Frankly, the pitch Ginko makes his whistle at isn't quite right... But it's not a swear or an insult, so Joker figures it's fine if he keeps trying. He whistles back encouragingly before smiling. "Not fish!" he announces, some of the two words he'd been best acquainted with in the few days he's spent with Ginko.

'Not' has been especially helpful in ruling things out, honestly. It's more than a little clumsy, but it gets the job done. That's the best he could ask for.