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Difficulty: HardWeaken

Marine Biologist: Over the past decade, the population of the silver-finned reef shark in the Salerian Sea has declined by nearly 40 percent. During this same period, commercial fishing operations have steadily increased their use of deep-water drift nets in the region. Since drift nets frequently entangle large marine predators, environmentalists have blamed the fishing industry. However, recent surveys show that the shark population has declined just as sharply in the northern marine sanctuary, where all commercial fishing has been strictly banned for twenty years. Therefore, the increased use of drift nets cannot be the primary cause of the shark's decline.

Which of the following, if true, most seriously weakens the marine biologist's argument?

  1. The silver-finned reef shark is a highly migratory species that spends several months each year foraging in the commercial fishing zones before returning to the northern marine sanctuary.Answer
  2. B
    A sudden shift in ocean currents has severely depleted the populations of smaller fish that make up the primary diet of the silver-finned reef shark across the entire region.
  3. C
    Illegal commercial fishing vessels have occasionally been spotted deploying drift nets near the outer boundaries of the northern marine sanctuary.
  4. D
    Several dead silver-finned reef sharks found washed ashore near the commercial zones showed distinct entanglement marks from deep-water drift nets.
  5. E
    The percentage of silver-finned reef sharks caught as bycatch in drift nets is significantly lower than the total number of other marine predators caught in the same nets.

Answer

The statement that the silver-finned reef shark is a highly migratory species that travels between the sanctuary and commercial fishing zones.
The correct answer exploits a crucial assumption in the marine biologist's argument: that the sharks living in the sanctuary stay in the sanctuary. If the sharks migrate into the commercial fishing zones for part of the year, they are exposed to the drift nets. This perfectly explains how drift nets could be the primary cause of the population decline even for sharks counted in the fishing-free sanctuary. By showing how the cause (nets) can still produce the effect (decline) despite the author's counter-evidence, this severely weakens the conclusion.

Step-by-Step Solution

1
Identify the conclusion of the argument.
The conclusion is that the increased use of drift nets cannot be the primary cause of the shark's decline.
This is the main claim the marine biologist is trying to prove based on the provided evidence.
2
Identify the premises supporting the conclusion.
The primary premise is that the shark population declined just as sharply in a marine sanctuary where fishing is banned.
Understanding the evidence helps identify the logical gap the author is relying on.
3
Identify the assumption connecting the premise to the conclusion.
The argument assumes that the sharks in the sanctuary are a distinct, isolated population that is never exposed to the drift nets used in the commercial zones.
If the sanctuary sharks are exposed to the nets elsewhere, the decline in the sanctuary could still be caused by those nets.
4
Select the option that attacks this assumption.
The option stating the sharks migrate between the zones directly attacks the assumption.
If they migrate, the sanctuary sharks are being killed by the nets when they leave the sanctuary, which perfectly explains why the sanctuary population is also dropping. This destroys the biologist's reasoning.

Key Concept

Identifying implicit spatial or temporal assumptions in causal reasoning.
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Topics

Weaken

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