Passage Summary: Fishermen are blaming chemical companies for fish with weird fins because the chemicals dumped years ago are known to cause mutations.

Conclusion: The fin deformities in the local cod population were definitely caused by the chemical waste dumped into the bay.

Reasoning: Chemicals were dumped in the bay, those chemicals are known to cause genetic mutations, and a small percentage of the fish now have mutated fins.

Analysis: This argument establishes a correlation and a possible mechanism, but it fails to rule out other explanations. To evaluate the claim, we need to know if these deformities occur naturally or if other pollutants are present. Since this is an 'EXCEPT' question, look for the one answer choice that provides information irrelevant to the causal link between the specific chemicals and the specific fin deformities. We want to know if the chemicals *actually* did it, not just that they *could* have.