Does "extended over" mean "more than"?
The spawning migration from salt water to freshwater began in early August and extended over 5-6 weeks.
No. "over {time period}" can mean "during the time period".
If we say "I went camping over the weekend" it means "... during the weekend" (from Saturday to Sunday)
Similarly here "began in early August and extended over 5-6 weeks" means it started in the first week of August (very roughly) and lasted for five or six weeks (until early-mid September). As noted in a comment, "extended" is a verb.