the process of review. When you spend more time with questions, particularly in the beginning, you will set yourself up for success on subsequent questions; when you place an emphasis on completing more and more questions, you will simply make more and more mistakes, many of which will likely overlap, and you will exhaust valuable study material in the process.

If you come up with further questions along the way, feel free to reach out to the community. We want you to succeed.

- Andrew

muralis18 · Answer

KarishmaB   GMATNinja

In option C -> The construction of the dam also includes the 'Fish-pass' right? So why can't we say that the Fish-pass stirred up the toxic material leading to fewer fish swimming upstream?

In option E -> I figured, the dam releases sufficient water for migratory fish below the dam to swim upstream. so even though only 300 fish go through the fish pass, there are some fish that swim upstream below the dam via other way tallying into a total of 1000's. So eventually it weakens the argument that fish-pass is defective.

Please let me where I'm going wrong with this approach.

KarishmaB · Answer

Even if we include construction of fish pass in construction of the dam, it does weaken our conclusion that the fish pass is defective. 
It is the construction of the dam that released toxic chemicals downstream (that likely caused fish to die) and hence led to fewer fish swimming upriver. The fish pass may not be defective then. The reason for fewer fish could be that there are fewer fish.

I did not understand your logic for (E). We are given that after the dam, only 300 fish swam upriver. We are also given that the dam would have blocked the migration route but for the fish pass. So I am not sure what you mean by 'below the dam and tallying a total of 1000s'

ArjunKumar25 · Answer

Hello,
I am having a difficult time understanding why E could not be a valid answer.
The arguments states "several thousand fish a day swam upstream during SPAWNING season." The later part of the argument says only 300 per day made the journey. Clearly, the fish pass is defective. 
Now that option E tells us that "During SPAWNING season the dam release ...." . Now, the several thousand number of fishes was recorded during the Spawning season. In the first season after project completion 300 was recorded. My thinking

sk05 · Answer

@gmatninja

Why is A incorrect? It can so happen that because the fish that have migrated don't return, there are fewer fish in the river now.

Isn't assuming that wish died because of toxic substances going too far? Nothing is mentioned that fish can die because of that.

MartyMurray · Answer

Notice that the passage says the following:

Before the construction of the dam and fish pass, several thousand fish a day swam upriver during spawning season. But in the first season after the project's completion, only 300 per day made the journey.

We see that the passage is saying that, in general, "during spawning season," several thousand fish swam upriver each day and that, during the first spawning season after the project's completion, only 300 fish swam upriver each day.

So, the argument is supported by a comparison between what occurred in previous entire seasons and what occurred during the entire first season after the project's completion.

Presumably, what (A) says, "Fish that have migrated to the upstream breeding grounds do not return down the Chiff River again," applies to both the previous seasons and the first season after the project's completion. So, (A) does not affect the support for the conclusion provided by the comparison between what occurred during the previous seasons and what occurred the first season after the project's completion.

In GMAT Critical Reasoning, we can apply common knowledge in our analysis of answer choices.

So, since it's common knowledge that toxic substances could cause fish to avoid a river, have trouble swimming upriver, or die, we can take what choice (C) says as indicating that there may be an alternative reason for the decrease in the number of fish that swam upstream and that, therefore, the argument's conclusion that the fish pass is defective may not be correct.

GMATNinja · Answer

Sorry that I'm late to the party, but here are some thoughts in case it's helpful!

The "several thousand fish a day" going upstream happened  before  the introduction of the dam AND the fish pass. So we have:

Spawning season  before  dam and pass: several thousand fish per day.
 
 Spawning season  after  dam and pass: 300 fish per day.

The author blames the pass. (E) tells us that the issue is NOT the dam. This points the finger back at the pass, which is exactly what the author is arguing.

By eliminating a potential issues with the dam, (E) actually supports the author's argument. That's why we can eliminate (E).

I hope that helps!

pearrrrrrr · Answer

Hi   KarishmaB  @ MartyMurray  @GMATNinja

If choice (D) were to be  "   Populations of migratory fish in the Chiff River have been declining  slightly over the last 20 years  after the dam was built"    , will this choice weaken the argument?

DmitryFarber · Answer

pearrrrrrr

No, changing the time frame wouldn't help D. It's true that we don't know when the dam was built, so at least knowing that a decline happened after the dam was built could make D a bit more relevant. However, a slight decline in population still doesn't explain why the number of fish swimming upstream is just a small fraction of what it was before the dam was built. Even if it said that populations had dropped by 90%, there'd still be another problem with D--we wouldn't know the direction of causation. After all, if author is correct and the fish pass is defective, then maybe fish are trying to make it upriver to breed and can't. In that case, we'd expect to see a drop in population, and that would in no way undermine the idea that the pass is defective.

pearrrrrrr · Answer

Hi  DmitryFarber 
Thank you so much for your answer!

If I’m not wrong, you’re trying to say that lower fish population isn’t the reason to say that the fish pass isn’t defective. Then, I wonder why you think choice (C) is correct. Isn’t it because of fewer fish?

Quote from KarishmaB 
“(C) The construction of the dam stirred up potentially toxic river sediments that were carried downstream. 
This provides us a reason why fewer fish are swimming upriver. Perhaps because there are fewer fish now. The construction introduced toxic substances in the river which should have killed many fish. Hence, the fish pass may not be faulty. The problem may be that we have fewer fish now. 
Correct. “

Or in your view, choice C implies that there're fewer fish because the fish died, not because they can’t swim upriver to breed.

Posted from my mobile device

DmitryFarber · Answer

Exactly. C gives us an alternative explanation for the situation. Sure, if the fish died, there would be fewer of them. But C gives us a clear possibility of what may have  caused  that situation other than a defective fish pass.

To prevent a newly built dam on the Chiff River from blocking

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GMAT Critical Reasoning (CR) Questions

To prevent a newly built dam on the Chiff River from blocking

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615 to 715 in 15 Days (Ria's Strategies)

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