According to the article, WHY did the U. S. Fish and Wildlife Service think the sturgeon was more than 100 years old?

Based on its girth and size.

Explanation: According to the U.S. Fish and Wildlife Service, the lake sturgeon which is currently an endangered species can live up to 55years for males and 70 to 100years for females.

However, the size and girth is an important criteria used by fish experts in deducing the number of years a sturgeon had used.

Thus, the caught sturgeon is about 7feet long and along with its girth suggested it had spent over a century roaming in the water body.