wow i didnt know they can get 15inches in only 1 half year.
Year and a half... Eighteen months. Normal growth is a half an inch a month, maybe a little more for a few months of the first year, combine that with a typical 3-4" fish, and you've got a 13-14" fish easy. If this guy was a quick grower, 15" isn't out of the question.
That being said, he looks more like a foot or 13" than 15" to me, but I suppose he could be 15".