There would be a lot more tails swishing through the ocean space, and sharks would be twice their size.
Although cat diets would severely decrease and sandwiches would never be the same.
Bluefin would easily grow to 3 feet long, possibly longer. Perhaps even closer to 6 feet, eventually.
And maybe their carnivorous nature would evolve as they grew, and they're start climbing the food chain when the small fry don't cut it anymore. They'd end up gorging on their dolphin buddies and defending themselves against sharks.
And they'd just continue to eat without the worry of nets scooping them out of the sea.
With not a tin can in sight, the majestic Bluefin tuna, once prized for its protein packed meat, would dominate the open sea like a mob and continue its feast through the open tides, hopefully never inviting itself to dinner on dry sand.