Skywalker,

Well my friend, these "gypsy" whales may have various reasons for hunting mammals vs fish -some logical other maybe.... They are quite competent in the brain department and sometimes make decisions on fun & ruthlessness but most often they hunt based on a probably a found niche over time. Kinda like how some farmers raise and slaughter cattle and others slaughter sheep for their wellbeing. Or maybe a better example would be grizzlies inland eat more berries, nuts, rodents and carrion and coastal griz or brown bears eat more salmon and some berries etc.! Somebody's got to keep those seals, sea lions, porpose etc. in check...Great white sharks are not found very often much north of our waters!
Good Fishing, Darin