Passage Summary: Scientists weighed a few fish caught by specific fishermen and concluded that those few fish represent the average weight of every single fish in the entire river.

Conclusion: The average weight of all trout in the Macawber River at the start of the season is approximately 1.6 kilograms.

Reasoning: In a sample of 90 fish, where each fisherman who caught at least two trout provided exactly two for weighing, the average weight was 1.6 kilograms.

Analysis: This argument suffers from a significant sampling flaw. The group of fish weighed is not necessarily representative of the whole river population because it only includes fish caught by humans, and specifically only from 'successful' anglers who caught multiple fish. Perhaps bigger, older fish are more likely to be caught, or perhaps the specific spots these anglers visited don't represent the whole river. You should look for an answer that identifies how this specific subset of trout might differ from the general population.