For the % increase of slapping the "most common", (to my knowledge) you just remove that % from the pool and recalculate the stats, so it's kinda redistributed.
ex : 50%-30%-18%-2% -> if you slap the 50% it'd become 60%-36%-4%

For slapping the "longest catch", that can complement the % if a long and a short are close, but keep in mind the % you may find probably don't take slapped fishes into acount.
You might also want to slap a short one sometime depending on the big fish window