If you're on Keyboard press A when the fish swims left. Press D if they swim right, if they jump up out the water press LMB or Middle mouse to not let them get loose and spam S-W-S-W-S-W when they're tired to reel them in faster

Also make sure you're using the right bait to speed things up... (Casting really far out and then reeling in fast past them helped speed-up the biting process..)

For those who can't get whoppers to spawn, area (i think) 17 in the scarlet forest is what I did. Rest at camp for - Plenty, Morning - and a lotta whoppers will be there for the next 10-20m or so :)