Passage Summary: Scientists compared old museum birds to living ones and found more mercury in the new ones, leading them to believe the fish in the ocean are now more contaminated.

Conclusion: Mercury levels in saltwater fish are higher today than they were 100 years ago.

Reasoning: Feathers from modern seabirds contain double the mercury of feathers from birds preserved in the 1880s, and birds acquire this mercury by eating fish.

Analysis: The argument assumes that the mercury levels found in the 1880s feathers today are the same as they were when the bird was alive. If mercury decays over time or leaks out of preserved feathers, the comparison is invalid. It also assumes that the birds' diets haven't shifted to more mercury-heavy fish species over the last century. Look for an answer that ensures the old feathers are a reliable, unchanged record of the past.