What was the question for this implicit rationale, and corresponding answer?
Nothing about a man fishing by himself implies that he has caught 10 fish so far or that he would catch more or any at all.
 The answer: it is not possible to tell
The question: Premise: "A man by himself fishing in a river that runs through a city."
Hypothesis: "The man caught 10 fish so far."
Do we know that the hypothesis entailed by the premise?