Oh no, they change that much and sometimes even more. The hump is a sexual selection trait - the bigger the hump, the more likely it is that a male will get to mate. While not all pink salmon get a hump that large, the Barry Whites of the humpy salmon world have a great big shoulder boulder that makes the lady fish swoon.

Here’s one of those fishy freaks: www.landbigfish.com/articles/default.cfm?ID=352