What does Lamarck have to do with it? These fish haven't been passing down traits they've developed during their lifetimes - we've been killing all the big fish, so smaller fish are selectively left to breed. That's Darwinian evolution.
In normal situations, I'd imagine that bigger fish tend to reproduce more often. But when some external force (e.g., thousands of fishers in boats with GPS and big nets) changes things, you get a different outcome.
If we preferred to eat fish that were darker in colour, they'd be getting lighter instead.
Either TaeKwonDood misunderstands evolution, or rushed to post his article a little too quickly...