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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?
| Text from Passage | Analysis |
| 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. |
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| Before the construction of the dam and fish pass, several thousand fish a day swam upriver during spawning season. |
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| But in the first season after the project's completion, only 300 per day made the journey. |
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| Clearly, the fish pass is defective. |
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The argument starts by explaining that a fish pass was built to help fish get around a new dam. It then compares the before and after numbers - thousands of fish per day before the dam versus only 300 per day after the dam and fish pass were completed. Based on this huge drop in numbers, the author concludes the fish pass must be defective.
The fish pass is defective.
The author uses a simple cause-and-effect argument: since fish numbers dropped dramatically after the fish pass was installed (from thousands to 300 per day), the fish pass must be the problem. The logic assumes that if the fish pass worked properly, fish numbers should have remained close to the original levels.
Weaken - We need to find information that would reduce our belief in the conclusion that the fish pass is defective
The argument makes specific quantitative claims (several thousand fish per day before vs 300 per day after) and a definitive quality claim (the fish pass is defective). We need to respect these numbers but find alternative explanations for the dramatic drop.
Look for alternative explanations for why fish numbers dropped so dramatically that don't involve the fish pass being broken. We need scenarios that show the reduction from thousands to 300 fish per day could be caused by something other than a defective fish pass.
This tells us that fish don't return downstream after migrating upstream. However, this doesn't help explain why fewer fish are making the upstream journey in the first place. We're concerned about the dramatic drop from thousands to 300 fish per day going upstream, not what happens after they reach their breeding grounds. This doesn't provide an alternative explanation for the reduced fish numbers, so it doesn't weaken the argument that the fish pass is defective.
This actually strengthens rather than weakens the argument. If other dams with fish passes in the region only caused small decreases in fish migration, but the Chiff River dam caused a massive decrease (from thousands to 300), this makes the Chiff River fish pass look even more defective by comparison. When similar technology works well elsewhere but fails dramatically here, it supports the conclusion that this particular fish pass has problems.
This provides a strong alternative explanation for the dramatic drop in fish numbers. If dam construction stirred up toxic sediments that flowed downstream, this could have killed fish or made them avoid the area entirely, regardless of whether the fish pass functions properly. The timing aligns perfectly - construction would stir up sediments, and toxic effects would be seen in the first season after completion. This directly weakens the argument by showing the fish reduction could be due to contamination rather than a defective fish pass.
A slight decline over 20 years cannot explain the dramatic drop from thousands of fish per day to only 300. Even with gradual population decline, we would expect to see perhaps hundreds fewer fish, not a reduction of \(\mathrm{90\%+}\) in a single year. This doesn't provide a sufficient alternative explanation for such a massive, sudden decrease in fish migration.
This tells us there's sufficient water for fish to swim upstream below the dam, but this doesn't address what happens when fish actually encounter the fish pass mechanism itself. Having enough water flow doesn't mean the fish pass structure, design, or operation is working correctly. Fish could still be unable to navigate through the actual fish pass even with adequate water levels.