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Difficulty: MediumIdentifying Necessary Assumptions

Marine biologist: The population of the rare blue-finned reef fish in the Coral Bay sanctuary will likely begin to increase over the next few years. Last year, authorities implemented a strict ban on all fishing in the sanctuary. Since the ban took effect, the number of blue-finned reef fish caught by illegal fishers has plummeted. This critical reduction in human-caused mortality provides the species with the necessary conditions to replenish its numbers.

Which one of the following is an assumption required by the marine biologist's argument?

  1. A
    The elimination of fishing mortality is the only factor required to guarantee the full recovery of the blue-finned reef fish population.
  2. B
    The percentage of blue-finned reef fish in the overall catch of illegal fishers has decreased significantly over the past year.
  3. The recent plummet in the number of blue-finned reef fish caught by illegal fishers was not primarily caused by a sudden, severe decline in the fish's overall population.Answer
  4. D
    The number of illegal fishers operating in the Coral Bay sanctuary has decreased significantly since the fishing ban took effect.
  5. E
    The recent ban on fishing will completely eliminate all instances of human-caused mortality for the blue-finned reef fish.

Answer

The argument must assume that the sudden drop in caught fish was not merely the result of the fish population having already collapsed.
The argument concludes that the fish population will recover because the number of fish caught has plummeted, assuming this drop indicates a reduction in mortality pressure. However, if the catch dropped merely because the fish are nearly extinct and there are simply none left to catch, the conclusion falls apart. Thus, the argument logically depends on the assumption that the drop in catch is not just a symptom of a collapsing population. Using the Negation Test, if the plummet in catch WAS primarily caused by a severe decline in the overall population, the conclusion that the species is poised to recover is completely undermined.

Step-by-Step Solution

1
Identify the core structural components of the argument.
Conclusion: The fish population will likely increase. Premise: The absolute number of fish caught by illegal fishers plummeted after a fishing ban.
Isolating the premise and conclusion is the first step to exposing the logical gap in the reasoning.
2
Analyze the relationship between the premise and conclusion for logical leaps.
The author assumes that a drop in the number of fish caught means the fish are surviving and will reproduce. However, an alternative reason for the drop in catch could be that the fish are already gone.
A necessary assumption often functions to defend the argument by ruling out a fatal alternative explanation for the premise's evidence.
3
Evaluate the choices using the Negation Test.
Negating the correct option yields: 'The plummet in catch WAS primarily caused by a severe decline in the overall population.' If true, the fish are dying off, which directly destroys the conclusion that they will recover.
The Negation Test is the definitive method for proving whether an assumption is strictly required by an argument.

Key Concept

Identifying Necessary Assumptions (Ruling Out Alternate Explanations)
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Topics

Identifying Necessary Assumptions

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