Is this a sentence fragment?
Although fewer than twenty-five California condors remained in the wild in 1982, their numbers increased to more than one hundred sixty by 2014.

 - no
 - yes
This is not a sentence fragment. It is a complete sentence because it expresses a complete thought.
Although fewer than twenty-five California condors remained in the wild in 1982, their numbers increased to more than one hundred sixty by 2014.