For most of Arlong's life, humanity has been a nuisance. They would come down to Fishman Island, seeking treasure, adventure, profit, and mermaids. They were ignorant and rude at best. At their worst, they were murderers, rapists, kidnappers, and slavers.
There were a few blissful years, Arlong could recall, when humanity's treacherous nature receded as a result of the Pirate King's influence. It did not go away fully, but it lessened enough that Arlong felt that maybe his baby sister would be able to have a life not steeped in fear and loathing.
Arlong is 19 when the Pirate King dies.
Arlong is 19 when the Great Age of Piracy begins.
Arlong is 19 when his country is flooded with weak, greedy, monstrous humans. Every single one of them is worse than the last, crass and unwashed and disgusting. Every single one of them commits crimes against fishmen. Arlong hates them more than anything.
The next few years are a blight upon Fishman Island. Towns are pillaged, women are raped, and families are ripped apart.
The Fishman District becomes overpopulated with orphans once again, but no one can tell how many there truly were. They were easy targets for slavers, and with no family there's no one to notice when they're gone.
People die and disappear without warning, and the future is bleak.
Until one day, Whitebeard comes. He comes with his battalion, swarming the underwater island with his ships. Countless vessels surround the kingdom, each armed to the teeth. There are whispers of a war between Whitebeard and the Ryugu Kingdom. The tension could be cut with a knife.
They shouldn't have worried. Whitebeard, more mountain than man, casts aside all of the pirates that plagued Fishman Island. He claims Fishman Island as his own territory, and with Neptune's blessing he promises to protect it as the Pirate King did.
The next few days are shaky. The seafolk are doubtful. Whitebeard's just a human. He's not even the King of the Pirates. Who is he to claim territory like that, and to promise to protect them?
The next group of pirates that step out of line are sunken by Whitebeard's First Division, and no one doubts any more. The humans, fearful of being the next one struck down, reign in their evil tendencies. Only the truly foolish dare to cause trouble.
Arlong still has to deal with shitty humans, but it's like those blissful years again. When there was hope of a peaceful life. He's far too jaded and scorned to believe it will last, but Whitebeard has earned Arlong's respect.
Whitebeard is the only human Arlong ever truly respects. Whitebeard protects Fishman Island for years, even when the seafolk kill any humans with prejudice. His protection is selfless. His protection is an armor. His protection never falters.
When Whitebeard dies, Arlong mourns with the rest of the world.